ASHRAE Journal - February 2015 - 56

TECHNICAL FEATURE

3. When replacing equipment on an existing circuit,
the new unit might not have the same MCA as the old
unit. If the new unit requires higher MCA, the conductors
might have to be changed. If the new unit has a smaller
MCA (which might be the case due to increased efficiency), it is usually OK to reuse the "oversized" conductors.
In most cases, equipment lugs are designed to accept up
to two wire sizes larger or smaller than the nominal size.
For example, a 60 amp circuit usually requires a #4 copper. Panel, circuit breaker, and equipment lugs would
normally accept wire as small as #8 or as large as #2.
If the MOCP of the new equipment is higher than the
existing breaker, the new unit might or might not start
without tripping the existing breaker. The breaker (and
maybe conductors) might have to be increased. If the
MOCP of the new equipment is smaller than the existing breaker, the breaker must be changed (to protect the
equipment) even if the conductors are "oversized." If the
existing wire size is OK for the existing larger breaker,
the breaker in the panel might not have to change. The
equipment can be protected with supplementary overcurrent protection (such as a fused disconnect) at the

unit, sized for the unit's MOCP.
4. Motors, like fan motors, have full load amp (FLA)
ratings. A motor's FLA is the maximum current it is expected to draw at full load and nominal voltage. The FLA
is used for sizing conductors and the thermal overload
devices (heaters) that protect the motor and conductors
against running overloads.
Refrigeration motor-compressors have rated load amp
(RLA) ratings. RLA for a motor-compressor is similar to
FLA for a motor but represents a slightly different function. Unlike motors, the current draw of refrigeration
motor-compressors depends on many factors, notably
suction and discharge pressures. The RLA of a motorcompressor is the current draw at the manufacturer's
rating point for the equipment.
RLA is not the same as the Branch Circuit Selection
Current described in code Section 440.22(C). Some
manufacturers mark equipment with a Branch Circuit
Selection Current, which is a recommended circuit
ampacity. RLA must be multiplied by the factors in
Section 440.22(A) or (B) (as discussed above) when using
RLA and FLA to determine minimum circuit ampacity.
5. Equipment ratings sometimes presume a specific
conductor material (copper or aluminum). Unless otherwise marked, the electrician must use copper conductors
for the final connections to the unit. When aluminum is
allowed, the lugs will be marked. The marking might be
an abbreviation, such as AL-CU or CO/ALR. The restriction to copper conductors, if present, applies only to the
termination the equipment, not to the entire circuit. A
unit that requires copper terminations could have aluminum conductors up to the line side of a disconnect at
the unit (assuming the disconnect is rated for aluminum
conductors) and copper conductors from the load side of
the disconnect to the terminals on the unit.
For more information on electrical topics of interest
to HVAC&R engineers, see "Practical Guide to Electrical
Engineering for HVAC&R Engineers" published as a supplement to the November 1996 issue of ASHRAE Journal.
Articles in that publication include more information on
circuit breakers, motors, and calculating heat gain from
electrical equipment.

References
1. Rosch, S. 1938. "The current carrying capacity of rubber insulated
conductors." Electrical Engineering 57(3): 155-167.
2. Zocholl, S.E. 2002 "Induction Motors: Part II - Protection." Schweitzer Engineering Laboratories, Inc. http://tinyurl.com/qc4xx7l.
www.info.hotims.com/54425-14
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