Not a separately derived system feed a motor

[binwork91]

Question: if generator only feed a motor, let's say an elevator. 208v 3P system. And ATS is 3P. I understand Not a separately derived system is show as below. Now, if it is feeding a elevator, since it is a motor, then i don't need a neutral. Then if there is a single phase to ground fault when it feed by generator, since there is not neutral conductor, how can fault go back to the generator? if there is other ATS, it can follow other ATS's neutral back to the generator. What if there is only one 3P ATS in the system and feed the motor without neutral?

 

[LarryFine]

You would still have a bond between the supply and motor EGC and the generator neutral.

 

[binwork91]

You would still have a bond between the supply and motor EGC and the generator neutral.

So it become a separately derived system?

 

[LarryFine]

So it become a separately derived system?

It seems that it would not be.

 

[Rock86]

I agree with Larry, it would not be a separately derived system. Your mark up shows the generator with a neutral. if the ATS was a 4-pole breaker and opened the neutral, then by the statement in the code, it would be a separately derived system.

 

[binwork91]

It seems that it would not be.

I found this.
It say not neutral at all consider as "separately derived system". Since there is only one 3P ATS, but it does not have neutral.

 

[augie47]

IF the generator output doesn't have a neutral then it will be operating as an ungrounded system and should have a ground fault monitor to detect phase to ground faults

 

[david luchini]

So it become a separately derived system?

Yes, that would be a separately derived system.

 

[jaggedben]

Question: if generator only feed a motor, let's say an elevator. 208v 3P system. And ATS is 3P. I understand Not a separately derived system is show as below. Now, if it is feeding a elevator, since it is a motor, then i don't need a neutral. Then if there is a single phase to ground fault when it feed by generator, since there is not neutral conductor, how can fault go back to the generator? if there is other ATS, it can follow other ATS's neutral back to the generator. What if there is only one 3P ATS in the system and feed the motor without neutral?
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View attachment 2567729

The fault current path is precisely why you still need to run a neutral in a non-separately derived system. Just because you don't run a neutral to the load doesn't mean you don't still run a neutral between the generator, ATS, and service. The part of the neutral you erased in your second pic is precisely the part you'd need to keep. The neutral to the load that you didn't erase is the part you can erase.

My guess is probably it is cheaper and easier to just run the neutral from the generator to the ATS and service, than to get a switched-neutral ATS and do it as a non-separately derived system (where you also have to ground and bond the neutral at the generator). But your mileage may vary.

 

[augie47]

The fault current path is precisely why you still need to run a neutral in a non-separately derived system. Just because you don't run a neutral to the load doesn't mean you don't still run a neutral between the generator, ATS, and service. The part of the neutral you erased in your second pic is precisely the part you'd need to keep. The neutral to the load that you didn't erase is the part you can erase.

My guess is probably it is cheaper and easier to just run the neutral from the generator to the ATS and service, than to get a switched-neutral ATS and do it as a non-separately derived system (where you also have to ground and bond the neutral at the generator). But your mileage may vary.

My post is based on the generator not actually having a neutral connection (terminal). If I does, I agree with you.....