ASHRAE Journal - February 2015 - 51

TECHNICAL FEATURE

insulation, and ambient temperature to determine conductor current carrying capacity. Rosch's calculation,
adjusted for factors like modern insulation materials,
remains the basis for the main ampacity table, Table
310.15(B)(16), in the NEC.
If keeping the insulation from melting were the only
consideration, we could all use 90°C (194°F) rated conductors and call it a day. However, panelboards, circuit
breakers, and contactors, have operating temperature
limits based, in part, on heat generated where the conductors terminate on those components. Under UL
listing conditions for electrical equipment for use in
ordinary locations (UL category AALZ), unless otherwise
noted, "the termination provisions are based on the use
of 60°C ampacities for wire size Nos. 14-1 AWG and 75°C
ampacities for wire size Nos. 1/0 AWG and larger." The
ampacity of higher temperature rated conductors can be
used as the base before applying adjustment factors for
ambient temperature and the number of conductors in
a conduit, but the resulting ampacity cannot exceed the
60°C or 75°C (140°F or 167°F) rating at the termination.

Minimum Circuit Ampacity (aka Wire Sizing Amps)
The circuit protection rules for conductors apply
directly to simple circuits like lighting and receptacle
loads. When a single circuit serves equipment, like HVAC
units, composed of several components, additional considerations apply. In simple terms, the circuit ampacity
has to be high enough for the unit to start and run all
components expected to run at the same time without
overheating the conductors that supply the equipment.
The NEC addresses this situation with Section 440.33:
440.33 Motor-Compressor(s) With or Without Additional
Motor Loads
Conductors supplying one or more motor-compressor(s) with
or without an additional load(s) shall have an ampacity not less
than the sum of the rated load or branch-circuit selection current
ratings, whichever is larger, of all the motor-compressors plus the
full-load currents of the other motors, plus 25 percent of the highest motor or motor-compressor rating in the group.
Although the sentence structure of Section 440.33 is
convoluted, the rule is straightforward: Take the largest
motor, add 25% to allow for the short-term load increase
when that motor starts, and add the remaining loads.
The circuit ampacity from Table 310.15(B)(16), after
applying any adjustments for factors like ambient temperature and number of current carrying conductors

TABLE 1 Conductor ampacity choices for 10 ton unit.
COPPER

ALUMINUM

WIRE SIZE

60°C

75°C

90°C

60°C

75°C

90°C

100

115

in the same conduit, must at least equal the calculated
amount.
For example, consider a 10 ton (35 kW) rooftop unit
operating at 208 volts. The unit has two compressors, two
condenser fans, a 3 hp (2.2 kW) supply fan, and a power
exhaust. The manufacturer lists a minimum circuit ampacity of 60.1 amps, which matches the following calculation:
Compressor #1
1.25 × 16.3 amps =
20.4 amps
Compressor #2
16.3
Condenser fans
2 at 3.5 amps
7.0
3 hp (2.2 kW) supply fan
10.9
Power exhaust
5.5
Minimum Circuit Ampacity
60.1 amps
Table 1 has some choices [extracted from Table 310.15(B)
(16)] to feed this circuit.
The smallest acceptable wire size for this circuit is
#4 copper or #3 aluminum. #6 copper or #4 aluminum
with 75°C (167°F) insulation is not a proper choice. The
conductor ampacity of 65 amps exceeds the calculated
minimum circuit ampacity of 60.1 amps, but remember that conductors #1 and smaller are limited to their
60°C (140°F) rating (unless the equipment is otherwise
marked) at the terminations.
Suppose the conductors will run through a hot environment that requires derating to 80% of table ampacity.
#6 copper with 90°C (194°F) insulation has an ampacity of 75 amps. Eighty percent of 75 amps is 60 amps, so
that choice fails on two grounds: the minimum circuit
ampacity is 60.1 amps (there is no provision to allow
rounding down), and a #6 cannot be terminated at more
than 55 amps. #4 copper with 75°C (167°F) insulation
(80% × 85 = 68 amps) is fine.

Maximum Overcurrent Protective Device (MOCP)
The minimum circuit ampacity calculation tells you
how big the wire must be to carry the starting and normal operating current for the circuit. The NEC also has
rules for sizing the fuse or circuit breaker that protects
the circuit from running overloads, short circuits, and
FEBRUARY 2015

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