Parallel feed from mcc

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steveng

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i have a question concerning a parallel feed originating from a mcc switchboard, the switch is a 30 amp 480 3ph wye double tapped feeding 3 motors, 2-10hp and 1- 7.5hp , only 2 motors run at the same time.

is this permitted by the nec?

should the motors be seperately fed?
 
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This is not a situation with "parallel" conductors. I am not fond of the motor rules, as they cause me headaches. :roll: But let me suggest you start with 430.53, and see if you can figure out what it says.
 
You CANNOT run parallel small conductors in order to reach an ampacity level when below a certain size, for instance 3 x 16ga wire to reach 30A capacity for the circuit. I don't have my NEC here to check, but the minimum size for paralleling on the same circuit is something like 1/0 I think (310.4?).

Your situation is different because you are feeding separate loads, a "tap rule" situation, but there are restrictions and rules to be followed here as well.
Tapt Rule Link

In addition EACH motor controller downstream would probably need its own SCPD (fuses or breaker) as well as an over load relay. That is what 430.53 will discuss.
 
as Jraef says, not what I would call a "parallel run"
That said, if you are feeding the three motors off the one 30 amp circuit, If it is a circuit breaker feed, the only problem I see is a question as the the circuit actually carrying the load.
Each 10 hp motor has a Code FLA of 14 amps by 430.52 a 30 amp breaker woukd be fine as GFSC protection.
Like wise for the 7.5 hP.
So (1) 30 amp breaker coud be legal for all 3, however, I doubt it would be sufficient if it was the (2) 10 hp motors on line.
 
125% of the largest motor on one feeder would make one of ten horse motors 17.5 amps for whatever its worth IMO I believe this is a violation.

Mark
 
augie47 said:
...snip...
Each 10 hp motor has a Code FLA of 14 amps by 430.52 a 30 amp breaker would be fine as GFSC protection.
Like wise for the 7.5 hP.
So (1) 30 amp breaker could be legal for all 3, however, I doubt it would be sufficient if it was the (2) 10 hp motors on line.
Click to expand...
D'oh! I was thinking only about the wiring rules, didn't even bother checking the math, he's right...

Even though you might run the feeders to each of he the 10HP motors with 30A rated wire (which would negate the 125% rule issue on the conductors), when both are running together under full load you would exceed the 80% rating of the breaker (24A). I don't know for sure because I have never checked, but I don't think you can get 100% rated 30A breakers and even if so, it's doubtful that it is listed as such in that MCC.
 
i am describing a method already installed

i am describing a method already installed

i am describing a wiring method already installed and working, recently when doing some maintenance and testing we uncovered this method, and found a
30 amp fused disconnect from a 480vac 3 ph mcc which had parallel #10 conductors feeeding 3 motor starters,

L1 had blown and after looking into the problem, #3 motor is bad, however both 10hp motors run together fine on the 30 amp fuses. 12.9 fla ea


the original wiring was for 2 motors, the third motor was added later and just tapped the existing motor starter, to provide power to the #3rd motor starter.

thanks for the replies.;)
 
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