"DonkeyHody" wrote in message m...

Are you sure about that one. All of the references I am aware of limit
the overcurrent device to 30A on a #10 wire.

Anyone know for sure?

It is all a function of the insulation used.

No, it's not. Theampacity may be higher than 30 A, but code limits the
evercurrent device to 30A. See the tables in section 310. Section 310
states that #14 = 15A, #12=20A and #10=30A. I agree that the rated
ampacities may, however, be greater than those limits.

My question is, can anyone cite the section of the NEC that allows a larger
overprotection that is stated in section 310?

I have never ran across that exception and I have been an Electrical
Engineer for 24 years working in, among other areas, electrical construction
engineering. I am still looking at the code book.

Well shucks. Y'all made me get out of my chair and go look it up.
Check out Section 430, Part D - Motor Banch-Circuit Short-Circuit and
Ground-Fault Protection.
Paragraph 430-52
(b) All Motors. "The motor branch-circuit and ground-fault protective
device shall be capable of carrying the starting current of the
motor."
(c) Rating or Setting.
(1) "A protective device that has a rating or setting not
exceeding the value calculated according to the values given in Table
430-152 shall be used."
EXCEPTION NO. 1 (Paraphrased)
Exception 1 allows you to go to the next size up if you are between
sizes.
EXCEPTION NO. 2 (Quote)
"Where the rating specified in Table 430-152 as modified by Exception
1, is not sufficient for the starting current of the motor:"
"(a) The rating of a nontime-delay fuse or a time-delay Class CC fuse
shall be permitted to be increased, but shall in no case exceed 400%
of the full-load current.
"(b) The rating of a time-delay (dual element)fuse shall be permitted
to be increased but shall in no case exceed 225% of the full-load
current.
"(c) The rating of an inverse-time circuit breaker shall be permitted
to be increased but shall in no case exceed 400 percent for full-load
currents of 100 amperes or less or 300 percent for full-load currents
greater than 100 amperes."

The commentary in the handbook states: (Quote)
"It should be noted that it is not necessary to size the
branch-circuit conductors to the percentages (150 to 300) permitted
for the branch-circuit short circuit and ground-fault protective
devices." . . .
"During a short-circuit or phase-to-ground condition, the extremely
high current causes the protective fuses or circuit breakers to open
the circuit. Excess current flow caused by an overload condition
passes through the overload protective device at the motor controller,
thereby causing the device to open . . ."

Well, now you caught me without my code book but that seems to imply that it is a dedicated, hardwired motor, being supplied by a
motor controller that incorporates both instantaneous (like an MCP) and overload protection AND that it pertains to the wiring
between the motor controller and the motor.

I could be wrong, but I doubt you could convince a residential electrical inspector to pass an installation with #10 wire being
protected by a 40A breaker.

--
Al Reid


DonkeyHody
Even a dumb old country boy gets it right once in a while.