Wiring air conditioning and heat pump motors
01/01/2009
There can be confusion among mechanics, electricians and engineers over the wiring/ breaker needs for air conditioning and heat pump applications. Proper sizing of the wires and breakers or fuses are covered in Table D12 of the Canadian Electrical Code. It shows the wire size and breaker (or fuse) to use with all motors including hermetic compressor motors. However, the technician should keep in mind that wire sizes are based on 125% of the motor full load current (FLA), NOT the breaker size. The code requires two over-current protection devices. It reads: Each motor has to have an over-current device, (thermo trip device) and an over-current device (fuses or breaker). Fuses if used, are shown in table D12 for motor full load amps, in both time delay and standard fuses. The proper breaker size is shown if fuses are not used. The motor overload device is normally selected at 125% of FLA of the motor. The breaker or fuse doesn't determine the wire size on a motor circuit as the overload device limits the maximum current (amps) of the motor circuit. Contactors and starters should be selected on their horsepower (hp) rating, not their full load amps. When there is more than one motor in a unit, the amperage draw, as far as Table 12 is concerned, is based on the largest motor load times 125 percent plus the name plate amps of the remaining motors. When it comes to non-motor loads such as lighting, electric heat, etc., the breaker size determines the wire size, and the connected load must not exceed 80 percent of the breaker rating
