Sizing Conductors, Part IV
by Charles R. Miller
Published: September 2011
Continuous Loads and Termination Temperature Limitations
Sizing conductors is not a difficult
task, but more is involved than referencing just one section, one
article or even one chapter in the National Electrical Code (NEC). To
select the correct conductor size, it is necessary to reference several
sections throughout the Code. Before referencing the NEC, certain
information is needed. Gather it by answering some key questions.
What is the lowest temperature rating of
any connected termination, conductor or device? What is the connected
load, or what is the calculated load in accordance with Article 220? Is
the load or any part of the load continuous? What will be the maximum
ambient temperature? How many currentcarrying conductors will be in the
raceway or cable? Will the conductors fall under the smallconductor
rules in 240.4(D)? Will the conductors be feeder taps or transformer
secondary conductors? Will the conductors supply any motors?
In the first two parts of this series, I
covered termination temperature limitations; the third part covered
continuous loads. Requirements for termination temperature limitations
are in 110.14(C). Continuous load requirements are in 210.19(A)(1),
215.2(A)(1) and 230.42(A).
Conductors that supply continuous loads
must also be sized to comply with the termination temperature provisions
in 110.14(C). For example, a branch circuit will be installed in a
nondwelling occupancy to supply power to a 16ampere (A) continuous
load. The circuit will be supplied from an existing 20A circuit breaker,
and the temperature rating is not marked on the breaker. What is the
minimum size THHN copper conductor required to supply power to this
branch circuit?
Because this branch circuit load is a
continuous load, multiply the load by 125 percent [210.19(A)(1)]. The
conductors must have an ampacity of at least 20A (16 125% = 20). Table
310.15(B)(16), formerly Table 310.16, provides allowable ampacities for
insulated conductors. Although a THHN conductor has a temperature rating
of 90°C, the ampacity must not exceed the ampacity in the 60°C column.
Section 110.14(C)(1)(a) covers circuits
rated 100A or less or marked for 14 AWG through 1 AWG conductors. While
it is permissible to install a conductor with a higher temperature
rating, the ampacity shall not exceed the 60°C ampacity of the conductor
size used [110.14(C)(1)(a)(2)]. If any termination is either 60°C or
unknown, the conductor’s maximum ampacity is listed in the 60°C column,
regardless of the insulation rating of the conductor. The ampacity of a
12 AWG conductor in the 60°C column is 20A. Conductors supplying power
to this branch circuit must be at least 12 AWG (see Figure 1).
It is not necessary to use the ampacity
from the 60°C column if the equipment is listed and identified for use
with such conductors [110.14(C)(1)(a)(3)]. This means the ampacity of a
75°C (or a 90°C) conductor can be based on the 75°C column if all the
terminations are rated at least 75°C. For example, a 208volt (V),
singlephase branch circuit will be installed in a nondwelling occupancy
to supply power to a 40A continuous load. The twopole circuit breaker
has a temperature rating of 75°C, and the termination on the other end
also has a rating of 75°C. What are the minimum size THHN copper
conductors required to supply power to this branch circuit? Because this
branch circuit load is a continuous load, multiply the load by 125
percent [210.19(A)(1)]. The conductors must have an ampacity of at least
50A (40 125% = 50). Because all of the connection points in this
example have at least a 75°C rating, the conductor’s ampacity can be
based on the 75°C column. The ampacity of an 8 AWG conductor in the 75°C
column is 50A. Conductors supplying power to this branch circuit must
be at least 8 AWG (see Figure 2).
While one branch circuit could supply both
continuous and noncontinuous loads, the combination of loads is more
often found in feeders and services. The minimum feedercircuit
conductor size, before the application of any adjustment or correction
factors, shall have an allowable ampacity not less than the
noncontinuous load plus 125 percent of the continuous load
[215.2(A)(1)]. After calculating the minimum ampacity, select a
conductor from Table 310.15(B)(16). The conductor selected must not
exceed the termination temperature provisions in 110.14(C). For example,
Article 220 was used to calculate the loads for a feeder in a store.
The calculation results for the continuous loads were 40A, and the
results for the noncontinuous loads were 47A. The feeder will be
supplied from an existing 100A circuit breaker and the temperature
rating is not marked on the breaker. What are the minimum size THHN
copper conductors required for this feeder? Start by multiplying the
continuous loads by 125 percent. The minimum ampacity for continuous
loads is 50A (40 125% = 50). Now add to this number the noncontinuous
loads (50 + 47 = 97). The conductors must have an ampacity of at least
97A. In accordance with 240.6(A), the minimum rating for this feeder is
100A. Because the temperature rating on the circuit breaker is unknown
and the circuit is not rated over 100A (or marked for conductors larger
than 1 AWG), the Table 310.15(B)(16) ampacity must not exceed the 60°C
column. The ampacity of a 2 AWG conductor in the 60°C column is only
95A, which is less than the calculated load. In the 60°C column of Table
310.15(B)(16), a 1 AWG conductor has an allowable ampacity of 110A.
Conductors supplying power to this feeder must be at least 1 AWG (see
Figure 3).
Requirements for serviceentrance
conductors are similar to the requirements for branch circuits and
feeders. The ampacity of serviceentrance conductors before the
application of any adjustment or correction factors shall not be less
than the sum of the noncontinuous loads plus 125 percent of continuous
loads [230.42(A)]. For example, Article 220 was used to calculate the
loads for a service in a store. The calculation results for the
continuous loads were 88A, and the results for the noncontinuous loads
were 130A. All the terminations will have a temperature rating of 75°C.
What is the minimum size THHN copper conductor required for these
serviceentrance conductors? The minimum ampacity for continuous loads
is 110A (88 125% = 110). Now add the noncontinuous loads to this number
(110 + 130 = 240). The conductors must have an ampacity of at least
240A. In accordance with 240.6(A), the minimum rating for this service
is 250A. Because of 110.14(C)(1)(b), the Table 310.15(B)(16) ampacity
must not exceed the 75°C column. At times, it is permissible to install
4/0 AWG conductors on a 250A service but not in this example. The
allowable ampacity of 4/0 AWG conductors is less than the calculated
load. In the 75°C column of Table 310.15(B)(16), a 250 kcmil conductor
has an allowable ampacity of 255A. Conductors supplying power to this
service must be at least 250 kcmil (see Figure 4).
If any of the conductors in the previous
examples were connected to overcurrent devices listed for operation at
100 percent of its rating, multiplying the continuous loads by 125
percent would not have been necessary: for branch circuits, see
210.19(A)(1) Exception; for feeders, see 215.2(A)(1) Exception No. 1;
and for serviceentrance conductors, see 230.42(A)(2).
Next month’s Code In Focus continues the discussion of sizing conductors.
MILLER, owner of Lighthouse Educational
Services, teaches classes and seminars on the electrical industry. He
is the author of “Illustrated Guide to the National Electrical Code”
and NFPA’s “Electrician's Exam Prep.” He can be reached at
615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.
