Generators

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Would like some input please.

I have two generators that are my only source of power (NO UTILITY POWER), How do you properly connect a THREE POLE transfer between the two? All the wiring diagrams that I have been able to find for this topic shows utility power on one side and a unbonded generator on the other. Nothing that matches my conditions. Only one generator runs at any one time, if I had a four pole transfer switch then I would bond both. But without a UTILITY source if one generator is unbonded and is in service then I do not have a ground fault current path.
 
Generators

Would like some input please.

I have two generators that are my only source of power (NO UTILITY POWER), How do you properly connect a THREE POLE transfer between the two? All the wiring diagrams that I have been able to find for this topic shows utility power on one side and a unbonded generator on the other. Nothing that matches my conditions. Only one generator runs at any one time, if I had a four pole transfer switch then I would bond both. But without a UTILITY source if one generator is unbonded and is NOT in service then I do not have a ground fault current path. Please disregard my first posting for this question as all can tell I am new at posting
 

iwire

Moderator
Staff member
Location
Massachusetts
Pick one of the generators, name it 'utility' set it up with a bonded neutral, grounding electrode system and wire as shown.
 
Generators

If I would proceed as you describe then I would not have a ground fault current path unless generator 1 that has the main bonding jumper installed was running. So then when the maintenance personnel shut down Gen 1 and start up Gen 2 nothing in the distribution would be bonded back to the generator. I dont see how Gen 1 can serve as a return path for ground fault current if it is not in service? Please enlighten me.

Thanks
 

slect

Member
Location
Florida
Generators-Iraq

Generators-Iraq

We use the same set up.. 2 gensets to 3-pole manual xfer switch..
N--G on one leave the other one unbonded and check for absence of internal bond... That is assuming a 5-wire system.. if you have a 4 wire you must add and EGC from each gen to Xfer switch then MDP.. eliminate all other bonds so you dont create and parallel paths.... ( violating 250.6)...
 

slect

Member
Location
Florida
Generators-Iraq

Generators-Iraq

if you google that setup you will find schematics with easy to understand diagrams.. Also.. we are trying to identify the source of current on the egcs at many sites here in Iraq.. HVAC units, auto transformers, etc in units.. the question becomes if they are leaking why dont the rcds trip ? There is also some induced current from the phase conductors.. have you run into this problem ?
 

iwire

Moderator
Staff member
Location
Massachusetts
If I would proceed as you describe then I would not have a ground fault current path unless generator 1 that has the main bonding jumper installed was running.

Running or not the bonding and fault path would remain.


If you physically removed the generator with the bonding then you are correct there would be no fault path for gen 2.
 

dbuckley

Senior Member
The right way to do this is to have the N/G bond at the transfer switch, so you run five wire (L/L/L/N/G) from each genset to the transfer switch, where you have a neutral bar, a ground bar, and a link between the two being the N/G bond, so all solidly connected together, and a three pole transfer switch.

Others can chime in as to the compliance of this with the regulations, but given I'm guessing from your username you are in Iraq and, and thus this is probably a pair of 400V gensets, then doing it a way that is compatible with regs in 400V-land should be acceptable.
 

iwire

Moderator
Staff member
Location
Massachusetts
The right way to do this is to have the N/G bond at the transfer switch,

That could meet the NEC rules as well but IMO there is no 'right way' only peoples preferred way.

Under the NEC we could.

1) Bond at one generator, isolate the neutral but not switch it at the transfer switch

2) Bond at the transfer switch only.

3) Bond at both generators and use a four pole transfer.

All of the above will work and be safe, which option to use is a design decision that may be influenced by what you have on hand to get the job done. :)

He has diagrams that will work for option one so that may be the way to go for him.
 
Generators

I understand now because I was so concentrated on the lack of the bonding jumper at the second generator did not see the current path thru the neutrals at the transfer switch.



Thanks for all the help
 

Cold Fusion

Senior Member
Location
way north
The right way to do this is to have the N/G bond at the transfer switch, so you run five wire (L/L/L/N/G) from each genset to the transfer switch, where you have a neutral bar, a ground bar, and a link between the two being the N/G bond, so all solidly connected together, and a three pole transfer switch. ...

That could meet the NEC rules as well but IMO there is no 'right way' only peoples preferred way. ...

Bob - You're right, of course. However, db's response points out an excellect design issue that gets important real quick.

With a bond in one gen only, there is no easy to disconnect (take completely off line) that gen, to pull electrical maintenance.. A system fault puts the tech at risk. And 4W transfer switches just aren't common. And, depending on the size of the gens, the transfer switch may well be two CBs in the Switchboard.

The following discussion is not concerned with the NEC, rather the best way to install for safety and maintenance. And it probably exceeds the NEC as well.

Tiger -
From your posts, you appear to have a pretty good handle on the issues - which is good, you will get this.

Most of my work is multiple generation, both isolated and parallel, mostly located off-grid. I highly recommend following db's design:

Assuming 3phase:
Wire each gen 5W to the transfer switch, solid neutral. If I could, I'd put the N-G bond in the main panel after the transfer switch.

Additionally, for safety in maintenance, install a neutral disconnect switch on each gen. It only has to be large enough to carry the design neutral current. If all you have are fused disconnects, slug the fuse slot - No Fuses In The Neutral. Lock this disconnect open as part of your LOTO when you are doing electrical maintenance.

If you are concerned with leaving it open when the maintenance is completed, wire one of the un-used poles onto the gen start circut. That way the gen won't run with out the switch being in the closed position. Aux contacts on disconnects are not common, and the disconnect will be a two or three pole - so there will be extras.

I don't know what you have for generation, so assuming you are 208V - 480V, 200A - 400A, the neutral disconnect will likely be in the 100A - 200A range. I'm sure someone will give you grief for using a 4/0 disconnect lug for a #14 control wire. Not to worry, there are several methods to fix this. The simplist is to unbolt the lugs and use crimp terminals on the control wires big enough to fit the lug mounting studs. If that is not available, I'm sure you will figure out a safe connection.

Good luck. Stay safe. Write back and let us know how the job comes out.

cf
 

slect

Member
Location
Florida
Generators-Iraq

Generators-Iraq

The Govt. TF Safe inspectors have approved both ways.. The way I choose to do it depends on:
1) available material
2) see one !
3) conditions and distances to gensets from xfer switch
4) skill level of workers
5) time given to complete the project

Thanks for your input...
 

iwire

Moderator
Staff member
Location
Massachusetts
Bob - You're right, of course. However, db's response points out an excellect design issue that gets important real quick.

Yes without a doubt it could turn out to be an issue.

With a bond in one gen only, there is no easy to disconnect (take completely off line) that gen, to pull electrical maintenance.. A system fault puts the tech at risk.

Brought that up in post 8.:)



The following discussion is not concerned with the NEC, rather the best way to install for safety and maintenance.

I think that calling it the best way assumes a lot that has not been stated, but I defer to your much greater experience then mine. :)
 

RUWired

Senior Member
Location
Pa.
Although i agree that each of the suggested methods will work, i believe that each will require a code change to allow each method to meet the NEC.

By installing the system bonding jumper in the transfer switch without an OCPD from each source in the transfer switch, will not meet article 250.30(A)1.

By wiring one of the generators not as a SDS can only be allowed when the grounded conductor of the generator is solidly connected to a grounded conductor from a service supplied system. By having two generators as alternate sources and neither connected to a service supplied system, each must be treated as a SDS.(250.20(D).

(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30(A). Where an alternate source such as an on-site generator is provided with transfer equipment that includes a grounded conductor that is not solidly interconnected to the service-supplied grounded conductor, the alternate source (derived system) shall be grounded in accordance with 250.30(A).

FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the grounded conductor is solidly interconnected to a service-supplied system grounded conductor. An example of such situations is where alternate source transfer equipment does not include a switching action in the grounded conductor and allows it to remain solidly connected to the service-supplied grounded conductor when the alternate source is operational and supplying the load served.

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Rick
 
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dbuckley

Senior Member
I think the root problem here is the definition of a service and how it works (or more accurately, how the definitions work) in the absence of a serving utility.

These gensets are effectively the PoCo, and they provide power which is distributed to one or more service entrance(s), each of which would be on some structure. Thus what comes out of the gensets switchgear should not be thought of as a feeder, but as a service drop.
 

hurk27

Senior Member
Although i agree that each of the suggested methods will work, i believe that each will require a code change to allow each method to meet the NEC.

By installing the system bonding jumper in the transfer switch without an OCPD from each source in the transfer switch, will not meet article 250.30(A)1.

By wiring one of the generators not as a SDS can only be allowed when the grounded conductor of the generator is solidly connected to a grounded conductor from a service supplied system. By having two generators as alternate sources and neither connected to a service supplied system, each must be treated as a SDS.(250.20(D).

(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30(A). Where an alternate source such as an on-site generator is provided with transfer equipment that includes a grounded conductor that is not solidly interconnected to the service-supplied grounded conductor, the alternate source (derived system) shall be grounded in accordance with 250.30(A).

FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the grounded conductor is solidly interconnected to a service-supplied system grounded conductor. An example of such situations is where alternate source transfer equipment does not include a switching action in the grounded conductor and allows it to remain solidly connected to the service-supplied grounded conductor when the alternate source is operational and supplying the load served.

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Rick


I think we have to look in the definitions to see that can have a "derived system" without a utility service:

Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system

The other generator to each of the generators can cause each to be a derived system if the neutrals are all solidly connected, I would say the other generator is "another system"
 

RUWired

Senior Member
Location
Pa.
I think we have to look in the definitions to see that can have a "derived system" without a utility service:
I agree with that.


hurk27 said:
The other generator to each of the generators can cause each to be a derived system if the neutrals are all solidly connected, I would say the other generator is "another system"

If you were to apply that though then each of the two gen sets would be none seperately derived systems because they are each tied to another system.
The definition of SDS does not change the wording in 250.20(D). If the wording had said "or to another alternate source", then things would be a little clearer.

Rick
 

RUWired

Senior Member
Location
Pa.
These gensets are effectively the PoCo, and they provide power which is distributed to one or more service entrance(s), each of which would be on some structure. Thus what comes out of the gensets switchgear should not be thought of as a feeder, but as a service drop.
I see your point but i don't know how the utilities work mulitple gen sets in parallel. IMO, I would think in this case if one of the generators failed that had the system bonding jumper in place and needed replacing , i doubt that they would shut down the other gen set to install a bonding jumper or even remember to do so.

Sticking with NEC, they are quite explicit in the definiton of feeder conductors and service conductors. To change their meaning to suit every situation would be tiring.

Rick
 
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