54 ELECTRICALCONTRACTOR | OCT. 14 | WWW.ECMAG.COM
COD E IN F O C U S BY CHARLES R. MILLER
While there are many ampacity
tables, Table 310.15(B)( 16) is used most
often. The ampacities shown in this table
are based on no more than three current-
carrying conductors in a raceway, cable
or earth (directly buried).
The ampacities listed in Table
310.15(B)( 16) are based on an ambient
temperature of 30°C. When there are
more than three current-carrying conductors, the conductor’s ampacity must
be adjusted. Likewise, if the ambient
temperature is something other than
30°C, the table ampacities shall be corrected. It may even be necessary to apply
both a correction factor and an adjustment factor.
Before selecting a conductor, the
load must be determined. Article 220
provides requirements for calculating
branch circuit, feeder and service loads.
As stated in 220.18, the total load shall
not exceed the branch circuit rating,
and it shall not exceed the maximum
loads specified in 220.18(A) through (C)
under the conditions specified therein.
The first specifications in this section
pertain to motor-operated and combination loads. The second sentence in
220.18(A) pertains to air conditioning
and refrigeration equipment. Where a
circuit supplies only air conditioning
equipment, refrigerating equipment or
both, Article 440 shall apply. Article 440
applies to electric motor-driven air conditioning and refrigerating equipment
and to the branch circuits and controllers for such equipment. Selecting
conductors and overcurrent protective
devices for air conditioning equipment
is not the same as it is for motors. With
a hermetic refrigerant motor-compres-sor, the nameplate on the equipment
usually provides the information necessary for selecting the conductor size
and the overcurrent protective device
rating. In accordance with 440.6(A),
it is permissible to use the nameplate
information in determining the rating or ampacity of the disconnecting
means; the branch-circuit conductors;
the controller; the branch-circuit short-circuit and ground-fault protection; and
Sizing Conductors, Part XLI
Air conditioning and refrigerating equipment
ARTICLE 310 IN THE NATIONAL ELECTRICAL CODE (NEC) covers general
requirements for conductors and their type designations, insulations, markings,
mechanical strengths, ampacity ratings and uses. Ampacities for conductors rated 0
to 2,000 volts (V) shall be as specified in the Allowable Ampacity Table 310.15(B)( 16)
through Table 310.15(B)( 19) and Ampacity Table 310.15(B)( 20) and Table 310.15(B)
( 21) as modified by 310.15(B)( 1) through (B)( 7) [310.15(B)]. Section 310.15(B) also
states that the temperature correction and adjustment factors shall be permitted to
be applied to the ampacity for the conductor temperature rating if the corrected and
adjusted ampacity does not exceed the ampacity for the termination temperature
rating in accordance with the provisions of 110. 14(C).
FIGURE 1 AIR CONDITIONING EQUIPMENT
In accordance with 440.6(A), it is permissible to use the nameplate information in
determining the rating or ampacity of the disconnecting means; the branch-circuit
conductors; the controller; the branch-circuit short-circuit and ground-fault protection;
and the separate motor overload protection.
An outdoor air conditioner will be installed. The minimum circuit ampacity shown on the
nameplate is 35.8A, and the maximum rating for the overcurrent device is 50A. The branch-circuit conductors supplying power to this air conditioner will be THWN copper, and the
conductors will be installed in a raceway. What is the minimum size conductor required
to supply this air conditioner and what is the maximum rating for the overcurrent device,
which will be a circuit breaker? The voltage drop will not exceed the NEC recommendation.
All terminations will be rated 75°C. The maximum ambient temperature will be 30°C, and
no more than three current-carrying conductors will be in the raceway.
Check Table 310.15(B)( 16) for a conductor that has an ampacity of at least 35.8A.
10 AWG (in the 75°C column) = 35A = Too small
8 AWG (in the 75°C column) = 50A ✔
Because the maximum rating for the overcurrent device shown on the nameplate is 50A, the
maximum rating for the circuit breaker is 50A.
The minimum size conductor required to supply this air conditioner is 8 AWG.
The maximum rating for the overcurrent device is 50A.
Continued on page 56 →