40 amp breaker on a 30 amp AC unit

[Dan Mages]

Good evening,

Wifey and I had an inspection done on the house we are buying. The inspector noted that the 30 amp AC unit is connected to a 40 amp breaker. He explained that under normal operations, the AC unit may pull more than 30 amps and would trip a 30 amp breaker so most installers use a 40 amp breaker. He also said that while this is a common practice, it is also a poor practice. I am not an electrician nor an HVAC technician, so a better explanation would be appreciated. Is this really an issue that needs to be addressed? If so, what is the correct fix?

Thanks!!

Dan

[Mike Henderson]

I think it would depend on the size of the wire in the circuit. If the wire will support 40 amps safely, there's nothing wrong with having a 40 amp breaker.

Mike

[Von Bickley]

I think it would depend on the size of the wire in the circuit. If the wire will support 40 amps safely, there's nothing wrong with having a 40 amp breaker.

Mike

Mike has got it right. If the wire is #10 copper, a double 30 amp is the max. size breaker.

If the wire is #8, you will be O.K. with the double 40......

[Tom Veatch]

... He also said that while this is a common practice, it is also a poor practice. ...

Don't see anything "poor" about it as long as the wire ampacity is not less than 40 amps and the outlets are rated for the circuit voltage and not less than 40 amps. If that's not the case, it's not "poor practice", it's a code violation and the inspector should have red tagged it.

If what he had refererence to was using a 40 amp breaker for a circuit where the load is no more than 30 amps, that's no more "poor practice" than plugging a vacuum cleaner which draws 5 amps into a 15 amp circuit. The breaker is there to protect the wiring and wiring devices, not whatever might be plugged into the circuit.

[Phil Thien]

I am no electrician, but I have rented lots of commercial spaces with old compressors. The fix has always been to add a hard-start kit (which is really just a big cap) at the compressor, not to install a larger breaker.

The hard-start kits don't last forever. I'd have an electrician look at it. If he can back it off to a 30amp breaker by adding a hard-start kit, I think it would be the right way to go.

[Tom Veatch]

...If he can back it off to a 30amp breaker by adding a hard-start kit, I think it would be the right way to go.

If the wiring and devices are properly sized and rated to handle 40 amps I don't see any advantage gained by replacing the breaker. On the other hand, if the wiring and devices are not properly sized and rated, I agree that the breaker should be downsized.

[Russ Filtz]

Agreed, breakers are there to protect the wiring, not the load or equipment. Heavy wiring, you're fine. Why go to the expense of new breakers/capacitors if you don't have to. Check what gauge the wire is and ask an electrician.

[Brian J Holmes]

Agree with everything posted so far but here's a bit more information:

Large appliances with motors tend to spike to a higher amperage draw at start up as the motor needs to overcome inertia and friction to start moving which momentarily takes more amperage to do. once running the motor should pull a constant amount of power to continue running. All motors should be marked on their plate with two numbers RLA=run load amps and LRA=locked rotor amps. The RLA number is what the circuit needs to be sized for along with the standard percentage of "headroom" which is usually 20% i.e. a motor that pulls 24 RLA amps would require a 30amp circuit. The circuit itself should be put together with a wire gauge and breaker sized for the expected load but MUST match. i.e. 14 gauge wire with up to a 15amp breaker, 12 gauge up to 20A, 10 gauge up to 30A, 8 gauge up to 45A, and 6 gauge up to 60A

Generally speaking with a new motor with bearings and windings in good shape the start-up spike is minimal and the 20% headroom easily accommodates it. However on some older motors the increased friction cause the spike to be either larger or longer and can sometimes trip a correctly sized breaker which is what hard start kits are for. They basically add a larger or additional capacitor to provide that initial extra amperage necessary to get the motor moving.

I'd be willing to bet that the circuit is wired to support a 40a breaker and the 40amp circuit was installed to meet the "load < 80% of circuit capacity" rule for instance if the RLA on your unit was 25amps the electrician could not use a 30amp breaker as the sustained load on the circuit would be more than 80% so he had to upgrade to a 40amp circuit.

As everyone else has said check the wire gauge and the RLA on your AC unit and if the wire gauge is 8 or lower don't worry about it.

[Rollie Meyers]

Good evening,

Wifey and I had an inspection done on the house we are buying. The inspector noted that the 30 amp AC unit is connected to a 40 amp breaker. He explained that under normal operations, the AC unit may pull more than 30 amps and would trip a 30 amp breaker so most installers use a 40 amp breaker. He also said that while this is a common practice, it is also a poor practice. I am not an electrician nor an HVAC technician, so a better explanation would be appreciated. Is this really an issue that needs to be addressed? If so, what is the correct fix?

Thanks!!

Dan

This is a old thread but with A/C equipment the rules are a lot different (NEC art. 440) then what most folks are used to, and one can find 10 or 12 AWG wire on a 40 amp circuit breaker and still be code compliant (This issue gets home inspectors all the time) It all has to do with the nameplate data on the unit and is more then I care to try to explain ATM, but bottom line the installation discribed in the 1st post is fine and there is nothing to worry about.

[Phil Thien]

Agreed, breakers are there to protect the wiring, not the load or equipment.

You know, I hear this so often it makes me pause and wonder, why not properly size the breaker so it protects the wiring AND the equipment?

That is, just because the wire can support 40A or 100A or even 200A, why not size the breaker to provide a little extra protection for the gear on the other end?

Or is that just not done?

[Bruce Wrenn]

Often you will find a larger breaker, with proper sized wiring. Then you have a FUSED disconnect with smaller fuses. The fuses are rated for us with HVAC equiptment, and are slower to blow.

[Matt Ocel]

Just another Home Inspector trying to validate his fees.

[Tom Veatch]

...
That is, just because the wire can support 40A or 100A or even 200A, why not size the breaker to provide a little extra protection for the gear on the other end?
...

Because the codes/code enforcement officers cannot control what might be plugged into a branch circuit.

The receptacle must be sized to the breaker. I.E., if you've got a 50 amp breaker, the outlet must be a NEMA X-50 receptacle. That means anything with an X-50 plug can be plugged into the circuit. There's nothing
to stop you, the consumer, from putting a 50 amp plug on a 5 amp load. So, how are you going to size a breaker to protect a 5 amp load today and the 50 amp load that might be plugged into the circuit tomorrow.

Circuit breaker protection extends only to the receptacle on a branch circuit because the load isn't fixed to a specific amperage. It can vary from day to day depending on what's plugged into the circuit.

Even on a hard wired load like a NEC 430 motor circuit, if you know the load isn't going to exceed 20 amps, why go to the extra expense and effort of putting in 8 ga. wire when 12 ga. is acceptable and perfectly safe. And for a branch circuit, ever tried to connect 8 ga wire to a 20 amp receptacle? Since the receptacles have to be sized to the breaker, that's what you'd need to do if you put a 20 amp breaker on a 50 amp circuit.

[Rod Sheridan]

There are two issues with the sizing.

1) If the HVAC unit does have a full load current of 30 amperes, the breaker would have to 40 amperes because the load is thermostatically controlled.

2) The HVAC unit will have a maximum protective device rating listed on the nameplate, and it may list different values for fuse or circuit breakers. This cannot be exceeded, as the manufacturer has determined that those listed protective devices, do not exceed the equipment withstand ratings.

Unfortunately, some home inpectors aren't that familiar with electrical codes.

Regards, Rod.