3 Phase to Single Phase

[tony1224]

hey guy's I'm trying to help a frieng hook a 3 phase generator to single phase at hes house.
I know that L0 is ground or neutral ,one of the other legs is 120 volts,my question is how to hook the other L1 or L3 when they are 208 volts or 240 volts how do I go by hooking or where do I get the other 120 volts.Any help please

 

[robertsears1]

What kind of generator is it? Need more information.

Robert

 

[jamawieb]

Normally you would just use 2 of the hot legs and leave 1 hot leg disconnected. LO is neutral. Most electronics will run fine at 208 but you should be able to adjust the voltage up if you have a military set. Just know that this is not a safe way to do this for a 3phase generator head because you have to make sure your legs are balanced pretty well.

 

[tony1224]

Normally you would just use 2 of the hot legs and leave 1 hot leg disconnected. LO is neutral. Most electronics will run fine at 208 but you should be able to adjust the voltage up if you have a military set. Just know that this is not a safe way to do this for a 3phase generator head because you have to make sure your legs are balanced pretty well.

thanks for the reply,but my question is how do i connect that one leg if it is 208 volts or 240 volts 3 phase.The one leg 120 volts is fine .but the other leg is 208 volts that is 328 volts, how do you hook the one that is 208 volts

 

[155mm]

???? you dont get 208 out of 1 leg,

GND is ground
LO is neutral
L1 is 120v, measured from L1 to L0
L2 is 120v, measured from L2 to L0
L3 is 120v, measured from L3 to L0

the 208v is when you pull 2 legs, measured from L1, L2 or L2, L3 or L1, L3.... all depends on which generator you are using, we will go back to the second post and have you clarify why type of generator you are using, or what size/manufacture if it is civi.

The only time you would get 200+ volts off of a single leg if the voltage reconnection board was set up to 240/416, you do NOT want to hook that up to a house, that would let the smoke out of everything.

 

[tony1224]

???? you dont get 208 out of 1 leg,

GND is ground
LO is neutral
L1 is 120v, measured from L1 to L0
L2 is 120v, measured from L2 to L0
L3 is 120v, measured from L3 to L0

the 208v is when you pull 2 legs, measured from L1, L2 or L2, L3 or L1, L3.... all depends on which generator you are using, we will go back to the second post and have you clarify why type of generator you are using, or what size/manufacture if it is civi.

The only time you would get 200+ volts off of a single leg if the voltage reconnection board was set up to 240/416, you do NOT want to hook that up to a house, that would let the smoke out of everything.

Thank you you are a big help ,I think I can work with that,I will try later to post what type of generator.
Thank you

 

[snowtrac nome]

you wont see 240 volts on 3 phase as there is a loss of voltage in three phase you can normally bump it up to 220 but most electricians wont like it the other option is to get a transformer to step your 208 3 / up to 240 1/ I have seen this used in the past

 

[DieselAddict]

Please don't try to make a generator output 240v when its set to 208. Use it as 208. Most appliances are rated for 240/208. Look at the labels. It may cause some derating for the appliance but it should work fine.

 

[155mm]

Please don't try to make a generator output 240v when its set to 208. Use it as 208. Most appliances are rated for 240/208. Look at the labels. It may cause some derating for the appliance but it should work fine.

yeah, that doesnt work so well, single leg voltage ends up being like 140v, and if i remember correctly on MEP's 8xx's you will get an overvoltage fault before you get there.... I know you do on the 240/416 side for sure.

 

[Guyfang]

Could have looked at the book to begin with.

 

[Jericho]

GuyFang is correct , All the tOs/TMs you should need if its MIL are likely here , BUT IF YOU ARE MEASURING ACROSS L1 AND L2 THINKING ITS 208 ON EACH LEG THEN PLEASE GET AN ELECTRICIAN TO ASSIST YOU.

 

[doghead]

Guys, we(he) does not even know what generator model.

This is not the kind of thing that lends well to speculation.

We needs some basic facts.

Also a concern, how is he isolated from the grid?

 

[155mm]

and maybe pull the meter so you dont back feed power down the line

 

[Daybreak]

Howdy,

There is a time and place to call a electrician.

Now is the time.

 

[rustystud]

Howdy,

There is a time and place to call a electrician.

Now is the time.

No kidding ! No one even mentioned the phase deferential between the three hot lines. If he had a 240 circuit there (say a dryer or stove) and it tried to use power from those two legs of the three phase generator it would not go well. There is so much more to converting a three phase generator to single phase. For one, is it a 12 wire or an 8 wire generator. This is something that should never be considered lightly.

 

[Triple Jim]

I'm not sure why you said that, Rusty. Running a 240 dryer on two legs of 3-phase power, giving 208 volts, works fine unless the motor is so voltage sensitive that it's not happy with 208. That would be unlikely, since I've never heard of a dryer specifically made for 208.

Or did we find out that the generator doesn't make normal 3 phase 120/208 power?

 

[DieselAddict]

I know it can be confusing to run residential single phase appliances on 2 phases of a 3 phase source but I assure you its OK (verify using appliance's data label). In the industrial world we do it all the time. A specific example is with freezers. In the biotech industry we use LOTS of freezers. They are 208/240v single phase units. The main difference in power source is that they pull a little more amperage when using 208.

You can see using the attached residential AC unit data plate that it is rated for 208/230 single phase. Since there is no such thing as true 208 single phase what they are saying is that you can run the unit off of two phases of 208 three phase.

A word about load imbalance.. Its not good to run a high load imbalance BUT its not the apocalypse I see generally communicated. Yes, its hard on the windings but its actually harder on the voltage regulator. In an emergency situation you do what you have to do to get by. You can always take the third phase and run a few extension cords around to power some of the 120v loads. Run all the freezers and fridges in the garage off of that extra phase. Anything like that helps.

 

[Triple Jim]

Or you can spread the 208 loads around if they're not all wired together. There are three combinations that give you 208: 1-2, 2-3, and 1-3.

 

[rustystud]

I'm not sure why you said that, Rusty. Running a 240 dryer on two legs of 3-phase power, giving 208 volts, works fine unless the motor is so voltage sensitive that it's not happy with 208. That would be unlikely, since I've never heard of a dryer specifically made for 208.

Or did we find out that the generator doesn't make normal 3 phase 120/208 power?

In a house with the normal third leg being grounded to the buss bar, if the two phases are not separated they can cause problems. I know if you have two completely separated circuits, or even three you can use three phase power. But if you look at the sine wave coming off a three phase generator you will see they are not 180 degrees from each other. It's more like a 60 degree separation. Electronics do not like that. Maybe I'm totally wrong here, but I do remember having a computer getting fried by using the two legs off a three phase generator.

 

[Triple Jim]

The legs are 120 degrees apart with 3-phase. In essentially all cases, when you use two legs and get 208v, the load can't tell about the 120 degrees between phases because it's not looking at the two 120v legs compared to neutral separately. It just sees two wires with 208VAC on them. Clothes dryers use the 208 for the heating elements and sometimes run light bulbs off 120 (one of the phases to neutral), but even then the heating element just sees normal AC, at 208 instead of 240.

If you had a computer that was powered by two legs from a 3 phase generator, it was seeing 208 VAC, not 120 VAC, if it was the usual 3 phase 120 volts to neutral like a MEP003A generates. You would connect a typical home computer made to run on 120v to one leg and neutral.