430.62(A)

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DetroitEE

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I've read through 430.62(A), and I'm a little bit confused on what it's saying, specifically when the feeder ground fault/short circuit protection device is a circuit breaker and the branch circuit ground fault/short circuit protection device is a fuse.

Does this article prohibit sizing the branch circuit fuse at 175% the FLA and the feeder circuit breaker at 250% the FLA when serving a single motor? The way I'm reading it, it sounds like the feeder circuit breaker rating cannot exceed the branch circuit fuse rating.

So, in other words, if I have a 480V, 75 HP motor, (FLA is 96A), would it be a violation to install a 175A fused disconnect next to the motor, and a 225A circuit breaker in the distribution panel feeding the fused disconnect?
 
Are you concerned with feeder (by definition) protection or branch circuit (by definition) protection as it relates to the conductors supplying the motor?

The reason I ask is that you use an example of a single motor.

Pete
 
I'm just concerned with properly sizing the circuit breaker in the distribution panel that is feedeing the fused disconnect next to the motor. Is the wiring between a distribution panelboard and a fused disconnect considered a feeder or a branch circuit if it is only serving a single motor? Maybe that's where I'm getting tripped up.
 
I think it's important to note that 430.62(A) specifies "conductor size based on 430.24" as part of the wording.
If you size the breaker based solely on the conductor size there is no reason the breaker could not be a 250 amp or 400 amp etc. as long as the fusible disconnect is sized for the proper GFSC motor protection and the conductor has an ampacity conforming to the breaker.
The way I read it, if you size the conductor based on the motor (430.24) then you would need to size each OCP device based on 430.52. The fact that 430.52 allows a different value for fuses and breakers, IMO, you could size the fuse at 175% and the breaker at 250%.
 
I agree with Gus's take and I will add that if the breaker at the distribution panel is sized to provide the GFSC then a fused disconnect would not be necessary. It could simply be a non-fused disconnect at the motor location.

The reason I kept saying "by definition" is that if you had a breaker at the distribution panel and a fused disconnect at the motor you would have, by definiton, a feeder. With the example I gave above, sans the fuses in the disconnect, it would be considered a branch circuit.

Pete
 
I stumbled upon this post and am now totally confused as well. Using the origional poster's example, I would have thought the conductor amps would be (96Ax 1.25) = 120A, the CB in the panle would be (96A x 2.5, then next size up) = 250A cb. The fused disc. would be inline and be rated for (96x1.75, then next size up) = 175A fuse.

However, after reading the debate, I am questioning myself as well. I think the CB in my calculations above would be a violation. It would be ok if there were no fuses.

I think it needs to be as follows:

wire between motor and fused disc = branch ckt = (96Ax1.25) = 120A.
wire between fused disc and CB = feeder = (largest FLC X1.25, + remaining motors FLC) = (96X1.25, +0) = 120A.
Fuse size = (96Ax1.75, then next size up) = 175A fuse.
CB in panel = feeder CB = (largest branch ckt protection device + remaining motor's FLC, then next size down) = (175A+0, then next size down) = 150A CB.

However, reading augie47's post, he was saying the CB could remain a 250A CB.

Can somebody else chime in or can augie47 please help me out anc clarify. Can the CB remain 250A?


I usually use a fused disc because of the kA rating being limited to 10 but lets not get into that at this point.

Thanks
 
Remember, Gus can be and is often wrong :)

that said...
430.52 would allow your SCGF device to be 250% if its an Inverse-time breaker, or in your case 96 x 2.5 or 240 rounded up to 250.

430.62 allows your feeder to have a OCP not greater than that calculated by 430.52 (+ sum of others =0)
If 430,52 allows a 250 amp breaker, 430.62 allows a 250 amp breaker.

You use breaker calculations (250%) for breakers and fuse calculations (175%) for fuses. Your fuse size does not dictate the breaker size.
 
thanks, but I'm still not seeing it. I must have once because I agreed with cb at 2.5% and fust at 1.75% up til I found this post. I'll need to re-read.
 
For conversations sake, assume you have two breakers instead of a fuse. 430.52 would size the one at 250 amps and 430.62 would allow the 2nd to be 250.
There is nothing that requires the feeder breaker to be any different if you make the branch protector a fuse.
 
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