120/208 generator as stand-by for 120/240 residential service

I have an opportunity to pick up a 45K natural gas powered generator and 150 amp ATS for next to nothing. I would like to have it installed as a stand-by unit for my house and shop but I'm getting conflicting information on whether I will encounter any issues.

My main concerns are my A/C condensing unit motors, which are labeled as 208/230v, my well pump which is on 240v and my pool pumps (which have to run in the winter to prevent freezing) but I haven't had a chance to check the ratings on. I have gas heat with 120v blowers, gas hot water heater and I wouldn't use the electric dryer during an outage, so they should be non-issues. I know it might take a little longer to bake a cake in the electric oven, but again, that's no big deal.

I know that all of the 120 circuits will be fine.

The opinions I have been given so far are:
1-Use it and everything should be fine
2-Use it but I would have to install buck-boost transformers on the A/C units, well & pool pumps
3-Stay far away from it, its more trouble than its worth

Any advice will be GREATLY appreciated and if anyone here has done this, what were your results?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I will allow this since it is not a how to thread.

IMO, the a/c will be fine but running the pool pump and well for extended periods of time will not be the best thing for them. They will run hotter and both will run below the +- 10% tolerance usually allowed for a motor.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I have an opportunity to pick up a 45K natural gas powered generator and 150 amp ATS for next to nothing. I would like to have it installed as a stand-by unit for my house and shop but I'm getting conflicting information on whether I will encounter any issues.

My main concerns are my A/C condensing unit motors, which are labeled as 208/230v, my well pump which is on 240v and my pool pumps (which have to run in the winter to prevent freezing) but I haven't had a chance to check the ratings on. I have gas heat with 120v blowers, gas hot water heater and I wouldn't use the electric dryer during an outage, so they should be non-issues. I know it might take a little longer to bake a cake in the electric oven, but again, that's no big deal.

I know that all of the 120 circuits will be fine.

The opinions I have been given so far are:
1-Use it and everything should be fine
2-Use it but I would have to install buck-boost transformers on the A/C units, well & pool pumps
3-Stay far away from it, its more trouble than its worth

Any advice will be GREATLY appreciated and if anyone here has done this, what were your results?
I'm assuming you are talking about a 3 phase machine since you mention 208 volt.
This is a bad idea for a number of reasons. Since you will only be able to use 2 of the phases and the neutral you will only be able to use about 26 KW of it's capacity. There will be issues with the 3 phase transfer switch as well, as you will have nothing connected to the 3rd phase.
If you intend to permit this many AHJ's would take exception to this idea.
The good news is I don't think the voltage would be a problem as most 240 volt loads will be fine on 208 volts.
My vote would be option 3 for sure.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
k46tank:
You have not, as far as I can see, specified what the output voltage of the generator is.
If it is 208V/ 120V three phase your electrician will have bigger problems than just the voltage in setting up the ATS.

if you have only 120V from the generator, none of your motors will survive.

Tapatalk!
 
I will allow this since it is not a how to thread.

Thank you for allowing this thread. I meant it as a discussion of the positives/negatives of backing up a 120/240v service with a 120/208v three phase generator and in no way a how to.

I would hope that since the well pump only runs in short cycles, and if it is a 208/230 motor, it would survive the 208v, minus the voltage loss due to the resistance of the wiring run from the panel to the pump.

I am concerned about the imbalance of utilizing two phases of the generator to supply the two legs of the residential service and not using the third leg of the generator. I know there would be a way to split up the 120v circuits but it would be a complicated affair and involve an additional panel for the third phase.......maybe I could install some external lighting that would only be illuminated while the genset is running....lol.

I did just check the pool pumps and they are 115/230v and operating on 240v which means that they wouldn't survive the 208v back-up. So, even though I would be getting an amazing deal on this genset, I just don't think it would be practical for me to use it.

So it looks like option #3 it is. I'll be back to looking for a good deal on a 120/240 single phase genset.

Thank you for the input and advice. I hope that my questions will help someone in the future.
 

broadgage

Senior Member
Location
London, England
As the proposed generator is said to be 120/208 volts, it should be fine for most loads.
Lighting and small appliances will get 120 volts as normal and wont know the difference.

240 volt loads will only receive 208 volts, most will be fine since 120/208 volt services are common especialy in apartments, and many appliances are listed for both voltages.

Double check the voltage of motors, if dual rated for 208 volt or 240 volts service they will be fine.
If straight 230/240 volt then I would think twice, the motor will draw more amps and run hotter, this MIGHT be OK if the motor is oversized and/or listed for continous duty but only used short time. Likewise it is not amps as such that kills motors but heat, if the motor is listed for tropical or hot weather but used in winter temps below freezing it will probably be fine.

Depending on the age, condition, and cost of the motors, it might be worth replacing them with dual rated units suitable for both 208 volt and for 240 volt service.

Since the generator is presumably 3 phase, 4 wire, you wont be able to fully utilise it.
 

ceb58

Senior Member
Location
Raeford, NC
Aside from the potential problem with the pool and well pump your ATS would/could be an issue. The ATS could "see" the single phase utility voltage and not "see" the third leg. It may think you have lost a phase and start and transfer to generator. If the third phase from the generator was not connected it would/should shut the generator down on a voltage or phase fault.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Another consideration: is the generator a 208/120V generator, or is it a '12 lead' generator connected for 208/120V? The installation electrician may be able to configure the generator for straight 120/240V single phase operation.

-Jon
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Another consideration: is the generator a 208/120V generator, or is it a '12 lead' generator connected for 208/120V? The installation electrician may be able to configure the generator for straight 120/240V single phase operation.

-Jon
Yah, I would guess if it is 3 phase it is likely a 12 lead machine. If so, it could be configured as 120/240 single phase. Problem is though, it would still only be usable at 26 KW and the OP would still have an issue with the transfer switch.
Of course, he could do this and ditch the transfer switch for a new single phase switch. He'd then have a genset with a healthy sized motor, but usable.
 

kwired

Electron manager
Location
NE Nebraska
Yah, I would guess if it is 3 phase it is likely a 12 lead machine. If so, it could be configured as 120/240 single phase. Problem is though, it would still only be usable at 26 KW and the OP would still have an issue with the transfer switch.
Of course, he could do this and ditch the transfer switch for a new single phase switch. He'd then have a genset with a healthy sized motor, but usable.
Good chance it uses more gas than an appropriate sized motor powering a 26 kW generator as well.
 

kwired

Electron manager
Location
NE Nebraska
Good chance it uses more gas than an appropriate sized motor powering a 26 kW generator as well.
I'm a sucker for a good deal. If the same offer were made to me I'd take the generator.
I may as well, but may find out the hard way just how much fuel it may take if ever in a situation again where power is out for 10 days.

A few years back we had ice storm take down a lot of rural distribution, I was out for about 10 days. I only was using a 8000 watt portable generator, had limits on what could run at one time compared to "normal" and was looking at gasoline consumption that would have been around $900 after a months time, well over what I would have paid the POCO for energy from them, bump the prime mover to one that can power a 45kW generator and I have to assume it only gets worse if you don't need at least 25kW of load to be powered at one time.
 
I would not use it for powering a well pump as-is. The low voltage coupled with the hassle and expense of replacing a well pump make it not worth the risk.

However, I have only seen one 120/208Y genset that could not have been reconnected for 120/240 and it was about a 1970 model.

Since this is not the place for a how-to, I suggest you dig in and see about reconnections.
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
HVACR Units

HVACR Units

Most modern HVACR units are specifically designed to be ok on 208-240 which I have to say is a wise move by the industry.
Also most all of them have a 208 and a 240 volt tap for the control transformer IF they are 208-240 based AHUs rather than 115V gas fired fan drives.
Other motors however such as the one on our plant air compressor are rated for one or the other voltage and they are not advised to run except on the rated voltage. One would have to look at all the ratings on all the equipment to make a determination.

SC
 

kwired

Electron manager
Location
NE Nebraska
Most modern HVACR units are specifically designed to be ok on 208-240 which I have to say is a wise move by the industry.
Also most all of them have a 208 and a 240 volt tap for the control transformer IF they are 208-240 based AHUs rather than 115V gas fired fan drives.
Other motors however such as the one on our plant air compressor are rated for one or the other voltage and they are not advised to run except on the rated voltage. One would have to look at all the ratings on all the equipment to make a determination.

SC
True, many general purpose motors also are rated 208-230 (typically single phase) or 208-230/460 (three phase), and have roughly 10% higher current rating if used at 208 instead of 230 volts. Those motors are designed for either nominal voltage, and a lot of hermetic compressors are also rated for 208-230 volts (single and three phase) and can be used on either.

Add: I guess the last sentence kind of says exactly what you said in a way, my point was many GP motors are rated for both voltages - But not all are.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Is there situation where you would actually want to write 120/208Y? And if so, what would it be?
For situations where you are feeding, say, an apartment sub panel fed from a 208Y/120 service.
But I think we need clarification from on the proper way to express this. Is it 120/208 or 120/208Y? I'll sure Jim Dungar will have the answer.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
For situations where you are feeding, say, an apartment sub panel fed from a 208Y/120 service.
But I think we need clarification from on the proper way to express this. Is it 120/208 or 120/208Y? I'll sure Jim Dungar will have the answer.
I had not thought of that.
My vote would be 120/208 three wire and leave it at that.
 
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