Commercial mains wiring: 12ga wire for 30 amp load?

You have 2 issues. Foiust, is the motor supposed to be running on 230 or 208? That couild be the extra current. If the motor is really exceeding the nameplate amps I would investigate futher.

The second question is ampacity of the conductor 12ga THHN is only good for 30a if you have all 90c terminations and that is just not going to true so you have to use 25a. That is still 125% of the 20a you measured so that is OK.

For you "20a voters", article 430 allows you to use the 310.16 ampacity on a dedicated motor circuit, not 240.4(D) which says 20. It also allows a breaker sized to 175% of FLA so the 30a breaker is OK.

It is my understanding that wire must be matched to the breaker. So, if the breaker is 30A, the wire should be rated for 30A as well. That means 10 gauge.

i

It does not have to be matched. Only on how big the breaker can be. The wire can be bigger than the breakers rating.

greg

I agree.

i

The breaker should be greater than the cable capacity so that the cable doesn't burn out to protect the breaker.

Cheers.

Ken

That was funny!

i

Eh? The breaker trip rating should be _lower_ than the current capacity rating of the wire.

...Jim Thompson

NO, not according to NEC and wiring protection practices. You size the breaker to be 80% of the rated FLA of the conductor. You are protecting the wire with the breaker, not the motor...If the wiring overheats you got a fire. John

Exactly!

Typically you protect a #12 wire with a 20 amp or less breaker, and a #10 wire with a 30 amp or less breaker. For #14 wire a 15 amp breaker is the maximum, but a 10 amp breaker provides an added margin of safety.

The terminal design on many breakers intentionally make it difficult (although not impossible) to attach a wire that is undersized for their rating.

Harry C.

Nonsense- if you hook say 40A 6ga to a 20A breaker, that 40A will trip the breaker almost immediately...

Even without a load?

:-)

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Damn! That was what I meant! Thanks Jim.

Ken

- you > best be using at least 10AWG wire! Even larger if your 50' or more from the > source. Why would you Jeopardize burning out the motor from a large voltage > drop in the wire? Stall current (starting current to break inertia) is > typically 20* running I... >

Motor Circuits are unique animals. In general the Over Current Protective Device for a motor branch circuit is sized to permit the motor to start without nuisance tripping of the breaker or fuse in question. The OCPD serves only to provide fault protection to the circuit. Overload protection is provided by the Motor Overload Protective Device. This may be a thermal protector on the motor or it may be heaters sized to the motor running current and installed in the motor starter. The circuit conductors are sized at 1.25 times the full load current of the motor it serves based on the ampacity in the NEC table. The note to the table reads "except as otherwise permitted by this code" and it is that phrase that allows for full current ampacity of conductors to be used in sizing motor branch circuits and feeders.

In other words the requirement to protect number twelve copper conductors with an OCPD not greater than twenty amperes does not apply to motor circuits.

-- Tom Horne

I always use #10 for 30A circuits, particularly if you're running a motor load which will pull a large inrush current. On a long run the motor can stall before it gets up to speed if the wiring is too small.

Where did the poster to whom you are commenting, say he would put a 40A load on a 20A breaker?

He said, "The wire can be bigger than the breakers rating.

Thus spake snipped-for-privacy@aol.com:

Nameplate: 230V, 16.5A.

I presume that if an autotransformer boosts the voltage

10%, it must draw 10%(?) more current in order to do so. Is this correct? (Nothing is free.) FBt

Correct, except that there will also be an additional bit of current draw due to losses in the autotransformer, which will probably be over

95% efficient:

For 208V in, 230V out at 16.5A out to motor and 95% autotransformer efficiency:

Iin = (16.5*(230/208))/.95 = 19.2A

Which is slightly under your measured 20A, which indicates that you are running slightly over 100% load on your motor if my voltage guesses are right. Hopefully the nameplate service factor is at least

1.15 continuous (if the motor runs continuously).

BTW congratulations on responding to one of only 2 posters who understand your situation (the other being Tom Horne of the Tacoma Park VFD). But consider that anyone responding to your question here cannot be aware of all pertinent circumstances such as possible derating requirements for high ambient temperature or more than 3 current carrying conductors in a conduit, which a competent local electrical designer would take into consideration, as well as considering the length of the wiring run and resulting voltage drop.

Since you did not seem to understand the very basic fact that the 90 C wire ampacity rating is provided only to allow for various derating requirements, and the usable ampacity cannot exceed the rating for the conductor at the max temp ratings of the connections at either end (probably 75 C max for your circuit breaker, which will be labeled with temp rating), I suggest you find a competent local electrical designer or inspector to examine your installation. The $100 or so could save a lot more in the long run.

There are also other exceptions where the characteristics of an overload protected hard wired load are allowed to protect the conductors instead of the circuit breaker, such as for welding machines with limited duty cycle. And then there are loads which do not require overload protection at all, such as certain emergency fire pumps, where the wiring burning up due to overload is deemed less hazardous than the pump shutting down early in a fire emergency. The NEC is a complex beast, not easily understood in its entirety!

Since he went with an autotransformer, he should derate the wiring for NEC "general use" and that puts 12ga THHN at 20A. He needs to reinstall at 10ga- maybe- depending on circuit run length. 10ga would be good for up to 100', 8ga would be recommended for over 150', 6ga for up to 300' etc at 2.5% voltage drop..This may explain why he went with an autotransformer- burned the old motor out with overload at low voltage and upped the breaker from 20A motor time delay to 30A to eliminate LRA trips. He should therefore reinstall the 20A breaker with the 10ga or larger and get rid of the autotransformer unless the motor specifically calls out 230V which is rare these days.