Motor Conductors
Motor Conductor Sizing - One Motor
The conductor for a single motor must have a table ampacity of at least 125%
of the flc of the table value of the motor. Now this part of the calculation
takes into consideration nothing more than the 125% requirement. There will be
more to this calculation later. First the 125% aspect.
The flc of the motor is found from the table depending on voltage, phase and
horsepower. It is this number times 125% which will yield the minimum ampacity
for the needed conductor.
Once the calculation has been made, then look into the table to find a
conductor of the temperature and type needed to meet this ampacity requirement.
Example. Say you have a 5 HP 240 volt single phase motor. What is the
minimum allowable 60 degree conductor to supply this motor?
Well, from the flc table we find the flc to be 28 amps. Times 125% is 35
amps. Checking table 310.16 we see that a #8 AWG would be needed in the 60
degree column. A #10 has an ampacity of only 30 amps and would not be at least
35 so it is too small.
Note, here is a complication. The code allows that if the calculated value
for a single motor conductor does not match a rating of an available conductor,
then the next size smaller can be used. So for a single motor, a conductor can
be selected which is the next size smaller. This is not true for feeders for
more than more than one motor, as will be seen in the next example. So a #10 is
alright, after all.
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Motor Conductor Sizing - More Than One Motor
When there is to be a feeder supplying more than one motor, the flc of the
largest motor at 125% is added to the 100% flc's for any other motors in the
group supplied by the same feeder. Then the conductor selected must have an
ampacity of at least this number.
Example
Say you need to size a feeder for four 5HP 240 volt single phase motors.
Remember they are 28 amps each flc.
So one of them is 28 times 125% or 35. Then there are three others at 28
amps each or 84 amps. So it is 35 amps plus 84 amps for a total of 119 amps. Now
go to the ampacity table and look in the 60 degree column for a conductor which
has an ampacity of at least 119 amps. a 1/0 AWG will do nicely. It has an
ampacity of 125 which will handle 119 amps. The #1 AWG is only 100 amps so can
not be used.
Tip: For a feeder for more than one motor, you can not go down to the next
smaller size after a calculation is made. You must go up.
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