Motor feeder s.c and ground fault protection

[cppoly]

Section 430.62 is for feeder short circuit and ground fault protection for motors. I'm getting mixed up on definitions here. This section assumes that feeder conductors supplying motors are tapped. Then the protection for the feeder is the largest protection of the largest motor plus the sum of the remaining motor FLCs.

Article 100 defines a feeder as the conductors from the service entrance equipment to the branch circuit OCPD. So if you have a 42 pole branch circuit panel, the feeder conductors would be the conductors coming into the panel and the branch circuit conductors would be for each circuit breaker.

The rule in section 430.62 for sizing feeder protection of motors doesn't make sense for a panelboard with motors on branch circuit circuit breakers. For instance, the feeder OCPD wouldn't be 20A + 5A + 5A for a 200A panel with 3 motors on the branch circuits. Am I missing something?

 

[Smart $]

... Am I missing something?

Well, yes


The following is a generic description...


With motor circuits there is no branch circuit overcurrent device. That's why it's called a motor feeder. With motors, what you typical call an OCPD is the GF/SC protection device. What you typically call overcurrent is overload with respect to motors... and dealt with separately from GF/SC.

Figure 430.1 will confuse you until you have a better understanding. It shows the motor feeder ahead of the motor feeder GF/SCPD. This is not always the case. In many cases the GF/SCPD will be ahead of the motor feeder. And in some cases, there is no motor branch-circuit GF/SCPD.

A panelboard feeder is just that, and not a motor feeder (in all but very rare cases). Motor feeders will be supplied by the panelboard breakers.

 

[cppoly]

A panelboard feeder is just that, and not a motor feeder (in all but very rare cases). Motor feeders will be supplied by the panelboard breakers.

Well yea that's part of the problem I'm seeing. How do you know that? Shouldn't there be a definition in article 430 that distinguishes a motor feeder vs the standard article 100 feeder definition?

 

[augie47]

Let's go back to your example for a moment. You listed 3 motors, two apparently have a 5 amp FLA and the 3rd a FLA permitting a 20 amp GF/SC device so let's assume 8 amps.

You could indeed size the feeder based on those numbers and have a feeder GF/SC device at 30 amp

Your feeder size, based on 430.24 would be a #12 (8 x 1.25 +5 +5 )

If for some reason you decided to use a 200 amp panelboard and install those 3 breakers and supply it with a @12 PROTECTED at 30 amps, you would be compliant. Any breakers added to the panelboaed would require a re-evaluation of the feeder and feeder protection.

If you are wanting to utilize all 42 circuits in the panelboard, you would have to take all the other loads into consideration.
The end result might be a 200 amp panelboard, protected by a 200 amp breaker and fed with a conductor rated less than 200 amps.

 

[cppoly]

If you are wanting to utilize all 42 circuits in the panelboard you would have to take all the other loads into consideration.
The end result might be a 200 amp panelboard, protected by a 200 amp breaker and fed with a conductor rated less than 200 amps.

My confusion is here. 200A breaker feeding 200A panelboard with motor branch circuits on the panelboard. The feeders are supposed to be protected by a maximim g.f./s.c. of 30A, so the 200A wouldn't work. Section 430.62 sounds like it was written to have motors not to be shared on panelboards.

 

[augie47]

I'm sorry, I don't understand the confusion.
Each motor would need the proper gf/sc for it's branch circuit. That can be accomplished by branch circuit breakers in a panelboard.
If you have a 200 amp panelboard protected by a 200 amp overcurrent device (integral with or ahead of the panelboard )and fed by a 200 amp conductor there is no reason you can't use it for your branch circuit breakers.
I rarely see it applied, but if you wanted to take advantage of 430.24, you could use a smaller conductor.

Likewise, if you wanted to install a 30 amp breaker in that 200 amp panelboard and supply a feeder for the motors you listed, you could do that provided each motor had the correct gf/sc protection installed for it's branch circuit. If we followed 430.24 and 430.62 for the feeder to your 3 motors that would have little effect on sizing the OCP and conductor for the 200 amp panelboard. With a bigger motor and proper calculations that main feeder to your panelbaord might change.

On a weekly basis I see 200 amp panelboards fed by 200 amp conductors with breakers installed for motor and non-motor branch circuits and occasionally for sub-feeders.

If I can add anything to explain it better please try to describe the problem again.

 

[Smart $]

My confusion is here. 200A breaker feeding 200A panelboard with motor branch circuits on the panelboard. The feeders are supposed to be protected by a maximim g.f./s.c. of 30A, so the 200A wouldn't work. Section 430.62 sounds like it was written to have motors not to be shared on panelboards.

First, read 430.62(A) carefully. It says where the feeder conductor sizes are based on 430.24. If you have 3 motors with FLC's of 8A each. Under 430.24 the conductor would be sized to 8A ? 125% + 8A + 8A = 26A. The max branch inverse time circuit breaker rating for protecting any one of the motors is 250% or 20A (see 430.52). Now because the feeder conductor was sized to 430.24, the protective device cannot be greater than 20A + 8A + 8A, or 36A. So you have a 26A conductor protected at 36A (max).


However, 430.62(B) says if the feeder conductor has an ampacity greater than required by 430.24 (26A), the protective device rating can be based on the ampacity of the feeder. Say you run a 200A feeder to the 200A panel. You can protect the feeder at not more than 200A.

 

[templdl]

This has always been my understanding of the referrence to ground fault when it comes to Art 430.
SC and GF protection is accomplished by the MCP. It is common for a winding of a motor to go to ground as it fails and as such if an MCP's mag pick up is set just above the inrush current of the motor such that it doesn't nuisance trip if it does trip it is reacting to a winding that is failing. You want to take the motor off line before you end up with a catastrophic phase to ground fault that may progress into a phase to phase fault.
The original intent of Westinghouse developing the MCP was to prevent fires that fuses were not able to respond to. Secondly, the MCP responds to faults or short circuits. I don?t recall that art 430 makes any reference to what class of GF that is required. It is the requirement of an instantaneous trip device such as an MCP that is being referred to. I believe that 430-52 allows for the use of a TM breaker in place of the MCP but my preference is to always use an MCP as a part of a listed combination motor starter, MCP+contactor+OLR.

 

[cppoly]

First, read 430.62(A) carefully. It says where the feeder conductor sizes are based on 430.24. If you have 3 motors with FLC's of 8A each. Under 430.24 the conductor would be sized to 8A ? 125% + 8A + 8A = 26A. The max branch inverse time circuit breaker rating for protecting any one of the motors is 250% or 20A (see 430.52). Now because the feeder conductor was sized to 430.24, the protective device cannot be greater than 20A + 8A + 8A, or 36A. So you have a 26A conductor protected at 36A (max).


However, 430.62(B) says if the feeder conductor has an ampacity greater than required by 430.24 (26A), the protective device rating can be based on the ampacity of the feeder. Say you run a 200A feeder to the 200A panel. You can protect the feeder at not more than 200A.

That hits the spot! I see now that 430.62 was written specifically for motor feeders by saying the feeders are sized with 430.24. I know what motor feeders are and they are also sized differently than feeders defined in article 100. So really this only applies where you have feeders supplying multiple motors that are tapped. Each motor tap will be considered to have its own branch circuit SC/GF protection and the combination of all motor taps being supplied from feeders will have the combined SC/GF rating based on the largest motor + remaining FLCs. Also 430.62(B) allows the feeders for a 42 pole panelboard (in my original example) to be protected by OCPD in accordance with their ampacity since they are not sized according to 430.24.

Thanks