Cleanup on aisle 2 - motor loads

Thank you all for your input. This should be helpful to future inspectors faced with the same question.

Motors and a dedicated hardwired load. No ability to put anything else on the circuit by end user.

This is one of those instances where an installation is code compliant even when it appears to go against conventional thinking of sizing the conductor to the size of the OCPD. Sort of like having a 50 amp single receptacle on a 40 amp range circuit which is also code compliant. :slight_smile:

Okay now you have thrown that out you need to explain it for dumbies like me. I understood the A/C before the OP. Do I need to start a new thread?

Always the facts, but yet I seem to find stuff like this quite often.
The label may be correct, but when the compressor is at the end of its life, It could be pulling more than 29 amps. Worn motors or compressors pull more juice, and hard start capacitors indicate this. The OCPD will not prevent a smaller wire from getting baked.

Sorry Chuck, I disagree. The minimum amps is what is needed to run the motor. The NEC specified what wires are rated for.
I guess the picture is worth something.
My last compressor shorted out so bad it blew the breaker apart internally and black pieces were falling out when i removed it.

If you do, please consider a thread title that will make it “Searchable”!
Thank you in advance. :slight_smile:

Sean, just curious… Do you feel the heavily corroded conductor in your pic is capable of carrying it’s max rated amps, or even 50% of max? Not disagreeing with anything you said, just sometimes things are not as obvious as one thinks, or doesn’t.

I just threw that out there as an example of counterintuitive thinking. You can start a new thread if you like. :cool:

I would be inclined to attribute the heat damage to high resistance at the terminals and the damage to the disconnect most likely due to high heat from the poor connection. It is too localized at the terminal to attribute to excessive load without actually measuring it (There is no visible damage to the insulation except right at the terminals). That also does not appear to be an overcurrent protection device.

Thanks Robert I reread it again after I posted and you wrote receptacle not breaker. My bad. :wink:

The internal overload will however. Even if the compressor has a locked rotor the overloads will open before the breaker. If you turn an AC unit off then back on (like in a momentary power outage) the current draw to get the compressor running again is so high that the overloads trip.

I would be skeptical of the picture, as an FPE breaker, and addition to the “Non-60-auto” label which might mean that is a switch instead of a breaker.

If your last compressor short circuited destroying a breaker something else is wrong. A breaker will interrupt its maximum rated current without falling apart, usually 10,000 amps rms.

When a compressor does short circuit the breaker will trip and that is the point of the NEC. A short circuiting compressor is exactly like the wire between the unit and breaker failing, so the breaker trips for short circuit.

An over load like a locked rotor or dirty condenser is cleared via over loads.

Picture it like this:

A 40 amp breaker on #12 protects the wire only between the panel and AC. Once in the AC unit an over load (similar to a 20 amp breaker) prevents the wire from seeing more than 20amps.

Gentlemen,

If and when you view smaller conductors on larger OCPD’s , expect overload protection on the serving end.

The normal branch and feeder circuits have short circuit and ground fault protection are supplemented by *overload *protection.

OL’s are mainly thermal cutouts , although magnetics exist as well.

If they are not readily available, look at either the motor and or manufacturers instructions

~S~

You are right, its not the OCPD, but the wires going to this disconnect are much larger.
I understand that the chicken or the egg may have come first, just as Jeffery stated, but we will never know. I personally think the wire should be sized appropriately.
I asked this same question years ago to a buddy who does hvac. He said technically yes, but they always make it habit to size according to the breaker amperage.

I agree with this one… being in electrical field for a while… question is what would I put in my own house. Personally I would upside the wire larger than minimum code and be done with it. Yes it costs a bit more, but in the whole scheme of things not a lot.

Now from a contractor perspective doing a few hundred homes in subdivisions over the years… that little would add up… so can see why they always go by minimum code to do the job.

My breaker got fried because the compressor puked. It had a dead short with high ohm readings when tested by a professional. When it tied to turn on, it just said no more, two capacitors feeding it also probably didn’t help.

Don’t you mean a low ohm reading for a dead short?

But in any case wire size would not have mattered. The breaker did its job and stopped a short circuit.

I too would always want to size with the larger conductor for my own work / home. However, production home builders and their lowest bid subs will typically use the minimum allowable and I wouldn’t call it out as a defect for conductor sizing in an inspection if it meets the minimum requirements. I would be flagging another new home almost every day and would lose my credibility in no time.

Gentlemen>

The applicable codes are 430.52 and 430.32 for motors, most of which are juxtaposed to 440

A typical scenario would be 430.52 allowing an inverse time breaker to be sized 250% of the motor FLA

>> 16FLA x 2.5= 40A OCPD

The OL protection is detailed in part 3 of 430.

430.32 being the norm, most motors OL’s are typically set @ 125%

so 16FLA x 125%=20A OL’s

The end game here is the conductor will mitigate starting currents , but never see accumulative heat beyond the motor FLA

~S~

That’s it, it’s merely a design issue. If one wants to design to the minimum code requirement then can do so and there should be no questions asked.

Regarding the OP running a 50 amp, #6 NM cable when the unit is only drawing 23.5 amps is a waste of money.