Motor Overcurrent Protection
Motor overcurrent protection of motor circuit conductors may seem
overly large to a working electrician. Just thought you should be warned. But
code is code. Here are the details.
The calculation technique is to find the flc table value for a motor. Then
multiply the flc by the percentage which corresponds to the type motor and type
overcurrent protective device.
Example: What is the inverse time circuit breaker needed for a 5HP 230
volt single phase motor?
Well the flc for the motor from the table 430.248 is 28 amps.
So the conductor would be 125% of 28 or have an ampacity of at least 35
amps. A #10 THW would do just fine with a breaker rated at 75 degrees C.
Now on to the overcurrent protection. The inverse time breaker for this
single phase motor can be not greater than 250% from NEC 430.52. That's not more
than a 70 amp breaker.
There is a sentence in 310 which says the table values of ampacity are not
to be used for conductor selection when article 430 has been complied with. So
if the conductor is set at at least 125 % and the breaker is not more then 250%,
310 can be ignored.
Now picture a #10 conductor attached to a 70 amp circuit breaker. That sort
of rubs you the wrong way, right? Well on license examinations use the letter of
the law. In this case, if asked what is the minimum size conductor, select #10.
If the question asks for maximum size breaker, select 70. Case closed.
Find practice problems HERE.
Find audio support HERE.
Find practice problems for motor feeders HERE.
Find audio support for motor feeders HERE.
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