ASHRAE Journal - February 2015 - 54

TECHNICAL FEATURE

motors are not so easy to get. The example in Figure 1 is
derived from Reference 2.
Section 440.22 of the electrical code provides rules for
sizing circuit breakers that serve refrigeration compressors so engineers do not have to gather data and plot
every case. Section 440.22 establishes the maximum
overcurrent protection device rating (MOCP) that the
code allows for motor-compressors.
440.22
(A) Rating or Setting for Individual Motor-Compressor
The motor-compressor branch-circuit short-circuit and
ground-fault protective device shall be capable of carrying the
starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor compressor
rated-load current or branch-circuit selection current, whichever
is greater, shall be permitted, provided that, where the protection
specified is not sufficient for the starting current of the motor, the
rating or setting shall be permitted to be increased but shall not
exceed 225 percent of the motor rated-load current or branchcircuit selection current, whichever is greater.
(B) Rating or Setting for Equipment
(1) Motor-Compressor Largest Load
Where a hermetic refrigerant motor-compressor is the largest
load connected to the circuit, the rating or setting of the branchcircuit short-circuit and ground-fault protective device shall not
exceed the value specified in 440.22(A) for the largest motorcompressor plus the sum of the rated-load current or branchcircuit selection current, whichever is greater, of the other motor
compressor(s) and the ratings of the other loads supplied.
Applying 440.22(B)(1) to the 10 ton rooftop unit, the
MOCP calculation looks like this:
Compressor #1
1.75 × 16.3 amps =
28.5 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
Maximum overcurrent device
68.2 amps
The standard breaker sizes in Section 240.6(A) present
a choice between 60 amps and 70 amps. Because Section
440.22 says "the rating or setting of the branch-circuit
short circuit and ground-fault protective device shall
not exceed" [emphasis added], the "rounding up" rule of
Section 240.4(B) does not apply. The calculated 68.2 amps
would have to be rounded down to a 60 amp breaker. The
time-current curve analysis shows that, in all likelihood,
a 60 amp breaker will trip when the compressor starts.
For that reason, Section 440.22 allows an increase up to
54

ASHRAE JOURNAL

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FEBRUARY 2015

225% of the motor-compressor rated load current. The
calculated MOCP for the rooftop unit becomes 76.4 amps,
making a 70 amp fuse or breaker a suitable choice.
As long as the wire is big enough to start the motor and
carry the load without overheating, why have an upper
limit on the fuse or circuit breaker size? For example, why
not use a #3 copper conductor and an 80 or 90 amp breaker,
just to be safe? The answer is that the installation would not
be safe. The larger the breaker size, the more current it can
handle for a given time before it trips, so the more potential
damage to the equipment where the short circuit flows.
To provide proper electrical protection, the fuse or circuit
breaker must open in case of an overload before the motor
or other device it is protecting sustains permanent damage.
After the fault is cleared, the equipment is supposed to be
ready to go back in operation. An oversized breaker might
not open before permanent damage occurs.

A Few Quirks
1. In most cases, the equipment manufacturer calculates and publishes the minimum circuit ampacity (MCA)
and maximum overcurrent protection device (MOPD) in
the product literature. Those values are also listed on the
equipment nameplate. Nameplate data trumps calculated or catalog data.
2. Most of the time, the overcurrent protection device
can be a fuse or a circuit breaker. Occasionally, the manufacturer's catalog or the equipment nameplate will say
"Maximum Fuse Size" instead of "Maximum Overcurrent
Protection" or "MOCP." In that case, unless there is a footnote that allows a circuit breaker, the MOCP must be a
fuse.
Sometimes the literature calls for "fuse or HACR
breaker." HACR (heating, air conditioning, and refrigeration) rated breakers have been specifically investigated and demonstrated to provide adequate protection
for HVAC&R equipment, which typically consists of one
or more compressors with other motors and electrical
equipment (such as internal wiring and controls). HACR
breakers often have faster response time to short circuits
than non-HACR breakers.
If a unit nameplate requires a fuse but the circuit feeding
it is protected by a circuit breaker, it is perfectly acceptable
to install a fused disconnect at the unit as supplementary
overcurrent protection. The conventional circuit breaker
(sized according to code rules) protects the conductors; the
fuse provides the faster action that the unit requires.

ASHRAE Journal - February 2015

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