Help with 430.110

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aksparky

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I am installing a fused motor disconnect for a 10hp, 208 volt 3 phase motor.
The nameplate says it is 30.8 FLA with a locked rotor current of 179 amps.

430.103 commentary in the 2008 handbook says the disconnect shall be capable of interrupting the locked rotor current of the motor which I believe the fuses will take care of.

430.110 says that I need to size the disconnect at 115% of the FLA,

430.110.C (1) Says the rating of the disconnecting means shall be determined from sum of all currents at the full load condition and also at the locked rotor condition. But this is for Combination Loads which I dont believe applies to single motors.

Question #1- If I size the disconnect at 115% of the FLA andf make sure it is rated for the 10HP wouldnt that comply with code? (Which would be a 60amp disconnect using Time Delay Fuses rated at 15HP).

Question #2- Under what circumstance would we have to use the Locked Rotor Condition to size a disconnect?
If I had to use ther LRA at 179 amps my disconnect would have to be a 200 amp disconnect for a 30.8 amp load which seems crazy, I would like to know because next time it might be a disconnect for a combination load.

Thanks in advance for any input
 
You are looking at two different things.
First is the ampacity rating of the disconnect (sum of FLA plus 115% of the highest motor)
Second is the HP rating of the disconnect (sum 115% times the LRA of all motors).
The first is pretty easy to figure out, 3 motors at 10FLA plus 15% equals 34.5 or so amps.
The second is a little trickier. it's based on the HP rating of the disconnect. So let's say a 10HP disco is rated for 30FLA and 170RLA. Which is good for a single motor in your situation. But let's take 2 - 10HP motors on same disco: The FLA is 60A, but the LRA is 340.
Now a 60A disco will take care of the ampacity, but it'll not take of the HP. There's a table of HP rating VS sum of LRA in the NEC, just can't quote it right now:)


aksparky said:
I am installing a fused motor disconnect for a 10hp, 208 volt 3 phase motor.
The nameplate says it is 30.8 FLA with a locked rotor current of 179 amps.

430.103 commentary in the 2008 handbook says the disconnect shall be capable of interrupting the locked rotor current of the motor which I believe the fuses will take care of.

430.110 says that I need to size the disconnect at 115% of the FLA,

430.110.C (1) Says the rating of the disconnecting means shall be determined from sum of all currents at the full load condition and also at the locked rotor condition. But this is for Combination Loads which I dont believe applies to single motors.

Question #1- If I size the disconnect at 115% of the FLA andf make sure it is rated for the 10HP wouldnt that comply with code? (Which would be a 60amp disconnect using Time Delay Fuses rated at 15HP).

Question #2- Under what circumstance would we have to use the Locked Rotor Condition to size a disconnect?
If I had to use ther LRA at 179 amps my disconnect would have to be a 200 amp disconnect for a 30.8 amp load which seems crazy, I would like to know because next time it might be a disconnect for a combination load.

Thanks in advance for any input
Click to expand...
 
Thanks for the reply,

Is 430.251(b) the section you are talking about?
Would this be correct for 3- 10HP, 208 volt, 3 phase, motors......
It would be the sum of FLAs which would be 30.8x3=92.4 amps + sum of all LRA which would be 179x3=537 amps (537+92.4=629.4 amps) So I then would compare to 430.251(b) and see that I would have to use a disconnect rated for 40 HP and rated for atleast 100 amps?

Again thanks for the input
 
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