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

NEC reference says that 12ga THHN wire can carry 30A max current. Motor load is currently drawing constant 20A (230v 1ph circuit). Breaker is 30A.
(A bit of history: Previous motor (running from 208v) was drawing less (15A?) current until machine broke. New machine's motor (running from 230v via boost autotransformer) is drawing 20A (measured at breaker). Motor spec plate says 16A.)
Is 12ga wire sufficient, or should I bump it up to 10ga?
Would the autotransformer account for the increased current draw?
FBt
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On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

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.
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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
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On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester

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.
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Glen Walpert wrote:

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.
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On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
That is what the NEC says to use for the circuit to the motor. They probably said 16 ".5" so you wouldn't use 14 ga wire and a 40a brealer <legal> That is usually a question on the inspector test. FLA with internal O/L protection, what is the min wire size & max breaker. Answer #14cu, 40a. Now days breakers are usually HACR so you don't need one that big to handle the locked rotor on start up but the code hasn't changed.
The 14ga = 15a, 12ga = 20a 10ga = 30a we all know is really 240.4(D) and is aimed at the circuits that are likely to have receptacles, where the installer has no control over what gets plugged in so the 80% safety factor is built into the breaker size limit. Folks will keep plugging things in till the breaker trips, then unplug the clock if that will let it hold.
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["Followup-To:" header set to sci.electronics.design.]

frequency? what mechanical load is it driving?

yeah 10% more voltage out than in means 10% more current in than out (+ losses).
--

Bye.
Jasen
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Thus spake Jasen Betts:

60hz. 4hp cap. start motor driving 2-stage compressor, light duty cycle (20-25%).
FBt
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["Followup-To:" header set to sci.electronics.design.]

Try a smaller pulley on the motor (or larger on the compressor)
That'll increase the duty cycle but reduce the load on the motor (and thefore the current it draws)
--

Bye.
Jasen
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I don't know what the fuss is all about };-)
For a 30 A load i'd use a 30A CB with 10AWG.... [on the safe side]
Induced currents from motor startup could easily overshoot the 19.*A calculated continuos draw value.
As far as useing a bigger breaker than the wires rating It Behooves you all that believe so to reconsider your Logistics., without much math (already given) Underrating a wire to a breaker would have no protection on a circuit that is optimum to the breaker but oblivious to the conductor ~ which may carry the full load of the breaker with forced electrical stress not suitable for such conductor of lower ampacity...
At the least, if yo're lucky stars are shining over you; Brown Out comes to mind.
Roy Q.T. Urban Technician [I don't make em, I just fix em]
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Roy Q.T. wrote:

Roy Sizing motor conductors to the size of the Circuit Over Current Protective Device that is needed to permit them to start is just a waste of materials. The overload protection of the motor provides the overload protection to the conductors that are sized for the running current of the motor. The circuit OCPD is only providing fault protection for the circuit so there is no reason to size it to provide overload protection to the conductors.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
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There is a reason they state that you require a 10AWG wire for a 30 amp circuit and that is because it is rated for 30A. #12AWG is rated for 20A continuous. However keep in mind that breakers, unless otherwise stated, are rated to run continuous at only 80% of their rating and over that the clock starts ticking.
Using a 30A breaker on #12AWG wiring is asking for trouble, you will be overstressing and overheating the wire which can lead to fire - good bye house. So it is not a waste of material, it is piece of mind. Code also calls for sizing the breaker and conductor such that you are not going to over load the conductor. Motors will typically have an inrush of 6 times FLA. If you motor has a FLA of 30 then you are going to need a larger feed.
Cheers ... Bob PEng.

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No, his point is that a dedicated circuit, that has motor overload protection for protecting the motor from drawing more than 16 amps continuous, also protects the supply cable from having more than 16 amps continuous draw. So the #12 is protected from continuous currents above 16 amps, just like the motor itself.
A motor connected to a 30 amp breaker better not have a FLA of 30. The 30 amp breaker is based on the requirements for a dedicated motor circuit whose motor has internal protection and draws quite a bit less than 30 amps.
daestrom
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According to the ampacity tables the ampacity of 12 AWG THHN at 75 degrees Centigrade is 25 Amperes. At 90 C it is 30 amperes. Since the breakers terminals are rated for 75 degrees C 20 amperes is 80% of the wires ampacity. Any motor with a running current of twenty amperes or less will have overload protection suitable for protecting the circuit from overload. The Circuit Overcurrent Protective Device is sized to permit the motor to start without nuisance tripping of the breaker or fuses. The US NEC specifically permits this. Saying that is an unsafe practice just demonstrates that your understanding of motor circuit protection is incomplete. The applicable sections of the code read:
240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). (D) Small Conductors. <B>Unless specifically permitted in 240.4(E) through (G)</B>, the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied. (G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).
Table 240.4(G) Specific Conductor Applications Conductor                    Article             Motor and motor-control circuit conductors    430, Parts III, IV, V,                         VI, VII As you can see Motor conductors are specifically provided as an exception to the over current limitations on small conductors because their ampacity is covered in section 430.
--
Tom Horne

Well we aren't no thin blue heroes and yet we aren't no blackguards to.
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You need to study NEC article 430. On motor circuits, the wiring is protected by the motor overload, since there are no other loads (nobody will be plugging in a space heater, for example). The breaker feeding the circuit is sized higher than the wire ampacity, to allow the motor to start. It provides ground fault and short-circuit protection only. This is perfectly safe.
Ben Miller
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
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On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

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

--


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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

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I agree.
i
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wrote:

The breaker should be greater than the cable capacity so that the cable doesn't burn out to protect the breaker.
Cheers.
Ken
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That was funny!
i
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