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?
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.
Eh? The breaker trip rating should be _lower_ than the current capacity rating of the wire.
...Jim Thompson
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I love to cook with wine. Sometimes I even put it in the food.
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
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.
- 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.
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.
Let me try another approach. If you attach a 10 guage wire to a 20 amp breaker, a 10 amp load will opperate more effeciently and have more voltage, and the wire will run cooler. The ground will also have a better connection to the main box.
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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