To switch the neutral or to not switch the neutral. That is the Question.

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Sleken

Member
Location
Oklahoma
Hello all.


Couldn't resist on the title. ;)

I'm being asked by a customer to start planning for a generator installation. They have already purchased the generator, but it has not yet arrived. It is a 250KW diesel with a nameplate 277/480V amperage of 375.9. I do not yet know if it has overcurrent protection.


The requested installation would supply three separate buildings, each with their own utility service. The customer is requesting manual transfer switches.


These services are all 277/480V.


My current plan is to feed a panelboard with the generator and split off to each of the three services. My main concern revolves around the transfer switches. I've been researching when the neutral is required to be switched. I don't think it needs to be in this situation but the more I read of the fifty bajillion articles on this topic, the more the little nagging voice in the back of my mind gets louder and I'd like some other opinions.

Thank you.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Generally, if the service neutral/ground bonding is done ahead of the transfer switch, the neutral should be switched; if after, unswitched.

In this case, you're talking about permanently tying together the neutrals of three separate services to use a solid, unswitched neutral.

Thus, my instincts suggest switching the neutral. I'm not sure where to suggest that you start here. I'm sure others will chime in.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I seldom see a switched neutral unless there is GF protection involved. Otherwise, the reason for not switching may simply be economics
 

Sleken

Member
Location
Oklahoma
I seldom see a switched neutral unless there is GF protection involved. Otherwise, the reason for not switching may simply be economics

No GF protection is in use here, since the individual services are under 1,000 amps.

The more reading I do, the more I am worried about interconnecting the neutrals of three services by using 3 pole transfer switches. Am I worried about nothing?

The cost difference between 3 and 4 pole is a little over 1k.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
No GF protection is in use here, since the individual services are under 1,000 amps.

The more reading I do, the more I am worried about interconnecting the neutrals of three services by using 3 pole transfer switches. Am I worried about nothing?

The cost difference between 3 and 4 pole is a little over 1k.

I have pondered this question as well. With 1 genset serving 3 separate building/services it has always seemed to me that you would need to set the genset up as a SDS and switch the neutrals. If you used solid neutral ATSs I think this would set up various parallel neutral/ground paths back to the source on the load side of the service disconnecting means in violation of 250.24(A)(5).
I don't recall ever seeing any white papers or discussion of this scenario. Would like to hear others thoughts on this.
 

__dan

Senior Member
With the multiple separately derived systems you have a neutral to ground system bonding jumper at each, and as you tie them all together solidly you are sure to get neutral current travelling the paralleled paths over the grounding system.

There is one way to try to do it cleanly with three pole switches. If the line to neutral connected load, at the dual fed bus, is only a small portion of the total, you can try to eliminate all of the neutral current by eliminating all of the line to neutral connected loads at the dual fed bus. You would move these loads to downstream of a new 480 delta primary, 480/277 Y transformer.

You would try to convert the dual fed bus to 3 phase 3 wire, 5 conductors with the neutral not used for any load, so there would be no worry of neutral current paralleled onto the grounding system.

That would be the value decision, the cost of four pole switches compared to the cost of moving the presently neutral connected load downstream of a new transformer that is line to line connected at the dual fed bus.

If the total neutral connected load at the dual fed bus is less than 20 or 30 kW, adding small transformers could be cost effective and yield added benefits. The available fault current downstream of the transformers is greatly reduced and the three pole switches will be more reliable over the life of the install (statistically over all such installs). The solidly connected neutral will have a zero failure rate, but the switched neutral, when it does fail, can turn a small catastrophe into a larger one. That is the clean way to if you do not need (to add) a huge transformer.
 
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