3 motors, 1 branch circuit.

[Dsg319]

I have 3 small HP motors.
A 1hp, and two .75hp.
Here is what I came up with.

largest FLC, 2.1x125%=2.6
Plus the sum of other two,1.6+1.6=3.2
Total, 2.6+3.2=5.8

Could use #14THHN but had #12 handy so will be using it.

But where I am getting hung up is sizing SC/GF protection. Each motor has its own starter with overloads.

But for the SC/GF Do I just use
430.53 (C)(4). Some info I forgot to add 480v 3ph squirrel cage.

So do I just take my Total added FLC of 5.8x250%=14.5 and use a 15ampere OCPD. Or am I still permitted to use 20amp OCPD since I used #12?

sorry I am having a brain fart……

 

[augie47]

430.52 would limit a breaker SC/GF device to 15 amp.
(If you are using fuses, the max would be lower due to 3 amp being a standard fuse size))

 

[Dsg319]

So I would only be allowed a 3amp fuse for a 3motor branch circuit with a total of 5.8FLC? Forgot to mention all will be using the same branch circuit.

 

[Dennis Alwon]

So I would only be allowed a 3amp fuse for a 3motor branch circuit with a total of 5.8FLC? Forgot to mention all will be using the same branch circuit.

No I believe Gus was saying 3 amp fuse for each unit which I disagree with since the overcurrent protective device can be 250% of the motor which would be 6 amps for the smaller units and 10 amps for the larger but then that would no longer be a branch circuit but a feeder to the motors.

 

[david luchini]

So I would only be allowed a 3amp fuse for a 3motor branch circuit with a total of 5.8FLC? Forgot to mention all will be using the same branch circuit.

The branch circuit could be supplied by a 15A c/b or 15A fuses per 430.53(A).

(Under 430.53(B) - If Smallest Rated Motor Protected, the OCP would be smaller for fuses. But you could comply with 430.53(A) with 15A c/b or 15A fuses, so there's no reason to look at 430.53(B)).

 

[augie47]

No I believe Gus was saying 3 amp fuse for each unit which I disagree with since the overcurrent protective device can be 250% of the motor which would be 6 amps for the smaller units and 10 amps for the larger but then that would no longer be a branch circuit but a feeder to the motors.

Is it not 175% for fuses ?? (1.6 x 175% = 2.8. next size up = 3) and then the 15 amp or 20 amp circuit would be a feeder and could be based on the conductor size.
I am in agreement that a 15 amp breaker will meet Code but since the OP did not state which type SCGF method was being used I mentioned fuses would have to be sized accordingly.

 

[david luchini]

Is it not 175% for fuses ??
I am in agreement that a 15 amp breaker will meet Code but since the OP did not state which type SCGF method was being used I mentioned fuses would have to be sized accordingly.

175% for dual-element fuses. 300% for nontime delay fuses.

But it's irrelevant per 430.53(A). The three motors can be on a 15A protected circuit (either c/b or fuses) per 430.53(A).

 

[Dennis Alwon]

Is it not 175% for fuses ?? (1.6 x 175% = 2.8. next size up = 3) and then the 15 amp or 20 amp circuit would be a feeder and could be based on the conductor size.
I am in agreement that a 15 amp breaker will meet Code but since the OP did not state which type SCGF method was being used I mentioned fuses would have to be sized accordingly.

See you are so much sharper than I am.. I should have looked at the table...LOL

I agree with David -I would not do it that way either but it is good to know

 

[Dsg319]

Sorry guys. Inverse time breaker. This is correct way of finding conductor size and SC/GF for multiple motors correct?

1-125% FLC of largest motor

2-Sum of all the other FLC

3-Get a total and select a conductor size at or greater than ampacity

4-size SC/GF. Take total FLC multiply by 250% for inverse time breaker.

 

[david luchini]

Sorry guys. Inverse time breaker. This is correct way of finding conductor size and SC/GF for multiple motors correct?

1-125% FLC of largest motor

2-Sum of all the other FLC

3-Get a total and select a conductor size at or greater than ampacity

4-size SC/GF. Take total FLC multiply by 250% for inverse time breaker.

Yes
Yes
Yes
No. SC/GF depends on branch circuit (430.53) or feeder (430.62).

 

[Dsg319]

Yes
Yes
Yes
No. SC/GF depends on branch circuit (430.53) or feeder (430.62).

Thank you! will take a look and get back.

 

[Dsg319]

Yes
Yes
Yes
No. SC/GF depends on branch circuit (430.53) or feeder (430.62).

For selecting SC/GF for the branch circuit supplying several motors is this correct.

480v 3ph squirrel cage induction motors.
1-1hp FLC=2.1
1-3/4hp FLC=1.6
1-3/4hp FLC=1.6

2.1x250%=5.25 Next standard size inverse time breaker is 15amp.

Plus the sum of other 2 motor loads.

15+1.6+1.6=18.2 (not permitted to go up so closest standard rating is 15ampere)

 

[david luchini]

For selecting SC/GF for the branch circuit supplying several motors is this correct.

480v 3ph squirrel cage induction motors.
1-1hp FLC=2.1
1-3/4hp FLC=1.6
1-3/4hp FLC=1.6

2.1x250%=5.25 Next standard size inverse time breaker is 15amp.

Plus the sum of other 2 motor loads.

15+1.6+1.6=18.2 (not permitted to go up so closest standard rating is 15ampere)

That is the correct method for a feeder SC/GF protection for that group of motors.

 

[Dsg319]

Tha

That is the correct method for a feeder SC/GF protection for that group of motors.

Thank you, do you mind to work it out and show me how to do the SC/GF branch circuit protection for that same group of motors sharing a single branch circuit.

 

[david luchini]

Tha

Thank you, do you mind to work it out and show me how to do the SC/GF branch circuit protection for that same group of motors sharing a single branch circuit.

There's really nothing to work out.
Per 430.53(A) you can put the motors on a single 15A branch circuit because each is less than 1HP, each has an FLC of less than 6, each has individual overload protection, and presumably, 15A doesn't exceed the rating listed on any of the controllers.

Or Per 430.53(B), the smallest motor is 1.6A. 1.6A*2.5=4A, so the smallest motor is protected by a 15A c/b (next standard size up.)