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Kaplan ITA's Monthly Inspection Tips - Free Electronic Newsletter

August, 2004
Conductors and OCPDs for A/C and Heat Pumps
Issue #28

Dear Inspector,

This month's tip focuses on understanding proper sizing of conductors and over-current protection devices (OCPD) for central air-conditioners and heat pumps.

The 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.

Sometimes, 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.

Minimum 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" rated insulation.

Minimum 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.

RLA (#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.

LRA (#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.

Hope this clears up any questions.

We'll talk next month,

Mike Casey
Kaplan Professional Schools
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