Single phase generator on a 3 phase wye service

[Barjack]

I know its possible to do this, but the way I would like to do it may not be legal.

This is a new building, service will be 3 phase wye, 800A gutter feeding 4 200A meterbases with disconnects. The disconnects each feed 200A panels inside the building. The main occupant is a doctor, and he has decided he wants a generator to back up the panel that feeds his computer equipment and other circuits he has deemed critical. As always, money is tight, and he doesn't want to spring for a 3 phase generator. The panel in question is mostly completed, and I do not like the idea of tearing it apart and installing a transfer switch/subpanel that would back up the necessary circuits. Would it be legal if I installed the generator transfer switch to back up A and B legs and arranged the breakers so that the backed up loads would fall on A and B? The only 3 pole load on the panel is the A/C, and I would install a NC 3 pole contactor to open it up when the generator turned on. I would also install a NC contactor on C phase to open when the generator is on.

I realize this sounds a little off the wall. You can probably guess I don't have much experience dealing with generators. Its just that at this stage, redoing the necessary circuits to back them up with a transfer switch/subpanel would be a major PITA. Would this work?

 

[iwire]

Off the wall? Yeah pretty much.


As far as a code violation likely 110.3(B) and possibly 408.3(E)

408.3(E) Phase Arrangement. The phase arrangement on
3-phase buses shall be A, B, C from front to back, top to
bottom, or left to right, as viewed from the front of the
switchboard or panelboard. The B phase shall be that phase
having the higher voltage to ground on 3-phase, 4-wire,
delta-connected systems. Other busbar arrangements shall
be permitted for additions to existing installations and shall
be marked.

Exception: Equipment within the same single section or
multisection switchboard or panelboard as the meter on
3-phase, 4-wire, delta-connected systems shall be permitted
to have the same phase configuration as the metering
equipment.

It does not indicate that the bus bars can be arranged "A, B, Nothing - A, B, Nothing", etc

Also what you suggest could really screw up 3 phase equipment someone might add to the panel after you are gone.

 

[Barjack]

Thanks for the reply. I considered not even posting this because of how absurd it sounds, but I needed other opinions. I guess I'm gonna have to tear apart a perfectly good panel and install a sub panel/transfer switch.

 

[augie47]

It sorta sounds like the panel in question is a sub panel.
Can you move the 3 phase load to another panel ?

 

[Barjack]

It sorta sounds like the panel in question is a sub panel.
Can you move the 3 phase load to another panel ?

It is a sub panel, but its 3 phase. I would consider changing it and installing a single phase panel, and moving the 3 phase load to another panel, but I already have all of the circuits in, and most of them are 3 phase MWBC's.

 

[Barjack]

Off the wall? Yeah pretty much.


As far as a code violation likely 110.3(B) and possibly 408.3(E)



It does not indicate that the bus bars can be arranged "A, B, Nothing - A, B, Nothing", etc

Also what you suggest could really screw up 3 phase equipment someone might add to the panel after you are gone.

Except that it would be single phase at that point, coming from the generator. I know that it is not a good design for future adding, but that really isn't my problem. I have to make it legal and work for today.

 

[iwire]

Except that it would be single phase at that point, coming from the generator.

408.3(E) is a requirement for panelboards, the panelboard is 3 phase and will remain a 3 phase panelboard even if the supply is single phase.

But regardless of 408.3(E), I see no way around 110.3(B).

I know that it is not a good design for future adding, but that really isn't my problem.

Understood.

I have to make it legal and work for today.

In my opinion you cannot comply with the NEC if you do what you suggest, but it will be the inspectors opinion that matters.

 

[Barndog]

Why couldnt you put a double pole breaker in the panel and intall a single phase sub panel off of that to a transfer switch to control the critical loads. I would think that would be easier then tearing out the panel and reworking the whole setup?

 

[ceb58]

I know its possible to do this, but the way I would like to do it may not be legal.

This is a new building, service will be 3 phase wye, 800A gutter feeding 4 200A meterbases with disconnects. The disconnects each feed 200A panels inside the building. The main occupant is a doctor, and he has decided he wants a generator to back up the panel that feeds his computer equipment and other circuits he has deemed critical. As always, money is tight, and he doesn't want to spring for a 3 phase generator. The panel in question is mostly completed, and I do not like the idea of tearing it apart and installing a transfer switch/subpanel that would back up the necessary circuits. Would it be legal if I installed the generator transfer switch to back up A and B legs and arranged the breakers so that the backed up loads would fall on A and B? The only 3 pole load on the panel is the A/C, and I would install a NC 3 pole contactor to open it up when the generator turned on. I would also install a NC contactor on C phase to open when the generator is on.

I realize this sounds a little off the wall. You can probably guess I don't have much experience dealing with generators. Its just that at this stage, redoing the necessary circuits to back them up with a transfer switch/subpanel would be a major PITA. Would this work?

One thing to be concerned with is you will be mixing voltages. You say the service is a wye so you have 208/120 3ph. The gen. will be 240/120 1ph. Are there any loads on two pole breakers? The thing that may really get you is 702.4 Equipment approval.

"All equipment shall be approved for the intended use."

 

[kwired]

I know its possible to do this, but the way I would like to do it may not be legal.

This is a new building, service will be 3 phase wye, 800A gutter feeding 4 200A meterbases with disconnects. The disconnects each feed 200A panels inside the building. The main occupant is a doctor, and he has decided he wants a generator to back up the panel that feeds his computer equipment and other circuits he has deemed critical. As always, money is tight, and he doesn't want to spring for a 3 phase generator. The panel in question is mostly completed, and I do not like the idea of tearing it apart and installing a transfer switch/subpanel that would back up the necessary circuits. Would it be legal if I installed the generator transfer switch to back up A and B legs and arranged the breakers so that the backed up loads would fall on A and B? The only 3 pole load on the panel is the A/C, and I would install a NC 3 pole contactor to open it up when the generator turned on. I would also install a NC contactor on C phase to open when the generator is on.

I realize this sounds a little off the wall. You can probably guess I don't have much experience dealing with generators. Its just that at this stage, redoing the necessary circuits to back them up with a transfer switch/subpanel would be a major PITA. Would this work?

I don't know what size generator you are intending to use or what the load calculation is for what is desired to operate on the generator, but if you were to do this you would need a 200 amp 3 pole transfer switch, but it sounds like you maybe only need 30 or 60 amps max feeder from the generator. I think it would cost much less to install an additional panel with only 30 or 60 amp feeder and transfer switch than it would for the 200 amp transfer switch.

If automatic transfer switch is to be installed you have no choice but to install the sub panel or install a generator and transfer switch that will handle the total load on the panel or have additional load shedding controls.

More information may need to be known such as how much standby power is needed and for how long. Computers and other data equipment may be able to use battery back up devices and if they can not be shut down will require battery or else they will be down from time of power loss until power is transfered to the generator.

 

[Barjack]

Thanks, everyone for your help. I'm going to have to grin, bear it, and install a sub panel/transfer switch somewhere inside.

I love it when the owner/GC changes things and thinks its just no big deal.

 

[hardworkingstiff]

I think some additional questions are in order.

Is the doctor going to try to operate his business while on generator power?

What happens to the computers and other critical equipment during the power loss to generator up time?

Seems to me a better understanding might lead to an alternate plan, like maybe some battery back-up (or UPS) for the critical equipment to allow for an orderly shut down?

So, what't the doctor trying to do?

 

[ceb58]

I think some additional questions are in order.

Is the doctor going to try to operate his business while on generator power?

What happens to the computers and other critical equipment during the power loss to generator up time?

Seems to me a better understanding might lead to an alternate plan, like maybe some battery back-up (or UPS) for the critical equipment to allow for an orderly shut down?

So, what't the doctor trying to do?

That's my question also. If the Dr. wants to keep the computers up and not loose date a gen. alone will not do it. Under the best conditions from power failure to gen. crank and transfer you are looking at 10-15 sec. If the computers are the main concern then they would need to be on a UPS system. You could install a 15-20 kw unit with the ATS that has the branch circ. in it but I still think the difference in voltage may be a problem. The transfer switch may see the utility 208 voltage as a drop from the 240v and try and crank the gen. Since there is about a 15% difference in voltage.