# Conductor size for A/C breakers

The house has two Condensing units- 2.5 ton and 4 ton on 30 amp and 45 amp breakers, respectively, in the panel. The 30 amp had #6 copper conductor and the 45 amp had #10 AWG conductor. I wrote it up in the report as improperly sized conductors. It appeared the breakers should have been switched to accommodate the conductor size.

The builder’s electrician says it is ok, citing NEC 440.22 - not exceeding 175% …

How does that rule apply to the improper conductor size for each breaker?

The conductors are sized based on the Minimum Circuit Capacity (MCA) listed on the nameplate of the unit. The OCPD or circuit breaker can be sized up to the Maximum OCPD listed on the unit. The conductor size and the OCPD size do not have to match. The OCPD is only providing short circuit and ground fault protection, the conductors are protected from overload by the thermal overloads built into the unit.

Here’s an example of a nameplate where the conductors can be #10 and the CB protecting the circuit can be up to 45 amps.

Thanks Robert. I get it on the 45 amp breaker with #10, but how about the 30 amp breaker on #6? The MCA on that unit would have been around 15-20 amps.

Robert,

What happen to the circuit breakers protects the conductors…

The AC unit will have over load protection built in the unit that protects the conductor

The inrush current of a motor will be about six times its running load current so to install a smaller breaker when the unit started the breaker would trip and the unit would be useless.

When dealing with inductive loads such as motors the general rules of 15 for #14, 20 for #12, 30 for #10, and so forth goes out the window.

430.32 says the over load device is to be set at a certain level that will protect the motor and conductors from heat due to over current.

[quote=“jwhitt, post:5, topic:75799”]

The AC unit will have over load protection built in the unit that protects the conductor.

[QUOTE]

I understand that part but what if the conductor gets damaged between the breakers and unit and create a short.

A light bulb was used as example…

I understand that part but what if the conductor gets damaged between the breakers and unit and create a short.

A light bulb was used as example…

If there is a dead short, then the breaker will trip. There would not be any excessive heat (except where the ungrounded conductor arcs to the grounded raceway/frame/conductor) that would cause the conductor to overheat and create any additional damage.

Jeff

Code compliant if the CB is listed for #6 conductors but I have no idea why they did it.

In the case of a motor driven compressor or motors in general it’s a matter of physics. In a simplistic example, a 20 amp motor with 20 amp conductor and a 20 amp OCPD probably wouldn’t even start. The NEC makes an allowance to oversize the OCPD to allow for the high starting current which only lasts for a few seconds and will not damage the conductors. It further allows thermal overloads that are built into the equipment to protect the conductors.

In general this is true but with motors or any other inductive load the overcurrent device is installed as a ground fault or short circuit protective device to protect the conductors. With motors such as the motor that drives the compressor it is the overload device that protects the motor and conductors from overload.

In general the fuse or breaker is sized to the conductor not the load such as with light bulbs. With individual loads such as with the compressor motor of an AC or heat pump it is a combination of fuse or breaker and running overload that protects the conductor.

Joseph W,

Thanks for the clarification!