Motor Feeder OCPD

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compumotor

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:rolleyes: This question has to do with Motor Feeder circuits OCPD. As an instructor for my electrical apprentice class I have been using Mike Holts Understanding the 2008 NEC. Great text book for class. I was using an example similar to his where I have design B motors, 20 HP-480 volt motor and a 15 HP-480 volt Hp motor. The FLC from table 430.248 are 27 for the 20 HP and 21 amps for the 15 Hp motors. I asked my students to use an Inverse Time Breaker, From Table 430.52. The example uses the higher FLC of 27 time 250 % = 67.7 amps, Mike Holts example rounds up this higher motor value to 70 amps. This is where 430.62 (A) does not ask to round this higher motor to the next size up. 70 amps + 21 amps for the 15 HP motor = 91 amps round down to 90 amp motor Feeder OCPD, some of my students want to add 67.5 amps + 21 amps = 88.5 amps and round down to 80 amp Motor Feeder OCPD.
I have looked on the net and have not found where the larger motor times the 250% needed to be rounded up, befor adding the additional motor FLC value. If anyone has come across this predicament, please can you share your findings.

Sincerely,
compumotor
 
I would say that you are correct. I believe that your students are using branch circuit rules and they should be using feeder rules.
 
440.22(b) (1)

440.22(b) (1)

Originally Posted by faranca77
Each breaker have thermal trip to disconnect the main panel at over load case, short circuit trip (electromagnet trip) to disconnect at the short circuit case.

For main panel feed more than motor-compressor I will apply Section 440.22(B) (1) to size the main breaker

And For main panel feeding lighting and receptacle circuits I will apply Section 215.3



1-Is Section 440.22(B) (1) in the national electrical code to size the main breaker according to thermal trip or short circuit trip?

If the answer according short circuit trip why the national electrical code don?t size according to the thermal trip???

If the answer according short circuit trip is this correct to size the breaker according to short circuit calculation which depend on impedance accumulation

I (sort circuit) =voltage (line to line)/total impedance/1.73



2-Is Section 215.3 in the national electrical code to size the main breaker according to thermal trip or short circuit trip?

If the answer according short circuit trip why the national electrical code don?t size according to the thermal trip???
 
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