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e 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
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