backup generator neutral ?

[sparky 134]

when two ungrounded conductors which share a neutral originate from two different panels ( one from the main panel and one from a generator backup panel ) in which panel should the neutral terminate. the situation is a residential setting and the backup panel is fed from the main panel and the generator via a transfer switch. there is a neutral from the main panel to the neutral terminal strip in the backup panel and also a neutral from the generator which terminates on the neutral strip in the backup panel. electrically speaking, it shouldn't matter which panel the neutral from the network terminates in since the neutral in the main panel and the backup panel are tied together.

 

[Ed MacLaren]

Re: backup generator neutral ?

in which panel should the neutral terminate.

I'm not quite sure what you mean by "terminate"?

The generator feeder can be configured as a Separately Derived System (SDS) - Diagram 2, or as a non-SDS - Diagram 1, depending on whether or not the generator neutral is bonded to it's frame.

The actual connections depend on which configuration is used.

Ed

[ February 17, 2004, 11:58 PM: Message edited by: Ed MacLaren ]

 

[mclain]

Re: backup generator neutral ?

Ed your diagrams are great and illustrate the difference in what the NEC calls a SDS and a non-SDS. It also points how what has been a hot subject on this forum before. In the SDS system the neutrals are still connected together by the GEC. As long as the NEC doesn?t recognized this as an electrical connection between the neutrals I guess it will be called an SDS.

Don

 

[Ed MacLaren]

Re: backup generator neutral ?

It also points how what has been a hot subject on this forum before. In the SDS system the neutrals are still connected together by the GEC.

You are correct, but contrary to popular opinion, this does not create a "parallel neutral" problem with regard to unbalanced load current flow.
The switching of the neutral creates two separate systems.

Ed

[ February 18, 2004, 07:40 AM: Message edited by: Ed MacLaren ]

 

[mclain]

Re: backup generator neutral ?

Ed I agree with you on this. The current will only flow back to the souce that is supplying it.

 

[sparky 134]

Re: backup generator neutral ?

diagram 1 would be the applicable drawing. the transfer switch is only a 2 pole. the neutal does not get switched. what i mean by terminate is that the 2 ungrounded conductors, which share a neutral, enter a junction box and from this box one of the ungrounded conductors pipes to the main panel and the other ungrounded conductor pipes to the backup panel. so my question is, which panel should the neutral from the network go to, or does it matter ?

[ February 18, 2004, 07:38 PM: Message edited by: sparky 134 ]

 

[Ed MacLaren]

Re: backup generator neutral ?

what i mean by terminate is that the 2 ungrounded conductors, which share a neutral, enter a junction box and from this box one of the ungrounded conductors pipes to the main panel and the other ungrounded conductor pipes to the backup panel. so my question is, which panel should the neutral from the network go to, or does it matter?

I don't understand the arrangement described.
What do you mean by "the neutral from the network"?

Ed

The cable layout would normally look like this-

[ February 18, 2004, 10:38 PM: Message edited by: Ed MacLaren ]

 

[sparky 134]

Re: backup generator neutral ?

the grounded conductor that is common to the 2 ungrounded conductors ( black, red, white ). for example, the black ( hot ) wire would go the main panel and the red ( hot ) wire would go to the backup panel. to which panel would the white ( neutral, grounded ) conductor go to. since a grounded conductor from the generator and a grounded conductor from the main panel both terminate on the neutral bus in the backup panel, does it make a difference where the neutral ( grounded ) conductor from the black / red network terminates ?

[ February 18, 2004, 10:44 PM: Message edited by: sparky 134 ]

 

[Ed MacLaren]

Re: backup generator neutral ?

for example, the black ( hot ) wire would go the main panel and the red ( hot ) wire would go to the backup panel. to which panel would the white ( neutral, grounded ) conductor go to.

You've lost me.

Here are the connections at the transfer switch, for the cable layout in my previous post. This is the way I would do it.

Ed

 

[sparky 134]

Re: backup generator neutral ?

the main panel, backup panel, transfer switch and generator wiring is done. my question is about the branch circuits that share a neutral BUT originate from different panels ( main panel and backup panel ). since the branch circuits originate from different panels should each branch circuit have its own neutral even though the neutral from the main panel and the backup panel are tied together on the neutral bus in the transfer switch. i apologize for the confusion...hope this explains it a little better.

 

[Ed MacLaren]

Re: backup generator neutral ?

my question is about the branch circuits that share a neutral BUT originate from different panels

Ah, you mean a three-wire branch circuit, only "half" of which is supplied from the generator? Sorry I was so thick.

The best solution would be to completely separate them and create two two-wire circuits, each with it's own neutral.

If that is not practical, run a neutral with each hot wire, in other words, to both panels.

If any part of this remains a three-wire circuit, sharing a neutral, you will need to make sure that it's two "hots" are supplied from different phases, to prevent overloading the shared neutral.

It is important that each hot wire is in the same cable or conduit as it's neutral, especially if it is metal-sheathed cable/metal conduit, so that their magnetic fields cancel, to avoid induction heating of the conduit or cable sheath.

Ed

[ February 19, 2004, 09:51 PM: Message edited by: Ed MacLaren ]

 

[highkvoltage]

Re: backup generator neutral ?

Ed. As a Jouneyman Lineman and a Jouneyman Wireman I prefer the second diagram. The reason is that in the first diagram you are back feeding the neutral to the utility. If the grounding system is better on the utility side i.e. at the transformer and there is a unbalanced load on the grounded conductor (neutral) you could be putting a lineman at risk. I always recommend that a 3 phase disconects or 3 phase lighting contactor be used so the neutral is disconnect on the system not being used. Also if a thunderstorms are in the area and the utility's gets struck it would destroy the generator. I have seen this happen once. Best regaurds and always enjoy reading your post.

 

[highkvoltage]

Re: backup generator neutral ?

Sorry for the bad typing. Good electrican/lineman Bad at typing. By the way IBEW trained in both fields after 10 years of school. I love them both and still work in both fields.

 

[jerryalan]

Re: backup generator neutral ?

i've installed several back-up generators and prefer using a three knife 200a transfer switch ahead of the main panel in order to totally isolate the two systems and provide for an upgrade to a larger generator once the customer realizes a 15kw won't do everything he wants it to do.
AEMC, poco in N. Ga. requires total isolation of generators and their equipment, i think they had a bad experience once.
although this requirement may not be supported by code it IS their equipment and their product . . .
i feel comfortable doing this and although the customer might complain about the cost this method hasn't killed anyone upstream yet, to the best of my knowledge . . .

 

[hurk27]

Re: backup generator neutral ?

By highkvoltage: The reason is that in the first diagram you are back feeding the neutral to the utility. If the grounding system is better on the utility side i.e. at the transformer and there is a unbalanced load on the grounded conductor (neutral) you could be putting a lineman at risk

I'm cornfussed HKV as how in the world can a three pole transfer switch back feed voltage into the system if there is no complete path for the current? Just having the neutral connected to the utility's neutral will not back feed the utility! You must have a complete path and as long as the switch is in the generator position there is no complete path. Transfer switch's are not make before break. other wise it would destroy most generators that don't have any sync circuit.

 

[ccha9219]

Re: backup generator neutral ?

Because the grounded and grounded conductors are common in the transfer switch,ther is a potential to get current back to the transformer.
I have seen this animal do stranger things.

 

[iwire]

Re: backup generator neutral ?

Originally posted by ccha9219:
Because the grounded and grounded conductors are common in the transfer switch,ther is a potential to get current back to the transformer.
I have seen this animal do stranger things.

How?

If only one conductor is common (the neutral) between the utility and the generator you do not have a circuit and can not have current.

 

[highkvoltage]

Re: backup generator neutral ?

Hurk27. If you are familar on how a pole transformers are bonded and grounded you would realize the point I was making. If you understand that the grounded conductor (neutral) in line work is just as deadly as the hot conductors you would also understand. When you have a back up power system and they only disconnect the phases and not the grounded conductor (neutral) that would leave the potential for the unbalanced load to be on the grounded conductor (neutral). Which if it is connected in some phyical means to the utility neutral and the resisiance is lower at the utility grounding verse the customers grounding system and the lineman goes to disconnect the grounded conductor (neutral) at the transformer he is also disconnecting the grounding or bonding. Thus you now have a PATH THAT COULD KILL A LINEMAN!!
That is why I perfer a 3 blade diconnect or contactor verses a two blade disconnect or contactor.

 

[Ed MacLaren]

Re: backup generator neutral ?

highkvoltage,

If the grounding system is better on the utility side i.e. at the transformer and there is a unbalanced load on the grounded conductor (neutral) you could be putting a lineman at risk

It sounds as if you are under the impression that the unbalanced load current, in the non-SDS configuration (Diagram 1) would be flowing back to the generator through the grounding system?

That can't happen. The only path to the generator winding center-tap is by way of the generator neutral conductor, which is solidly connected to the load neutral conductor.

Ed

 

[don_resqcapt19]

Re: backup generator neutral ?

highkvoltage
It sounds like you are thinking that the generator neutral current is trying to get to the earth. Its not. It can only return to its source, the generator.
Don