month's tip focuses on understanding proper sizing of conductors
and over-current protection devices (OCPD) for central air-conditioners
and heat pumps.
first thing we must realize is that the OCPD designations
from the standard branch circuit sizing table in section
310-16 of the National Electrical Code (NEC) we all have
probably memorized do not apply to hermetically sealed motors
(air conditioning and heat pumps). There is an exception
in section 440-4B of the NEC. The circuit conductor and
OCPD are sized per the appliance nameplate, NOT the tables
we all know. The reason for this exception is the startup
inrush current demanded to start the motor because it always
starts against a load (there is always pressure in the system).
Inrush current can be twice or more the RLA (run load amps)
for a few cycles (there are 60 cycles per second in alternating
current), say a fifth of a second or 12 cycles. This is
the reason OCPD's are "oversized." We must also
realize that conductor insulation may have a withstand capacity
of several to many seconds for current much higher than
its rating. With these facts in mind I have provided an
example of a typical nameplate and we will discuss the relevant
numbers to examine.
in the model number (number one in the photo) it is possible
to determine the BTU rating (1 ton of cooling = 12,000 BTUs)
by a number divisible by 12 in the model number. Judging
by the large run load amps and the 60 in the model number
this is a 5-ton unit (60/12 = 5). Sometimes the number is
divisible by 6, such as 30, indicating a 2.5 ton unit. This
has nothing to do with conductor sizing for home inspectors.
circuit ampacity (#2 in the photo) tells the electrician
what size conductor to use. In this case the minimum ampacity
is 38.0. Since there is no standard 38 amps conductor we
must go to the next size up which would be 40 amps per the
branch circuit table. This means the minimum size conductor
would be #8 copper or #6 aluminum for standard "TW"
and Maximum Fuse/Breaker HACR (heating, air-conditioning
refrigeration) (#3 in the photo). Many times there is a
difference between minimum and maximum sizes, in this case
none. The 60 AMP rating means the maximum or minimum size
OCPD (breaker or fuse) to be installed to protect the A/C
circuit with the 40-amp conductor. Yes, it is Code-complying
to connect 40 amp wire to a 60 amp breaker in the case of
air-conditioning and heat pumps. Section 440 of the NEC
allows up to a 225% over sizing of the OCPD.
(#4 in the photo). This is the Run Load Amps of the compressor
motor. This is normal operating amperage once up to speed,
not at startup.
(#5 in the photo). This is Locked Rotor Amps of the compressor
motor. This means amperage if the motor is locked (can't
turn due to malfunction) and not operating. As you can see
the OCPD will trip very quickly at LRA, or if current even
makes it that high.
this clears up any questions.
talk next month,
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