John Gilmer wrote:
wrote in message
...
On Mon, 25 Jun 2007 10:00:27 -0500, Bud--
wrote:

For motors the rules change. The breaker protects the wire from short
circuits, not overloads, as gfretwell said. Rather arcane rules for
motors are among the reasons why electricians have licenses.

Exactly right Bud.
In fact there is a question on every inspector exam I have taken about
a 115v 1HP motor with internal overload protection (16a FLA) and the
correct answer is "40a breaker, 14 ga wire".
I understand this is an anomally using all the exceptions and "round
up" rules but it is NEC compliant.

Really?

That's plain nuts!

Your run of the mill CB takes seconds to trip just from FLA currents.
There just isn't any need to put a 40 amp breaker on a motor circuit.

If there is something about the motor the keeps it from coming up to speed
in a second or two then fix that. But don't put a 40 amp breaker on #14
wire.


FLA (full load amps) are not the problem. Starting current/LRA (locked
rotor amps) are. I presume this was a typo on your part.

As a rule of thumb a motor draws a current (LRA) of about 6x its FLA
when it starts.

Looking at a SquareD current - trip-time curve, at 6x the rated breaker
current a breaker may be in it's "instantaneous" trip region - no time
delay.

If the motor was rated 80% of the breaker rating (more likely) the
starting current is (0.8)x(6) = 5.4x the breaker rating. The trip time
can be 0.3 to 2.5 seconds - not reliable.

Obviously the motor current goes down as the motor accelerates. But that
doesn't help in the instantaneous trip region. And in the 0.3 to 2.5 sec
range there is no guarantee the motor will start. If turn up a
thermostat then turn it down a refrigeration compressor it will likely
not restart immediately. That causes the LRA until the motor protector
opens the circuit. You would generally not want the circuit breaker to
open. And high inertia motor loads increase the time to start.

The NEC is really quite pragmatic. Rules like this come from field
experience as well as engineering considerations. Not only can a circuit
breaker be 250% of FLA, as in gfretwell's example - if the breaker does
not permit the motor to start the breaker can be increased to 300 or
400% of the FLA.

And as has been a couple times, the breaker provides short circuit
protection for the wire, not overload protection.

--
bud