AC wire size

One of the most commonly misunderstood issues in your electrical system is what size wire and circuit breaker you need for your central air conditioner or heat pump.  In this post I’ll explain the issue.

To determine what wire size is needed all you have to do (and in fact this is the only way to determine the necessary wire size) is to look on the nameplate of the central air conditioner’s outside condensing unit.  The manufacturer is required to do the calculations and list the results as the “minimum circuit ampacity” (National Electrical Code NEC section 440.4).  This number is all you need to determine the wire size.

The condensing unit (the outside part) of a central air conditioner system has two motors.  One is the compressor motor and the other is for the fan that circulates air across the coils.  The rule for equipment like this with two motors is to require a wire size rated for 125% of the larger motor plus 100% of the smaller motor.  (NEC section 430.24)

Here’s a typical air conditioner nameplate for a three ton air conditioner.  We can see that the rated load amps (RLA) for the compressor motor is 14.1 amps (blue arrow).  We can see that the full load amperage (FLA) for the fan motor is 0.7 amps (blue arrow).  So we do the math and we get:

(14.1  x  1.25) + 0.7 = 18.325

So that’s what the manufacturer specifies for the minimum circuit ampacity (red arrow — they’ve rounded down).  In this case a 12 AWG (American wire gauge) copper conductor has that ampacity and so that would be the minimum wire size required.

The nameplate also lists the maximum circuit breaker or fuse size that’s allowed (green arrow).

For most electrical circuits in your house the point of the fuse or circuit breaker is to protect the wire.  The breaker size and wire size must match up so that if too much current flows through the wire then the circuit breaker will trip off.  The NEC has charts that define how much current can flow through a wire (the wire’s ampacity) and the breaker must be sized to protect the wire at that current rating.  This breaker or fuse is called the overcurrent protection.

But for an air conditioner, when the compressor motor starts, the initial surge of electrical current might be five to six times as much as during steady-state operation – enough to trip off a circuit breaker.  This surge only lasts for about 0.1 second, so it’s not enough to cause overheating in the wire but it might be enough to trip off the breaker.

So central air conditioners have their overcurrent protection built into the appliance, and this is what protects the wire from too much current.  It will also allow the large inrush of current at startup without tripping off.  The circuit breaker is only there to provide protection against short circuits and ground faults.  So the circuit breaker and the wire size don’t need to match up according to the typical NEC requirements that all the other circuits in your house must follow.

Again, the only way to know what size circuit breaker to use is to go outside and look at the nameplate on the air conditioner.  In our example here the manufacturer has specified a maximum circuit breaker of 30 amps (green arrow).  Normally this is too big to protect the 12 AWG wire, but the wire is protected by the air conditioner.

So for a central air conditioner the wire size and breaker size don’t have to match up according to the typical rules of the National Electrical Code.  You look at the nameplate and you follow the size requirements that the manufacturer has provided for wire size and circuit breaker size.

In our example above, the nameplate specifies a breaker that’s type HACR.  An HACR breaker stands for Heating, Air Conditioning, and Refrigeration.  It’s designed to have a time delay to allow for that initial surge of current at startup without tripping off.  It used to be that HACR breakers were specialty items and cost more than standard breakers.  But today almost all circuit breakers are HACR type and they’re not any more expensive.  This nameplate specifies an HACR circuit breaker, so that’s the type that must be installed.  There are rules outlined in the NEC that the manufacturer follows to determine their maximum breaker size.