Service Overcurrent Protection (Motors)

[cruiser-x]

I have five motor branch Circuits (not Fire Pumps). Full load current on each are as follows: 34A ; 65A ; 96A ; 124A ; 240A. I'm trying to figure out the main minimum feed conductor and main overcurrent protective device(dual element;time delay fuse) thats feeds my circuits. Art. 430.62 NEC says that overcurrent device cannot be greater than largest motor FLC plus the sum of lessr motor FLC's? very confusing. Also , overcurrent device can be determined from feed conductor ampacity. I sure could use some help!

 

[augie47]

Take a look at 430.24 as far as sizing your conductor(s), and 430.62 for the Feeder Short Circuit device. Tell us how you get your answers and then you will get all types of help.
You are on the right track with the SCGF device...figure the one then add the other FLAs.

 

[Keith Chitwood]

To size the feeder conductors you will use 430.24 as stated earlier.

Judging by the 240 amp load I am assuming that you are installing three phase motors.

Steps:
1. Determine which of the three phases will have the highest amperage on it while achieving the best possible balance of your loads per 210.11(B)

2. From the most loaded phase, choose the highest rated motor in Full Load Current and multiply that by 125%.

3. Add the sum of the full load currents of all other motors connected to that phase. Use this required ampacity to size the feeder circuit conductors based on Table 310.16

4. For the Feeder OCP - determine the largest rating or setting of Branch OCP on that feeder and add the sum of the full load currents of all of the motors connected to that conductor. If this rating or setting does not match 240.6 GO DOWN to the next standard size fuse or circuit breaker.