10 Gauge Wire connected to a 40amp breaker...Need advice

This was a connection in the Main Electrical panel for an Air conditioner.

It was a 100 amp service…Everything else looked normal in the panel.

I have a guide for wire sizes and Breakers…

It says 10 gauge wire for a 30 amp breaker

8 gauge for a 40 amp breaker…

Is the 10 gauge ok… for the 40 amp breaker…used fro air conditioning?


What does the AC unit label say?

It’s possible that this is allowed. As Mike suggested, you’d have to look at the data plate on the condenser to verify.

Im to understand that in the case of a/c that its ok to “undersize” the wire to the breaker cause it only needs a 10 wire to run but at start up there is a surge that would trip a 30 amp so they upsize the breaker so it wont trip. The 10 wire can handle the extra current that lasts less than a second. But yes , you have to read the plate which states the max breaker

Thanks everybody …

That is correct, there are things like RLA (running load amps) and SLA (starting load amps) on certain appliances such as AC units. Typically the unbit will have a data plate on it that will tell you Max fuse/breaker size.


A 5 horse power 240 volt single phase motor is wired with #10 wire but protected by a 70 amp breaker

Reference 430.22 and 430.52 of the NEC

When dealing with AC or heat pumps look at the name plate for MCA and MAX fuse or HARC breaker for the conductor and overcurrent sizes.
These units have internal running overload that protects the motor and conductors. The fuses and breakers are installed for ground fault and short circuit purposes only

Thanks Mike. I never knew that. I always assumed, incorrectly, that the panel breaker was there to protect the compressor motor.

breaker does not protect equipment, OCPD fuse or breaker in a load center is protecting the wire feeding the equipment. So who cares what it operates, that’s for the installer to decide, what we care about first is what size wire they used and does the OCPD protect that wire. So a 40 amp breaker better have at minimum #8. If you are a code guy and want to do the math for each range and AC unit go ahead, but that is a little outside our scope.

well they actually protect from over current, due often to short circuit not necessarily ground fault. fuses work faster for smaller over currents but breakers will trip with small over current over a longer period of time.

well actually breakers and fuses protect from over current, due often from short circuit, not necessarily ground fault. fuses trip faster than most breakers with over current, but breakers will trip with small over current over a period of time.

If the manufacturer’s label says it can have a smaller gauge wire (minimum circuit ampacity) on a larger breaker (max. fuse or crk. breaker size)), so be it.

For instance:

You need to learn more about motor and compressor sizing rules. The correct information was already in this thread. Even though several years old it was still correct. The simplest is just to check the data plate on the equipment.

A breaker will hold 125% for several hours depending on how fast the overload occurs. The faster the rise the quicker the trip.

You should care about it because there are different rules for motor and compressor loads. If you use the standard sizing rules you are writing up things that are perfectly fine and allowed.

You went out of your way to dig up a 4 year old thread just so you could demonstrate how much you don’t know? If you call these out as defects, you can count on losing credibility when the electrician comes around behind you and sets the matter straight. You don’t have to be a “code guy” or do the math here. You just have to read the label. Getting things right is not outside our scope.

As others have stated this is incorrect. For an AC unit the OCPD provides the conductors with both ground fault and short circuit protection, it does not provide overload protection. Overload protection is provided by the overload device that is integral to the compressor.

According to the NEC overload protection is typically set at 115% of the motor FLC, the conductors feeding at motor are sized at 125% so the overloads will protect the conductors from overcurrent.

When sizing conductors for AC units the NEC requires you to use the nameplate data (if available) to size the conductors and the OCPD. The MCA is used to size the conductors and the minimum size OCPD for the branch circuit. The MaxOCPD can also be used to size the OCPD and is often larger than the ampacity of the conductors. This is permitted because the conductors are protected from overload by the overload device within the compressor. The OCPD can be any size between the MCA and the MaxOCPD.

Some of this seems strange because it violates what we consider to be the normal way conductors are protected (#14-15 amps, #12-20 amps, #10-30 amps, etc.).

And there are other factors that make this even more complicated, for example #12 conductors are normally limited to 20 amps but for motors and AC units their ampacity can be 25 amps according to T310.15(B)(16) when a few other variables are met. For example if the nameplate says:

MCA=25 amps, MaxOCPD= 40 amps you could have piece of conduit feeding the AC unit with #12 THHN conductors and a 40 amp OCPD protecting the curcuit.

So a final note when inspecting AC units wired with NM cable, conductors are sized to the MCA and the OCPD cannot exceed the MaxOCPD listed on the nameplate.

Yes this is complicated stuff. :slight_smile:

thanks Robert. Awesome info.

You’re welcome, not that I will listen but someone told me to stick to electrical information. :roll: