3-Wire System on a 4-Wire Service

[Lauren]

I have a 4-wire, 480V supply that is brought to a service entrance disconnect. I know I am required to bring the neutral to this point to comply with code (250.24). My question is, can I "abandon" it there if I do not need a neutral for single-phase loads? The three-phase system will be brought from the service entrance into a 3-wire switchboard (or 4-wire, depending on the answer to my question), and will feed 3-phase loads.

Second part of my question is, if I connect a generator to that 3-wire switchboard, how to I properly ground the neutral of the generator? Do I need to bring the generator neutral back to the service entrance as you would with a non-separately derived system? Or can I solidly ground the generator and create a separately derived system?

Third part...does the answer to "part 2" depend on if I am operating the generator in parallel or islanded with the utility?

 

[Dennis Alwon]

The code only requires the neutral at the service. Once you leave the service disconnect a neutral isn't required however it may be wise to install one if you are feeding a sub panel.

 

[Dennis Alwon]

I believe you have a choice to either have a separately derived system or not. I am not sure what the advantages are for having a separately derived system however most of the time a non-separately derived system is installed.

 

[augie47]

1. The grounded conductor (Neutral) does not need to run beyond the service if not needed.
2. The connection of the generator will depend in how the transfer swicth is wired. If the neutral it unswitched you do not bond it at the generator.

 

[Lauren]

I believe you have a choice to either have a separately derived system or not. I am not sure what the advantages are for having a separately derived system however most of the time a non-separately derived system is installed.

The only advantage I can see would be cost savings. The generator will not be located near the service entrance, so it's an additional cable that would need to be run.

 

[Lauren]

1. The grounded conductor (Neutral) does not need to run beyond the service if not needed.
2. The connection of the generator will depend in how the transfer swicth is wired. If the neutral it unswitched you do not bond it at the generator.

1. Thanks for the clarification
2. There isn't a transfer switch. The transfer will occur at the switchboard level (electrically operated breaker for the incoming utility and an EO breaker for the generator).

 

[ron]

Whenever possible, we bond the N-G at the service disconnect and leave it there and do not bring the neutral any further downstream (if there are little or no L-N loads). The N-G gets bonded at the generator and not brought downstream, and then you don't have any worries on what to do with it at the breaker transfer pair in the switchboard (3 pole or 4 pole breakers), since there is no neutral.
If there are a few L-N 277V loads, you just put in a small transformer to derive a neutral for those few loads.

 

[Lauren]

Whenever possible, we bond the N-G at the service disconnect and leave it there and do not bring the neutral any further downstream (if there are little or no L-N loads). The N-G gets bonded at the generator and not brought downstream, and then you don't have any worries on what to do with it at the breaker transfer pair in the switchboard (3 pole or 4 pole breakers), since there is no neutral.
If there are a few L-N 277V loads, you just put in a small transformer to derive a neutral for those few loads.

This seems to be describing exact what I was thinking. Just for clarification: the N-G at the service does not get connected to the N-G on the generator in this scenario...they are grounded separately?

Thanks!

 

[augie47]

IMO, with your generator neutral hard connected to your service gear (no transfer switch, IF you bonded the neutral at the generator you would be in violation of 250.24(A)(5). The generator would need a EGC but the neutral would remain unbonded at the generator

 

[Lauren]

IMO, with your generator neutral hard connected to your service gear (no transfer switch, IF you bonded the neutral at the generator you would be in violation of 250.24(A)(5). The generator would need a EGC but the neutral would remain unbonded at the generator

So it's either I need to bond at the service disconnect OR at the generator (but not both)...correct?

 

[augie47]

Your service neutral would be required to run to your service gear and bonded.
Based on 250.20 I can't see where you are required to use the generator neutral but I would be interested in other thoughts on the matter.

 

[ron]

This seems to be describing exact what I was thinking. Just for clarification: the N-G at the service does not get connected to the N-G on the generator in this scenario...they are grounded separately?

Thanks!

You run EGC's with your feeders to the switchboard where the transfer pair live, so bonding occurs as would normal.

 

[123ozzie]

Your service neutral would be required to run to your service gear and bonded.
Based on 250.20 I can't see where you are required to use the generator neutral but I would be interested in other thoughts on the matter.

Where in the code does it state that a neutral is required to run to your service gear? Just looked at a building that has a 12.5 to 480 v utility transformer feeding a multi meter bank. The meter bank has a neutral in it from the transformer, and has individual disconnects. One for each meter. The building in question has a 200 amp 480 v switch gear in it, with only 3 hots pulled from the meter bank. No neutral. I was called out because the gear doesn't have a cold water GEC in it (probably because there isn't a neutral to bond it to) Any thoughts?

 

[roger]

250.24(C)

 

[hillbilly1]

1. Thanks for the clarification
2. There isn't a transfer switch. The transfer will occur at the switchboard level (electrically operated breaker for the incoming utility and an EO breaker for the generator).

That’s still a transferswitch, it’s just a different style. Still prevents back feeding the utility. If it is a groundfault main, (If service rated transferswitch ) it’s most likely a four pole breaker breaking the neutral, in which the generator would be a separately derived system.