Single-phase generators supplying three-phase systems

[raludovico]

Why can't my single phase 20 kW generator provide power for a three-phase panel box with a load of 10 kW. All of the load circuits are 120 volt in the three-phase box. I understand that 1 phase will not work. This will be OK because of critical circuits only will be energized in the 3 phase panel box. There is no locked rotor amperage to be considerate. The loads have been calculated and measured over a 30 day period. Could somebody please show me in the NEC for any violation. Thank you

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[ron]

You will connect the two hots from the gen to two of the phases in the panel.

How do you pick up loads for the third phase in the panel?

 

[GoldDigger]

You connect the third bus to your choice of the other two.
But be careful not to overload the neutral of any three phase MWBCs when you do that.
Some breakers might have to be turned off when on generator.

 

[iwire]

Are you planning on a manual or auto transfer switch?

An ATS won't work as it will never think the generator is ready.

 

[topgone]

Why can't my single phase 20 kW generator provide power for a three-phase panel box with a load of 10 kW. All of the load circuits are 120 volt in the three-phase box. I understand that 1 phase will not work. This will be OK because of critical circuits only will be energized in the 3 phase panel box. There is no locked rotor amperage to be considerate. The loads have been calculated and measured over a 30 day period. Could somebody please show me in the NEC for any violation. Thank you

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Please post the nameplate, or cut sheet of your generator set, your loads arrangement/ connection wiring, etc. Then, perhaps we can help you with your problem.

 

[gar]

150929-2208 EDT

If I understand the question correctly, then the three phase source connected to the panel is a wye source. Also there are no loads requiring three phase power, in particular three phase motors.

All loads were stated to be 120 V.

Use a generator with 120 V output and adequate to handle the total load, and then when disconnected from the power company connect all the hot bus bars together with the generator ouput connected between the paralleled hots and the panel neutral bus bar. This will require a generator with a capacity somewhat over the total load.

If the generator is single phase 240 with a neutral and you do as GoldDigger suggested, then depending upon your load distribution you will have to increase the size of the generator.

.

 

[GoldDigger]

150929-2208 EDT
...
Use a generator with 120 V output and adequate to handle the total load, and then when disconnected from the power company connect all the hot bus bars together with the generator ouput connected between the paralleled hots and the panel neutral bus bar. This will require a generator with a capacity somewhat over the total load.

If the generator is 120 only and you connect all three bus bars to the generator, you are guaranteed to have a problem with any MWBC or partial MWBC that is wired to the panel. You could have up to 3X the rated neutral current.

 

[Haji]

Replace with 2 pole and 4 pole breakers supplying loads to have a measure of safety against overloading the neutral if you plan to provide single phase power to three phase and neutral loads.

 

[GoldDigger]

Replace with 2 pole and 4 pole breakers supplying loads to have a measure of safety against overloading the neutral if you plan to provide single phase power to three phase and neutral loads.

I do not see how changing the breakers in any way protects the neutral of an MWBC.
(Unless you replace existing breakers with ones of 1/2 or 1/3 the rating).

 

[K8MHZ]

Why can't my single phase 20 kW generator provide power for a three-phase panel box with a load of 10 kW. All of the load circuits are 120 volt in the three-phase box. I understand that 1 phase will not work. This will be OK because of critical circuits only will be energized in the 3 phase panel box. There is no locked rotor amperage to be considerate. The loads have been calculated and measured over a 30 day period. Could somebody please show me in the NEC for any violation. Thank you

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I added the bold because I don't see how a single phase genny can power more than just that, a single phase. Thus, wouldn't 2 of the 3 phases not work instead of just one?

What I envision is the three phase main being shut off and a single 120 volt breaker being back fed from the genny, powering up every third slot in the three phase panel.

Am I missing something?

 

[gadfly56]

I added the bold because I don't see how a single phase genny can power more than just that, a single phase. Thus, wouldn't 2 of the 3 phases not work instead of just one?

What I envision is the three phase main being shut off and a single 120 volt breaker being back fed from the genny, powering up every third slot in the three phase panel.

Am I missing something?

As previously stated, there are only 120 volt loads on the panel. Most generators output 240 volts, so one hot goes to Phase 1, one hot goes to Phase 2, and jump Phase 3 with either of the first two and you're good to go, MWBC's notwithstanding. As previously stated. What's not to understand? :?

 

[GoldDigger]

I added the bold because I don't see how a single phase genny can power more than just that, a single phase. Thus, wouldn't 2 of the 3 phases not work instead of just one?

What I envision is the three phase main being shut off and a single 120 volt breaker being back fed from the genny, powering up every third slot in the three phase panel.

Am I missing something?

Yes, you are missing something. Since the loads are all line to neutral, one can use the same 120V source to power up to all three panel buses. (Subject to shared neutral issues). Or if a single phase 120/240 three wire supply is available from the genny you can power two of the three buses with no neutral problems at all.
The key is that you are not actually supplying distinct phases, but the loads could care less about that.

 

[Onsite_energy]

They make a converter.... But it's not good for many amps .... 3 phase has 3 phases single has single start phase ..... Kinda like revving up and dumping a clutch to where 3 is like a smooth automatic..... Sorta speak ....


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[Onsite_energy]

Or Get 3 generators don't have to be 20kw cause of the shared loads just gotta sync em


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[Onsite_energy]

Is this 120/208 or 440v were talking about..... Guessing 120/208


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[GoldDigger]

Or Get 3 generators don't have to be 20kw cause of the shared loads just gotta sync em


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As long as they are feeding three separate buses each with only line to neutral loads, the only reasons I see to sync the generators are to avoid neutral overload and keep multiple motors in one piece of equipment or conveyor belt running at the same speed.

 

[Onsite_energy]

That's a good idea!! The belt never thought of that


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[topgone]

As long as they are feeding three separate buses each with only line to neutral loads, the only reasons I see to sync the generators are to avoid neutral overload and keep multiple motors in one piece of equipment or conveyor belt running at the same speed.

AFAIK, mwbc neutral conductor only caters to the unbalanced portion of the loading. As long as the neutral is sized the same as the hot leg conductors, I see no problem there. The only concern with mwbc is when the neutral conductor gets broken and your loads starts emitting holy smokes!

 

[GoldDigger]

AFAIK, mwbc neutral conductor only caters to the unbalanced portion of the loading. As long as the neutral is sized the same as the hot leg conductors, I see no problem there. The only concern with mwbc is when the neutral conductor gets broken and your loads starts emitting holy smokes!

Then there is something you do not yet K.

The reason the neutral carries only the imbalance is that the 2 or 3 hot leads are all at different phase angles and do balanced currents add as vectors to zero.
Once you connect all the hots to a single 120V source the currents add instead of cancelling.
Twice the rated neutral current may not start a fire immediately, but 3X probably will.

 

[Haji]

I do not see how changing the breakers in any way protects the neutral of an MWBC.
(Unless you replace existing breakers with ones of 1/2 or 1/3 the rating).

If you provide neutral 1/2 or 1/3 rating of the phase conductor, then what you say is correct. But normally same size neutral is provided as that of phase conductor for small sizes of cables..............