neher-mcgrath

certification means that your underground installation meets the highest standard and has been designed or peer reviewed by a Registered Professional Engineer (P.E.) who specializes in the Neher-McGrath Calculation.

 

neher-mcgrath

certification tells the end user that there critical environments underground installation are within there temperature parameters and there is minimal I2R energy losses in there underground system.

neher-mcgrath

As Critical Environment operators, owners and designers we know there are more than enough things to be concerned about in our buildings. The underground cable need not be one of them.

neher-mcgrath

certification increases uptime in your critical invirimment while saving money due to heat losses in your system.

 

 

NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH A Growing Trend NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH NEHER-MCGRATH Since 1985, the use of underground service conductors in Canada has grown annually by 1%, on average. Despite greater installation costs that range from 3-1/2 to 6 times thosneher-mcgrathe of overhead installations, this ongoing trend is sometimes motivated by purely aesthetic reasons, or by very practical ones, especially in the wake of the devastating effects of the 1998 ice storm on Hydro-Quebec's grid. Given the ever-increasing number, and high cost of underground installations, makes understanding the underlying theory and principles of the Electrical Code in this respect all the more important. Conductors as a Heat Generator All electrical conductors, except superconductors, generate heat as a result of resistance losses (I2R), magnetic effects (skin effect and Eddy currents), and dielectric effects (heating of insulating materials by the electric field, especially in medium and high voltage conductors). Since the conductor current determines heat losses, it must be limited in order to protect the insulator from overheating, hence allowable ampacity. Determining the allowable ampacity of a conductor is analogous to calculating heat loading for a building. One first calculates the thermal resistance of the walls, ceilings, doors, and windows (R factor), then the thermal load needed to maintain a given interior ambient temperature relative to an exterior temperature. With conductors, one first determines the thermal resistances of the electric insulation, armour, jackets and air contained within the cable. The maximum allowable ampacity is the current value that will generate sufficient heat to bring the insulation to its maximum allowable temperature, namely 90°C for an RW90, or 75°C for an NS-1 construction. Heat Transfer is the Key Heat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat TransferHeat Transfer Calculating the maximum allowable ampacity requires an understanding of heat transfer mechanisms, namely how heat is transmitted from the conductor metal to the ambient air through successive insulating layers. In free air, heat is dissipated by convection, whereas conduction is the mechanism used in buried conductors (Figure 1). This explains why the same conductor has different allowable ampacities depending on the type of installation used. Heat dissipation underground depends on the depth of burial, and on the thermal conductivity of the surrounding backfill. It is estimated however, that for conductors sizes AWG #0 and smaller, heat dissipation takes place at the same rate whether underground or in free air. Hence, the same allowable ampacities are used in both cases. Unlike conductors in ambient air, ampacity calculations for underground conductors must include the thermal conductivities of backfill material, concrete, conduits, and ducts to determine the rate of heat conduction from conductor metal to the ambient air (Figure 2). Despite these differences, all allowable ampacity values found in the Code are determined using IEEE method 835, which in turn is based on the Neher-McGrath method (see insert). The method is used whether the installations are in ambient air (Tables 1, 2, 3 and 4), or underground (Appendix D). The Electrical Code on Underground Installations The Canadian Electrical Code deals with underground installations by providing two types of guidelines: NEHER-MCGRATH METHOD* NEHER-MCGRATH METHOD*NEHER-MCGRATH METHOD* NEHER-MCGRATH METHOD* NEHER-MCGRATH METHOD* The allowable ampacity from following general formula: I = Tc - (Ta + sTd) Rdc (1+Yc) x Rca Where Tc = maximum insulation temperature Ta = ambient temperature sTd = temperature rise due to dielectric losses Rdc = direct current electrical resistance (1+Yc) = skin and proximity effects Rca = total thermal resistivity between conductor and ambient air * J.H. Neher of the Philadelphia Electric Company and M.H. McGrath of the General Cable Company presented their method at the 1959 AIEE conference in Montreal. This method is a combination of the various methods used until then by electric utilities and cable manufacturers in North America. 1. Specifications for directly buried conductors are described in sections 6-300 (Underground Consumer's Services); 12-012 (Underground Installations); 12-112 (Conductor Joints and Splices); and in Table 53 (Minimum Cover Requirements for Direct Buried Conductors, Cables, or Raceways). 2. Allowable ampacities are addressed in sections 4-004 (1[d]), 4-004 (2[d])(Ampacity of wires and Cables), and tables of values are provided in Appendix D for the standard configurations shown in Appendix B, section 4-004. These include: Single Conductors Directly Buried in Earth (Diagram B4-1); Single Conductors in Underground Ducts (Diagram B4-2); Multiple Conductors Directly Buried in Earth (Diagram B4-3); and Multiple Conductors in Underground Raceways (Diagram B4-4). Changes to the latest edition of the CEC have made underground ampacities a sensitive issue for many installers, who sometimes question their pertinence. Understanding the underlying motivation behind these changes will help clear any misunderstandings. Unlike its earlier versions, the 18th edition of the Canadian Electrical Code no longer allows the use of underground allowable ampacities based on open-air values. Rather, the latest edition deals with underground installations distinctly and explicitly. Allowable currents are limited to the values specified by section 8-104 (7) (Maximum Circuit Loading), out of concern for protecting connected equipment. The values in Appendix D tables therefore never exceed: • 85% of the values shown in Tables 1 and 3 if the equipment is suitable for continuous service at 100% of the protective device's nominal current; • 80% of the values shown in Tables 2 and 4 or 75% of Tables 1 and 3 if the equipment is suitable for continuous service at 80% of the protective device's nominal current. Despite these guidelines, the Code still allows users to calculate the allowable ampacity with IEEE method 835 (see article 4-004 of the CEC). However, the value obtained must remain within the limits established by section 8-104 (7) mentioned previously. Method As the title of this article implies, determining the allowable ampacity and conductor sizes for underground installations is relatively simple. However, to use the tables provided in the Code, the installation dimensions must comply with configurations B4-1, B4-2, B4-3, and B4-4 shown in Appendix B4-004, since the Code tables are calculated using these specific dimensions. The following steps should therefore be followed (Figure 3): 1. First, determine the conductor metal used, for example, copper, aluminum or ACM. For copper conductors, refer to tables D8, D9, D13 and D15. When using aluminum or ACM conductors, refer to tables D10, D11, D14 and D16. 2. Next, determine whether the load is continuous, or non-continuous based on the criteria given in CEC 8-104 (3) (Maximum Circuit Loading). 3. Determine which ampacity tables to use in Appendix D, according to the type of load and equipment used. 3.1 For any equipment other than a circuit breaker, service box, fusible switch, circuit breaker, or panelboard, refer to the "A" tables, e.g. D8A 3.2 For a circuit breaker, service box, fusible switch, circuit breaker, or panelboard, refer to the "A" tables for non-continuous loads, e.g. D8A 3.3 For continuous loads refer to the "B" tables, e.g. D8B. Within these tables select the appropriate column according to the equipment nameplate ratings: use the 80% column if the nameplate indicates the equipment can carry 80% of the nominal continuous load, or the 100% column if it can carry 100% of the nominal continuous load. 4. To obtain the conductor size, select the specific ampacity table and column based on the installation configurations (B4-1, B4-2, B4-3, B4-3) shown in Appendix B. If the conductor size is less than AWG #0, use tables 1, 2, 3 and 4 (see heat dissipation mechanisms discussed previously). Please note that allowable ampacities found in Appendix D tables are for 90°C insulation temperatures. 5. Calculate the voltage drop. The conductor size obtained in step 4 above is only a baseline value. Voltage drop must always be calculated to determine if the selected conductor size will maintain a voltage drop within the allowable 3% limit when energized. The Electrical Code – Your Best Source of Information Although it may seem a long and tedious chore, a careful review of Appendices B and D will provide specific information that goes beyond the scope of this article. These chapters provide de-rating factors to apply when using 60° or 75°C insulation, or when the backfill temperature exceeds 20°C. Other provisions cover installations that use a different number of conductors than those shown in Appendix B. Summary The Electrical Code deals with the most practical aspects of underground installations, ranging from the allowable ampacity to the required conductor size. However, since the Electrical Code only serves as a guideline, only the most common configurations are presented. When using a configuration not shown in the Code, the installer can contact the local inspection authority, conductor manufacturer, or electric utility to obtain further technical assistance. Unfortunately, since these resources are often limited, much wasted time and frustration can be avoided by simply following the configurations shown in the Code to obtain the ampacities and conductor sizes in the tables provided. 1 Type of Conductor COPPER ALUMINUM 2 Type of Load Continuous Non Continuous Continuous Non Continuous 3 Type of equipment 4 Allowable ampacity According to Diagrams 5 Calculate the voltage drop, using the same tables or formulae as for installations in ambient air. Tables Tables Tables Tables D8B D8B D8A D8A D9B D9B D9A D9A D13B D13B D13A D13A D15B D15B D15A D15A 80% 100% Column Column Circuit breaker, service box, fusible switch, circuit breaker, or panelboard. Markings indicate what the equipment can carry. 80% of the maximum load 100% of the maximum load OTHER THAN A circuit breaker, service box, fusible switch, circuit breaker, or panelboard ANY circuit breaker, service box, fusible switch, circuit breaker, or panelboard B4-1 B4-2 B4-3 B4-4 circuit breaker, service box, fusible switch, circuit breaker, or panelboard ANY circuit breaker, service box, fusible switch, circuit breaker, or panelboard Circuit breaker, service box, fusible switch, circuit breaker, or panelboard. Markings indicate what the equipment can carry. 80% of the maximum load 100% of the maximum load When electrical duct banks with significant amounts of conduits and conductors are routed belowgrade, heating calculations are performed to determine if any conductor derating is required. Factors include the following:  Number and size of conduits and conductors  Configuration of the conduits and conductors  Horizontal and vertical space between conductors  Amount of earth above conductors  RHO factor and the amount of the backfill material  Load factor of the conductors  Actual design load. When electrical conductors overheat past their rated use, burning or degrading of normal insulation that protects conductors can precipitate a short circuit condition. Electrical conductor heating calculations can be complicated, but there are software programs for determining the potential derating of feeder conductors in large electrical duct banks. Any conceivable underground duct bank configuration can be input into the software. Additionally, various wire sizes can be used within the duct bank configurations. Load factor is the ratio of the average load in kilowatts supplied during a designated period to the peak or maximum load in kilowatts taking place in that period. Load factor, in percent, also can be derived by multiplying the kilowatt hours (kWh) in the period by 100 and dividing by the product of the maximum demand in kilowatts and the number of hours in the period. For example, load factor = kWh in period/kW. Assume a one-day billing period for a total of 24 hours. A customer uses 15,000 kWh with a maximum demand of 1,500 kW. The customer's load factor would be 41.6%: ((15,000 kWh/24 hrs/1,500 kW)*100). The 41.6% load factor represents a standard commercial building. The load factor in a data center would be significantly higher. Thermal resistivity, as used in materials the National Electrical Code annex, indicates the heat transfer capability through a substance in the trench by conduction. This value is the reciprocal of thermal conductivity and is typically expressed in the units C-cm/watt. NEC Section 310-15(b) indicates calculations to determine actual rating of the conductors, and provides a formula that can be used under "engineering supervision." But the formula typically is insufficient: It doesn't include the effect of mutual heating between cables from other duct banks. For distinctive duct bank configurations, a system designer must use the Neher McGrath calculation method, which involves many calculations and equations. In addition, many of these calculations build on one another, so an error in one part of the calculation can result in a significant error in the final outcome. The hand calculations become even more complex if cable in the duct bank are of different sizes. The NEC tables for underground duct banks are limited. If the RHO or load factor values are different from what is stated, then the tables do not apply. The actual configuration of the conduits within a duct bank can be manipulated to reduce any potential derating, by placing the conduits with the most amount of heat dissipation at certain locations within the duct bank or separating conduits that will emanate the most heat. According to Fourier's law, heat flux is proportional to the ratio of temperature over space. In the air space within the conduit—the only area within a duct bank that does not conduct heat—convection occurs in lieu of conduction. Because the main method of heat transfer within a duct bank is conduction, the air within the conduit will have less of an effect. One of the major components of this calculation is the RHO (see chart for typical RHO values for various types of fill). There are methods of analyzing the actual thermal characteristics of the soil, such as with a thermal property analyzer. Also, the duct bank installercan use engineered backfill with these characteristics specifically designed. All heat created by an underground electrical cable must be dissipated through the adjacent soil. This is identified by the soil thermal resistivity coefficient (or thermal RHO, °C-cm/ W). This value can typically fluctuate from 30 to 500 C-cm/W. The use of a soil thermal RHO of 90 C-cm/W is standard practice. However, this is conservative for most moist soils in the U.S. This RHO value is commonly used for electrical distribution cables when the native soil is reused as the backfill, but select backfill generally has a lower RHO than native soil. The ability of the soil in the direct area around the conduits to transmit heat from the electrical cables establishes whether an electrical cable overheats. Enhancing the peripheral thermal surroundings and precisely defining the soil and backfill thermal RHO values can result in a 10% to 15% increase in cable ampacity, with 30% improvements noted in some cases. Most damp soils have an RHO of less than 90 C-cm/W. Moist sands, which are frequently positioned around electrical distribution conduits, may even have an RHO of less than 50 Ccm/ W. The dilemma is that many soils, particularly homogeneous sands, may dry considerably when heated. On the other hand, the thermal RHO of a dry soil can exceed 150 C-cm/W, and possibly reach levels of 300 C-cm/ W. The dry thermal RHO of properly designed and installed thermal backfill should be less than 100 C-cm/W, potentially as low as 75 C-cm/W. Soils found in barren areas are dry. The assumption of a moist soil in the calculations is certainly not conservative. In certain parts of the country, the soils have a high inherent thermal RHO. Soil that is not properly compacted in the cable trench will be less dense and have a significantly elevated thermal RHO. Even typical 480-V electrical distribution or low voltage cables that are continuously under full load may dry the soil. Inadequately compacted trench backfill also can be an issue. The thermal RHO of this soil can be much higher. In addition, loose soil will dry more easily, which increases the possibility of thermal runaway in a domino effect. Cables that are in close proximity to heat producing equipment and infrastructure will experience elevated ambient temperatures and can operate at a hotter temperature. The same effect can be developed when other cables are in close proximity. This effect is known as mutual heating. In the following examples, the duct configuration is the same, but load factor and RHO factor change. Modifying the values changes the amount of current that can be pushed through the electrical conductors without exceeding temperature limits. The amount of derating can be significant. Examples 1 and 2. In the first example, a 4,000-amp duct bank with a design load of 3,600 amps is simulated based on an earth RHO factor of 90, dirt RHO factor of 90 and a load factor of 100 (Figure 2). Dirt RHO is same as earth RHO if native backfill is used directly around the conduits. If select backfill is used, dirt RHO might be less. Dirt RHO of less than 90 can lead to less heating and less potential derating. In this analysis, 16 sets of 4-in. conduits with three #600 MCM copper conductors are required to feed the 3,600-amp load, each set locked in at 225 amps each (16 x 225 = 3,600). This essentially is a 60% derating. The 16 sets of conductors with no derating would equal 6,720 amps (4,000 / 6,720 = 60%). The maximum temperature of the hottest conductor is 66.73 C. Per NEC, feeders should stay below 75.0 C. In Example 2 (Figure 3), I reduce the number of conductors to 14, and the temperature rises to 79.67 C. Because the temperature is above 75 C, the example is not code-compliant. amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—does not heat up to more than 75 C. This example illustrates that most electrical duct bank installations will typically not require any derating based on the heating calculations. Example 7. In this example, I use a 4,000-amp duct bank with a design load of 3,000 amps and increase the load factor to approximately 85%. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—will not heat up to more than 75 C. This is another example that illustrates that most electrical duct bank installations typically do not require any derating based on the heating calculations. Example 8. The intent of this example is to illustrate the effect of allowing a 90 C temperature of the wire. I do not recommend allowing the wire/termination to go above 75 C. In this final example, I use a 4,000-amp duct bank with a design load of 3,200 amps and increase the load factor to approximately 90%. I am assuming a maximum of 80% loading on the breaker (3,200 amps) and not using the 100% rating of the breaker and I am using the full 90 degree rating of the THHN/THWN conductor. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)— will not heat up to more than 90 C. If this example used a non-power circuit breaker, the maximum termination rating is only 75 C, so the example would not be applicable. Additionally, based on my coordination with the breaker manufacturers, they will not guarantee that the breaker termination can go to 90 C even if the actual load is equal to or less than 80% of the rating. Therefore, I recommend, in all cases, not exceeding 75 C. Although most installation may not require derating, these heating calculations may be critical to ensure that the designed electrical duct bank is adequate to serve the anticipated loads based on the assumed load factor, RHO factor and duct bank configurations. This is especially true for critical facilities such as data centers. A data center typically has a load factor between 90 and 100 and the load typically is managed to the peak design load. Additionally, in a data center application, the actual metered demand load could be very close to the NEC calculated load. Not all feeders routed in electrical duct banks are going to require derating. In fact, most will not. If the "design load" is less than the rating of the conductors and/or if the load factor is lower than 100, in many cases derating may not be required. In reality, most commercial installations have a load profile with a load factor of less than 50% and an actual current draw of somewhat less than the full rating of the conductors. Additionally, there are certain NEC sections that reduce the calculated effect of heating. Another simplec way to look at AHJ requirements and good engineering judgment with respect to NEC load calculations is that there is significant amount of conservatism built in. If you provide your load calculations based on NEC 220, with good engineering judgment, some of the conservatism could be applied to the potential derating from the heating calculations. In other words, if the load calculation indicates that you need 3,600 amps on a 4,000-amp service, the load realized once the facility is in operation is probably less than half of that number. Therefore you may be able to use 10 sets of #600 MCM THHN (420 amps each, per 310.16), for a total ampacity of 4,200 amps. You may be able to do this even if the conduits are in an electrical duct bank, because the negative effects of mutual heating at 1,800 amps or less will not be as bad as if one actually placed 3,600 amps through them. With all of these factors and criteria involved, it is important to evaluate each electrical duct bank to determine if heating calculations are required and if any derating will apply. Because of the complexity of the analysis, acquire the services of a design professional who uses the appropriate software. Additionally, the calculations should be performed under "engineering supervision" and approval of a licensed professional engineer amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—does not heat up to more than 75 C. This example illustrates that most electrical duct bank installations will typically not require any derating based on the heating calculations. Example 7. In this example, I use a 4,000-amp duct bank with a design load of 3,000 amps and increase the load factor to approximately 85%. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—will not heat up to more than 75 C. This is another example that illustrates that most electrical duct bank installations typically do not require any derating based on the heating calculations. Example 8. The intent of this example is to illustrate the effect of allowing a 90 C temperature of the wire. I do not recommend allowing the wire/termination to go above 75 C. In this final example, I use a 4,000-amp duct bank with a design load of 3,200 amps and increase the load factor to approximately 90%. I am assuming a maximum of 80% loading on the breaker (3,200 amps) and not using the 100% rating of the breaker and I am using the full 90 degree rating of the THHN/THWN conductor. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)— will not heat up to more than 90 C. If this example used a non-power circuit breaker, the maximum termination rating is only 75 C, so the example would not be applicable. Additionally, based on my coordination with the breaker manufacturers, they will not guarantee that the breaker termination can go to 90 C even if the actual load is equal to or less than 80% of the rating. Therefore, I recommend, in all cases, not exceeding 75 C. Although most installation may not require derating, these heating calculations may be critical to ensure that the designed electrical duct bank is adequate to serve the anticipated loads based on the assumed load factor, RHO factor and duct bank configurations. This is especially true for critical facilities such as data centers. A data center typically has a load factor between 90 and 100 and the load typically is managed to the peak design load. Additionally, in a data center application, the actual metered demand load could be very close to the NEC calculated load. Not all feeders routed in electrical duct banks are going to require derating. In fact, most will not. If the "design load" is less than the rating of the conductors and/or if the load factor is lower than 100, in many cases derating may not be required. In reality, most commercial installations have a load profile with a load factor of less than 50% and an actual current draw of somewhat less than the full rating of the conductors. Additionally, there are certain NEC sections that reduce the calculated effect of heating. Another simplec way to look at AHJ requirements and good engineering judgment with respect to NEC load calculations is that there is significant amount of conservatism built in. If you provide your load calculations based on NEC 220, with good engineering judgment, some of the conservatism could be applied to the potential derating from the heating calculations. In other words, if the load calculation indicates that you need 3,600 amps on a 4,000-amp service, the load realized once the facility is in operation is probably less than half of that number. Therefore you may be able to use 10 sets of #600 MCM THHN (420 amps each, per 310.16), for a total ampacity of 4,200 amps. You may be able to do this even if the conduits are in an electrical duct bank, because the negative effects of mutual heating at 1,800 amps or less will not be as bad as if one actually placed 3,600 amps through them. With all of these factors and criteria involved, it is important to evaluate each electrical duct bank to determine if heating calculations are required and if any derating will apply. Because of the complexity of the analysis, acquire the services of a design professional who uses the appropriate software. Additionally, the calculations should be performed under "engineering supervision" and approval of a licensed professional engineer amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—does not heat up to more than 75 C. This example illustrates that most electrical duct bank installations will typically not require any derating based on the heating calculations. Example 7. In this example, I use a 4,000-amp duct bank with a design load of 3,000 amps and increase the load factor to approximately 85%. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—will not heat up to more than 75 C. This is another example that illustrates that most electrical duct bank installations typically do not require any derating based on the heating calculations. Example 8. The intent of this example is to illustrate the effect of allowing a 90 C temperature of the wire. I do not recommend allowing the wire/termination to go above 75 C. In this final example, I use a 4,000-amp duct bank with a design load of 3,200 amps and increase the load factor to approximately 90%. I am assuming a maximum of 80% loading on the breaker (3,200 amps) and not using the 100% rating of the breaker and I am using the full 90 degree rating of the THHN/THWN conductor. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)— will not heat up to more than 90 C. If this example used a non-power circuit breaker, the maximum termination rating is only 75 C, so the example would not be applicable. Additionally, based on my coordination with the breaker manufacturers, they will not guarantee that the breaker termination can go to 90 C even if the actual load is equal to or less than 80% of the rating. Therefore, I recommend, in all cases, not exceeding 75 C. Although most installation may not require derating, these heating calculations may be critical to ensure that the designed electrical duct bank is adequate to serve the anticipated loads based on the assumed load factor, RHO factor and duct bank configurations. This is especially true for critical facilities such as data centers. A data center typically has a load factor between 90 and 100 and the load typically is managed to the peak design load. Additionally, in a data center application, the actual metered demand load could be very close to the NEC calculated load. Not all feeders routed in electrical duct banks are going to require derating. In fact, most will not. If the "design load" is less than the rating of the conductors and/or if the load factor is lower than 100, in many cases derating may not be required. In reality, most commercial installations have a load profile with a load factor of less than 50% and an actual current draw of somewhat less than the full rating of the conductors. Additionally, there are certain NEC sections that reduce the calculated effect of heating. Another simplec way to look at AHJ requirements and good engineering judgment with respect to NEC load calculations is that there is significant amount of conservatism built in. If you provide your load calculations based on NEC 220, with good engineering judgment, some of the conservatism could be applied to the potential derating from the heating calculations. In other words, if the load calculation indicates that you need 3,600 amps on a 4,000-amp service, the load realized once the facility is in operation is probably less than half of that number. Therefore you may be able to use 10 sets of #600 MCM THHN (420 amps each, per 310.16), for a total ampacity of 4,200 amps. You may be able to do this even if the conduits are in an electrical duct bank, because the negative effects of mutual heating at 1,800 amps or less will not be as bad as if one actually placed 3,600 amps through them. With all of these factors and criteria involved, it is important to evaluate each electrical duct bank to determine if heating calculations are required and if any derating will apply. Because of the complexity of the analysis, acquire the services of a design professional who uses the appropriate software. Additionally, the calculations should be performed under "engineering supervision" and approval of a licensed professional engineer amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—does not heat up to more than 75 C. This example illustrates that most electrical duct bank installations will typically not require any derating based on the heating calculations. Example 7. In this example, I use a 4,000-amp duct bank with a design load of 3,000 amps and increase the load factor to approximately 85%. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—will not heat up to more than 75 C. This is another example that illustrates that most electrical duct bank installations typically do not require any derating based on the heating calculations. Example 8. The intent of this example is to illustrate the effect of allowing a 90 C temperature of the wire. I do not recommend allowing the wire/termination to go above 75 C. In this final example, I use a 4,000-amp duct bank with a design load of 3,200 amps and increase the load factor to approximately 90%. I am assuming a maximum of 80% loading on the breaker (3,200 amps) and not using the 100% rating of the breaker and I am using the full 90 degree rating of the THHN/THWN conductor. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)— will not heat up to more than 90 C. If this example used a non-power circuit breaker, the maximum termination rating is only 75 C, so the example would not be applicable. Additionally, based on my coordination with the breaker manufacturers, they will not guarantee that the breaker termination can go to 90 C even if the actual load is equal to or less than 80% of the rating. Therefore, I recommend, in all cases, not exceeding 75 C. Although most installation may not require derating, these heating calculations may be critical to ensure that the designed electrical duct bank is adequate to serve the anticipated loads based on the assumed load factor, RHO factor and duct bank configurations. This is especially true for critical facilities such as data centers. A data center typically has a load factor between 90 and 100 and the load typically is managed to the peak design load. Additionally, in a data center application, the actual metered demand load could be very close to the NEC calculated load. Not all feeders routed in electrical duct banks are going to require derating. In fact, most will not. If the "design load" is less than the rating of the conductors and/or if the load factor is lower than 100, in many cases derating may not be required. In reality, most commercial installations have a load profile with a load factor of less than 50% and an actual current draw of somewhat less than the full rating of the conductors. Additionally, there are certain NEC sections that reduce the calculated effect of heating. Another simplec way to look at AHJ requirements and good engineering judgment with respect to NEC load calculations is that there is significant amount of conservatism built in. If you provide your load calculations based on NEC 220, with good engineering judgment, some of the conservatism could be applied to the potential derating from the heating calculations. In other words, if the load calculation indicates that you need 3,600 amps on a 4,000-amp service, the load realized once the facility is in operation is probably less than half of that number. Therefore you may be able to use 10 sets of #600 MCM THHN (420 amps each, per 310.16), for a total ampacity of 4,200 amps. You may be able to do this even if the conduits are in an electrical duct bank, because the negative effects of mutual heating at 1,800 amps or less will not be as bad as if one actually placed 3,600 amps through them. With all of these factors and criteria involved, it is important to evaluate each electrical duct bank to determine if heating calculations are required and if any derating will apply. Because of the complexity of the analysis, acquire the services of a design professional who uses the appropriate software. Additionally, the calculations should be performed under "engineering supervision" and approval of a licensed professional engineer amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—does not heat up to more than 75 C. This example illustrates that most electrical duct bank installations will typically not require any derating based on the heating calculations. Example 7. In this example, I use a 4,000-amp duct bank with a design load of 3,000 amps and increase the load factor to approximately 85%. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)—will not heat up to more than 75 C. This is another example that illustrates that most electrical duct bank installations typically do not require any derating based on the heating calculations. Example 8. The intent of this example is to illustrate the effect of allowing a 90 C temperature of the wire. I do not recommend allowing the wire/termination to go above 75 C. In this final example, I use a 4,000-amp duct bank with a design load of 3,200 amps and increase the load factor to approximately 90%. I am assuming a maximum of 80% loading on the breaker (3,200 amps) and not using the 100% rating of the breaker and I am using the full 90 degree rating of the THHN/THWN conductor. The earth and dirt RHO are again set at the standard 90. The standard feeder schedule for a 4,000-amp service—10 sets of 600 MCM copper conductors (10 x 420 amps = 4,200)— will not heat up to more than 90 C. If this example used a non-power circuit breaker, the maximum termination rating is only 75 C, so the example would not be applicable. Additionally, based on my coordination with the breaker manufacturers, they will not guarantee that the breaker termination can go to 90 C even if the actual load is equal to or less than 80% of the rating. Therefore, I recommend, in all cases, not exceeding 75 C. Although most installation may not require derating, these heating calculations may be critical to ensure that the designed electrical duct bank is adequate to serve the anticipated loads based on the assumed load factor, RHO factor and duct bank configurations. This is especially true for critical facilities such as data centers. A data center typically has a load factor between 90 and 100 and the load typically is managed to the peak design load. Additionally, in a data center application, the actual metered demand load could be very close to the NEC calculated load. Not all feeders routed in electrical duct banks are going to require derating. In fact, most will not. If the "design load" is less than the rating of the conductors and/or if the load factor is lower than 100, in many cases derating may not be required. In reality, most commercial installations have a load profile with a load factor of less than 50% and an actual current draw of somewhat less than the full rating of the conductors. Additionally, there are certain NEC sections that reduce the calculated effect of heating. Another simplec way to look at AHJ requirements and good engineering judgment with respect to NEC load calculations is that there is significant amount of conservatism built in. If you provide your load calculations based on NEC 220, with good engineering judgment, some of the conservatism could be applied to the potential derating from the heating calculations. In other words, if the load calculation indicates that you need 3,600 amps on a 4,000-amp service, the load realized once the facility is in operation is probably less than half of that number. Therefore you may be able to use 10 sets of #600 MCM THHN (420 amps each, per 310.16), for a total ampacity of 4,200 amps. You may be able to do this even if the conduits are in an electrical duct bank, because the negative effects of mutual heating at 1,800 amps or less will not be as bad as if one actually placed 3,600 amps through them. With all of these factors and criteria involved, it is important to evaluate each electrical duct bank to determine if heating calculations are required and if any derating will apply. Because of the complexity of the analysis, acquire the services of a design professional who uses the appropriate software. Additionally, the calculations should be performed under "engineering supervision" and approval of a licensed professional engineer lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn lanecoburn vvvlanecoburn lanecoburn lanecoburn