Single Phase generator on Three Phase service

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david luchini

david luchini

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A customer has a single phase (120/240V) generator that they want to install at a building with a three phase (208/120V) service. There are no three phase loads in the building. Their thought is that when on generator, loads on the 3rd phase will be unavailable. I can't give them a reason why it wouldn't work. Are there any Code or technical issues that I'm not thinking of?
 
jumper said:
My old workplace in VA was set up that way. Although we did have three phase loads, a small genny just powered lights and a few receptacles from a single pgase gen panel. Worked fine.
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Thanks. To clarify, they're not looking to add a small single phase generator panel, they're looking to put the ATS between the main disconnect and the 3 phase building panel. In other words, a whole building generator, except that it wouldn't power the 3rd phase.
 
If only supplying 120 volt loads, no problem IMO.

If you have two pole (208) volt circuits involved - they will get 240 volts when on standby - something to consider.
 
As long as you can adjust the pickup and shutdown voltages it should work. You said no three phase loads, but watch out for single phase 208 loads.
 
dkidd said:
As long as you can adjust the pickup and shutdown voltages it should work. You said no three phase loads, but watch out for single phase 208 loads.
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ATS can possibly be looking for third phase before it will transfer to generator also.
 
kwired said:
If only supplying 120 volt loads, no problem IMO.

If you have two pole (208) volt circuits involved - they will get 240 volts when on standby - something to consider.
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Thanks...they do have some 208V loads. Mini split is listed for 208/230V, so it shouldn't be a problem. Some appliances (range, dryers) should be fine on 240V. Site pole lights would be my concern, but I'm not sure if they want those to stay powered.

Any concern with a 2 pole load being connected to the 3rd phase to drop out during generator run...That is they would be "half powered."
 
They would be better off setting a single phase 120/208 panel and have just what they want to power in that panel with a manual transfer switch feeding it, then you don't have to worry ablution damaging anything else in the system.

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david luchini said:
Any concern with a 2 pole load being connected to the 3rd phase to drop out during generator run...That is they would be "half powered."
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I cannot speak for all equipment but 240V photocells only break one leg and many HVAC contactors for outside units do the same, so I would say a lot of equipment would just shut off. It is hard for me to say without looking at each piece of equipment and seeing how it is wired.
 
david luchini said:
Any concern with a 2 pole load being connected to the 3rd phase to drop out during generator run...That is they would be "half powered."
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jumper said:
I cannot speak for all equipment but 240V photocells only break one leg and many HVAC contactors for outside units do the same, so I would say a lot of equipment would just shut off. It is hard for me to say without looking at each piece of equipment and seeing how it is wired.
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If you have a 208 volt load connected A-B, and another 208 volt load connected B-C then connected the 120/240 standby to A and C - you would have 240 volts between A and C with the two loads in series through their connection to B bus. Two identical heaters in that situation would only see 120 volts across them. Get unbalanced loads in series via B and you have some voltage to neutral on B and low voltage is sent to other 120 volt loads connected to B bus.
 
Take a close look at the generator...there are 12 lead generators, sold as single phase units, that are actually 3 phase generators. They are factory connected in a zig zag, or double delta to provide 120/240 single phase.
 
windingelectric said:
They would be better off setting a single phase 120/208 panel and have just what they want to power in that panel with a manual transfer switch feeding it, then you don't have to worry ablution damaging anything else in the system.
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JFletcher said:
Agree.
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Ditto.

To the OP, are you planning to connecto just 2 of the 3 phases to generator power or are you planning to feed 2 phases with one leg and the 3rd phase with the 2nd leg? It sounds like the customer is being a bit too cheap and you are trying to accommodate them. You really do need to pay attention to whether or not you have MWBCs because that can easily be an overloaded neutral if you are feeding all three phases from the single-phase generator.

You also most likely will have 2-pole circuits spread between AB, BC, CA connections and there is just no way to balance that out to work on a single-phase source w/out doing a BUNCH of work.

What are they trying to do? Operate fully, or just keep the lights and heat on? How big is the service and how big is the generator?

IMO, you are making a mistake trying to make this work. Something will not go right, and no matter how many warnings you give to the customer, they will think it's your fault because you should know better. You are setting yourself up for problems in the future if you do this. Better to give them a price to install a 3-phase unit that will do the job, or move the customer care about circuits to a single-phase panel and feed that panel with the generator.

Good luck no matter what you decide and I hope you let us know how it goes.
 
david luchini said:
A customer has a single phase (120/240V) generator that they want to install at a building with a three phase (208/120V) service. There are no three phase loads in the building. Their thought is that when on generator, loads on the 3rd phase will be unavailable. I can't give them a reason why it wouldn't work. Are there any Code or technical issues that I'm not thinking of?
Click to expand...

If you feed say, A and C phases from the generator, you will have:

~ All single phase 1p (120V) loads from A and C
and
~ All 2p 208V loads that are fed from A and C legs.

No 3 phase (which you are aware of), no 2p loads that are connected to AB or BC, and no 1p 120V loads from B phase.

What all do they want to power? Putting a lot of 2p loads across AC legs to be able to run them on the generator is asking for a huge imbalance when running on POCO power, unless as oldsparky52 mentioned you are willing (and able) to rebalance all 1p and 2p loads.

Even if you can wire the generator 3ph, it would be a good idea to check the current (and expected) load distribution to load the generator as evenly as possible.
 
kwired said:
If you have a 208 volt load connected A-B, and another 208 volt load connected B-C then connected the 120/240 standby to A and C - you would have 240 volts between A and C with the two loads in series through their connection to B bus. Two identical heaters in that situation would only see 120 volts across them. Get unbalanced loads in series via B and you have some voltage to neutral on B and low voltage is sent to other 120 volt loads connected to B bus.
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I have experienced somewhat similar with people trying to back feed their house panel with 120 volts from a generator. Forget the fact they weren't using proper transfer equipment for the moment - they maybe connect a double male ended cord to a 120 volt receptacle or some other method of just putting power onto one leg of the 120/240 single phase panel. This results in 120 volts on all circuits connected to that leg, but what happens when the thermostat closes in the 240 volt water heater? The 120 applied to line 1 passes through the resistance of the water heater element and is in series with all the 120 volt loads connected to line 2, and any active loads on line 2 see low voltage conditions - prompting them to call me to figure out what the problem is. I see similar happening if you have three phase panel and only supply it with single phase supply and there is any line to line loads connected to the "dead" leg, in fact it does happen if you have fused disconnects in the service/feeders and blow just one fuse.
 
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