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208/120 volt generator

Jack Innes

Subscriber
I am wondering if a 208/120 volt, 15 kw, 60 hz genset would be practical to use as a house backup unit.

My, perhaps too simple, understanding is that I could use 2 legs & neutral to each give 120 volts which could supply the house panel. This would run 120 volt requirements but due to phase shift would only supply 208 volts in a single phase form. I also understand that 208 volts will run a 240 volt motor but can be damaging. I have few 240 volt motors & can live without their use. Will it run a 240 volt water heater safely?

Would the 3rd & unused 120 volt leg need to have an isolated load (heater, lights) while the unit is running?

Am I missing something here, it seems too simple? I am not an electrical engineer for sure!

The unit has a Nissan diesel engine, 200 hours total running time & is up & running.

Thank you,
Jack
 

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pegasuspinto

Active member
You're probably OK. You might be careful which two legs you pick, you want one of the legs with sensing for the voltage regulator.

You can also turn the volts up a bit, 120/208 is nominal but you won't cause any issues going to say, 125/216. But the volts being low can help with marginal generator capacity on resistive loads. For instance a water heater running about 3/4 it's normal watts will probably still make enough hot water that no one will notice.

Finally, look at your 240 appliance tags, many of them are designed from the start to work on 208-240 volts.
 

Birken Vogt

Email NOT Working
Often it is not too hard to covert it to 120/240 true single phase as well. Rating will be something like 67% to 83% of nominal. But that is all it would be on 208 single 2 phase usage anyway. What 240 volt loads do you have?
 

Jack Innes

Subscriber
Often it is not too hard to covert it to 120/240 true single phase as well. Rating will be something like 67% to 83% of nominal. But that is all it would be on 208 single 2 phase usage anyway. What 240 volt loads do you have?
The only 240 loads would be the water heater, a 1 1/2 hp motor on a mill, 5hp motor on a lathe & a 1 1/2 hp motor on a compressor. These all are not in commercial use & a power outage would not be terribly inconvenient.

My main concern is the 120 volt well pump, furnace & refrigeration.
 

Birken Vogt

Email NOT Working
The mill and lathe will not care unless you load the motors to max output. So don't take deep cuts on generator power.

The compressor might. You would have to run it on 208 and measure the current the motor is taking compared to the max current stated on the nameplate. Below max current, it is fine. Above and you risk overheating the motor.

The water heater will take a little longer to get the job done, but the 120 volt loads will see no difference whatsoever.
 

PLCtech

Member
You will notice no difference. I have at least 100 motors running on 208 volts. I promise, the motors don't care. Your water heater won't care.
 

Rich Mc

Member
Check to see if the generator head is constructed as such that it could be wired for 120/240 by zig/zag or other configurations. How many leads in the generator housing(some maybe tied together) are available to the end user?
 

miltruck

Member
I have a 20 kw 208 3 phase 2-53 Detroit powered gen set that I use to power the house with no issues. The only 240 volt load is the 3 hp air compressor in the garage, so I just shut it off. The 120 volts runs everything fine, if fact due to the size of the engine and the low load I have to turn on all the lights in the house and garage just so the engine will get even a little warm. Looks funny driving down the road and the only house lit up like a Christmas tree is mine and everyone else looks like they are on candles.
 

Jack Innes

Subscriber
Check to see if the generator head is constructed as such that it could be wired for 120/240 by zig/zag or other configurations. How many leads in the generator housing(some maybe tied together) are available to the end user?
I will check this once the generator arrives here, thank you.
 

dkamp

eMail NOT Working
Yeah, it'll be just fine. You have there what's frequently referred to as 208/125Y... I have a Kohler flathead on propane set up just like that.

If you're looking for emergency power service, set it in a weather-protected shed, or inside a building, that'll provide you with shelter and service access under inclement weather conditions. Make sure you have more than ample battery power for cranking the engine, and a float charger system that does so. If your area is not heated, tt would be a good idea to have an engine heater on a switch that you can turn on, let's say when there's a nasty ice storm barreling in, so you've got a warm engine ready if/when the power lines come down.

Set up the power feed from your unit so that the unused leg goes to some overhead lights in your generator area, so you have illumination as SOON as you get it running. IF you have the ability, extend this line to say, a fixture on the outside of the building, mebbie even to a few critical places in your home (like a second sump pump, and an emergency light at your utility panel) for same reason.

You could use an automatic transfer switch... or An inexpensive, but code-viable manual-transfer option is to use an interlock breaker kit to back-feed your panel. This involves installing a circuit breaker into a particular location on your service panel, and an accessory on the front of the panel that allows either the main breaker, or the backfeed breaker, to be engaged to the main panel bus at a time, but not both.

IF you choose that route, it'd be a good idea to set up your heavier, non-critical loads so that they cannot start when the generator is running. There's many ways to do this, and having that third leg coming in can provide you with a 'emergency power' signal source to energize lockout relays for uneccessary loads.

Since it's a diesel, it'd be wise to have about a 10-gallon 'nurse tank' near the machine, that will warm rapidly once the engine is started. have your primary tank plumbed in with large-ID tubing, so that when really cold out, the thick fuel will still be able to flow into the nurse tank. It would not be a bad idea to get a magnetic stick-on engine heater and slap it on the bottom of the big tank, and bring the wire into that third power leg, and put it on some sort of a temperature control to be able to put some heat into the pickup area of the tank just-in-case.

Also... it would be a good idea to set up that tank so that you can dispense fuel from it, into farm machines, etc., regularly, so that your main supply is regularly refreshed. Use good fuel, especially in winter, as an emergency generator that won't start, is pointless.
:wave:
 
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