Chapter 12 Branch Circuits and Feeders 155
Copyright Goodheart-Willcox Co., Inc.
An air-conditioning branch-circuit load is determined
from the data provided on the nameplate affixed to the
unit. The nameplate will list the phase, voltage, frequency,
full-load current, and other pertinent information of the
hermetic motor compressor. The full-load current rating
shown on the nameplate serves as the basis for deter-
mining the branch-circuit conductor size, overcurrent
protection requirement, controller rating, and disconnect
size.
Sometimes the branch-circuit current rating is also
included on the nameplate. In such instances, use the
larger of the branch-circuit current rating or full-load
current rating for sizing the circuit. If a unit has two or
more motors, then the circuit ampacity must be computed
at 125% of the largest motor plus the sum of the other
motors. For a single-motor unit disconnect, use 115% of
the full-load current.
NOTE
A disconnecting means must be within sight of the unit
and it must be correctly sized at 115% of the total current
rating.
The overcurrent protective device cannot exceed
175% of the motor full-load current or branch-circuit
rating, whichever is larger. This can be increased to 225%
if the motor will not start or come up to full speed without
tripping an overcurrent protective device rated for 175%.
The label may include the rating of the motor in horse-
power. This rating must be converted prior to determining
the branch-circuit load rating. Use Tables 430.247 through
430.250 for converting horsepower to amperes. These
tables are contained in the Reference Section at the end of
the text.
Sample Problem 12-11
Problem: For an air-conditioning unit with a three-
phase, 230-volt, 20-hp motor, what is the required
size of the branch-circuit overcurrent protective
device?
Solution: Use Table 430.250 to convert horse-
power to amperes. 20 horsepower with a 230-volt
supply is equal to 54 amps. This load must be
increased by 125% for the conductor ampacity:
Conductor ampacity = 54 A × 1.25
= 67.5 A
The next highest standard overcurrent protective
device rating is 70 amps.
Feeders
The conductors between the service equipment and
the branch-circuit overcurrent devices are called feeders.
Article 215—Feeders provides information regarding the
safe and adequate sizing and installation of these conduc-
tors. This article also applies to subfeeders, which provide
power to branch-circuit panels but originate at power
distribution centers rather than the service equipment.
Feeder loading is dependent on the total power
requirement of the system. If it is possible for all
connected loads to operate simultaneously, then the
feeder must be of sufficient ampacity to meet that
demand. If only 75% of the connected loads will ever be
operating at the same time, then the feeder would be sized
larger than the service conductors.
Prior to installation, certain factors must be consid-
ered to ensure the feeder size, type, and overcurrent
protection is correct for the application:
• Material—The feeder can be copper or aluminum.
• Location—The environment around the feeder
(damp, hot, corrosive) must be considered.
• Wiring method—The feeder can be run in
conduit, cable trays, or other systems.
• Cable—Single or multiconductor cable can be
used.
• Type—Feeders can be paralleled or individual.
• Length—Voltage drop becomes a consideration
in long feeders.
• Derating factor—Conductor sizing includes
several factors: conduit fill, ambient temperature,
and connected load demand.
• Neutral—A neutral wire may not be necessary
with the feeder.
• Demand factor—Continuous loads will affect
feeder size.
• Protection—Various overcurrent protective
devices can be used with feeders.
Summary of Commercial Service Load
Computation Procedure
1. Determine the general lighting load based on the total
square footage multiplied by the volt-ampere load
indicated in Table 220.12.
2. If the lighting is a continuous load, as is true of most
commercial lighting, increase the load by 125%. That
is, multiply by 1.25. (Section 210.20(A))
3. For feeder and service loads, use Table 220.42 and
apply the lighting demand factor required for the type
of building.
4. After computing both air-conditioning and heating
loads, omit the smaller of the two. (Section 220.60)