Neutral....

[DG2]

Hello,

I need some help on an electrical problem as I am clueless. I want to verify the design. The current situation has an generator and utility feeding an ATS 1200A 4P (switching neutral). The load is an 1200A 3P circuit breaker (existing). What should happen to the neutral when connecting the ats to the 1200A breaker. Ground the neutral or leave it floating? Other possibility? I should also mention that upstream of the ATS on the utility side is a another breaker with GFCI (1200A).

Thanks.

 

[waross]

This has to do with the "One ground to neutral connection" rule. Typically the utility neutral will be grounded ahead of the ATS and the generator will have the neutral connected to ground. The neutral is switched so that both connections to ground are not in service at the same time.
I prefer a solid, unswitched neutral and one common ground point. With no neutral switching there is much less chance of open neutral issues.
BUT, I don't always get to say, and there are some installations where two grounds and a switched neutral may be preferable. If the generator is a relatively great distance from the ATS and possibly in a different building, two ground connections and a switched neutral scheme is more likely to be used.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DG2]

everything is in one building. So does this calls for grounding the neutral after the ATS? How about the GFCI breaker, will it trip?

 

[rockman7892]

I have a similar installation where we are installing a new ATS which as a normal feed from the Utility and the emergency feed from the generator. The ouput of the ATS feeds a 3phase 3-wire 480V MCC.

In the ATS there is a nuetral bus however we do not use a nuetral anywere on the MCC downstream. The nuetral from the utilitly transformer (wye secondary) is grounded at the transformer, and the nuetral from the generator (wye) is grounded at the generator.

Therefore I am thinking that we do not need to use this nuetral bus in the ATS switch at all and can ignore it. As long as there are two other places to land our equipment grounds from each source (I'm thinking there should be a seperate ground bus) then we can land our equpment grounds on on this ground bus and not worry about the nuetral bus.

After typing the last paragrah I'm begining to wonder should we have both source grounds tied together at all times and not worry about switching them or a nuetral?

 

[dpc]

The neutral goes to your service entrance equipment where it is bonded to the neutral bus and then the ground bus. You can't leave it floating.

If this is all downstream of the service entrance, then the neutral is connected to the neutral conductor that runs to your loads. If you have only a three-wire system, the neutral never should have left your service panel.

Is this a 480/277 V system? Do you have 277 V loads?

 

[DG2]

this is a 480/277V system. The existing circuit breaker after the proposed ats is 3 wire. What do you mean by "never left the service panel"?

 

[waross]

There are some ground fault detection issues that may be solved by using one ground at the ATS for grounding both the transformer neutral and the generator neutral.
All grounds should be bonded together. There should be only one connection from the neutral to the ground grid or system.
If you must ground the neutral at both the generator and the transformer, you must switch the neutral in the ATS so that the neutral is only grounded in one place at a time.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

If it is a 4 wire system, the neutral connects to the system neutral.

 

[DG2]

waross,

my situation has no transformer. Utility service is 480/277V. The neutral is designed to switch, but it is feeding a 3ph 3w breaker. what should happen to the neutral.

 

[rob46]

Please review the attached file which shows the neutral bonding requirements.

 

[DG2]

rob,

thanks for that link. I compared the design and they look similar to the diagram (4P). However, since four wire is going to the load which in this case is a 3W CB. What happens to the neutral?

 

[dpc]

No offense, but maybe you need to get a EE involved.

 

[jghrist]

If your neutral is brought to the ATS from the service entrance and from the generator, and is grounded at both places, you need to use the neutral switch on the ATS. Otherwise, the neutral will be grounded in two places and your GFCI may not operate correctly. It doesn't matter whether the neutral is taken to the MCC. The switch keeps the two neutrals from being connected together.

 

[rockman7892]

rob46 thanks for the drawing it helps.

I know its probably semantics, but why in the first diagram is the ground symbol shown going to the ground bus then bonded to the nuetral, and on the geneartor side it is shown going directly to the nuetral point then bonded to the ground bus. Again I'm almost positive this is just semantics but just wanted to make sure.

If both nuetrals are brought to the the ATS and are grouned in both places then I'm assuming that this may cause a GFI to trip because current from one source may return possibly on the nuetral to the other source therfore not summing to zero on the GFI. The grounds do not matter that they are tied together because they are only for protection and not any relay GFI equipment.

 

[waross]

Re the link to the connection drawings.
The connections shown in the first drawing may not be acceptable in many jurisdictions.

Bill
--------------------
"Why not the best?"
Jimmy Carter