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
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
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
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
For 208V in, 230V out at 16.5A out to motor and 95% autotransformer
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.
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.
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
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.
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
[I don't make em, I just fix em]
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.
"This alternating current stuff is just a fad. It is much too dangerous
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
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.
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
Motor and motor-control circuit conductors 430, Parts III, IV, V,
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.
Well we aren't no thin blue heroes and yet we aren't no blackguards to.
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
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