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37.5 KVA 3 phase generator converted to single phase 240V. What size breaker should I use?

Billybob16

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Sep 27, 2021
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I just installed 4 EG4 6000xp and want to hookup a diesel generator that was converted to single phase. It's rated at 37.5kva 120/208 60HZ on the name plate. It's my understanding that the conversion uses 2 of the 3 phase windings with the neutral at the center and that a rule of thumb is It's new KVA is aproximarly 2/3 that of original. Which makes it about 25KVA or appropriately 25KW, 25,000 watts.

I plan to feed it in to a panel and install 4 double pole breakers to feed the 4 6000xp inverters. If I calculate 25,000 watts at 240V I get 104 amps/4 gives me 4 - 25 amp double pole breakers. Is that right? Or is that 25 amps per leg and I should use more like 45 or 50 amp breakers?

Is anyone familiar with this?
 

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You have this phase converter? I think throwing away 1/3 of power is a waste. I would think this device should delay phases and make all three phases lined up, with more power.
 
I would expect neutral at the center to give 120/208 L1, N, L2.
I don't think simply rewiring a Wye connected generator would make split phase. With a 208 to 120/240V transformer it could.

With the generator running, measure voltages [L1,N], [L2,N], [L1,L2]
While you're at it, check each of them ground/chassis as well, useful shortly.

Some inverters will accept 120/208.
 
What do you want to use the generator for? Powering the house or charging batteries?

If you're just going to charge batteries with it, you could just use it in 3 phase with multiple chargeverters...
 
Typically you would use a Motor Protection Circuit Breaker sized to your generator to protect the generator.
You would use a fuse near the generator sized to the size of your wires to protect the wires.
You would use a fuse sized to the inverters for each inverter.
You also have to ground one of the generator outputs. Which exactly depends on the details of the "conversion".
 
I would expect neutral at the center to give 120/208 L1, N, L2.
I don't think simply rewiring a Wye connected generator would make split phase. With a 208 to 120/240V transformer it could.

With the generator running, measure voltages [L1,N], [L2,N], [L1,L2]
While you're at it, check each of them ground/chassis as well, useful shortly.

Some inverters will accept 120/208.
This is a generator that cam out of an air force base already converted to 240 with a neutral. It runs produces 2 120V lines with no problem. I want to protect the generator from being over loaded which is why I asked the question. What is the current limit of this generator?
 
You have this phase converter? I think throwing away 1/3 of power is a waste. I would think this device should delay phases and make all three phases lined up, with more power.
Thanks for your reply. I understand not wanting to waste energy. I don't have a phase converter and only want the generator to charge batteries and run loads during emergencies or prolonged cloudy days. I'm trying to figure out proper breaker size for gen protection.
 
What do you want to use the generator for? Powering the house or charging batteries?

If you're just going to charge batteries with it, you could just use it in 3 phase with multiple chargeverters...
My plan is to charge and run loads at the same time. It is already wired for single phase which makes it simple to run to a generator panel then to all inverters after proper breakers sized to generator.
Thanks for your reply.
 
Generators tend to protect themselves from overload by bogging down and reducing the output, bit like a solar panel does.
Breakers and fuses are used to protect the wires not the equipment, so use the appropriate size wires and protect accordingly. Seems like you could be pulling 50 to 70 amps so use 4awg from the generator to the panel with a 70a breaker and then 10awg to the inverters with 30a breakers.
Double check and confirm for yourself, this is just my back of the napkin calculations.
 
If it were me, I'd wire it up to simply directly charge the batteries, let the inverters the be ones powering the loads from the batteries. That way, when the batteries are getting low and there's not enough solar, you fire up the genny, you've got it set to the most efficient load for the generator (don't max it out, usually something around 70-80% is highest efficiency for a lot of generators), and then you can shut the thing back down and keep going on batteries.

Less fuel usage and less generator run-time so less noise too.
 
If it were me, I'd wire it up to simply directly charge the batteries, let the inverters the be ones powering the loads from the batteries. That way, when the batteries are getting low and there's not enough solar, you fire up the genny, you've got it set to the most efficient load for the generator (don't max it out, usually something around 70-80% is highest efficiency for a lot of generators), and then you can shut the thing back down and keep going on batteries.

Less fuel usage and less generator run-time so less noise too.

That’s what I’d do, depending on how much charging he needs, I’d do in in 3 phase even. Use the 3 phase to run multiple chargeverters and boom he’s set.
 
37KVa 3 phase was made to output 65 amps. That's what the windings are rated for no matter how you connect any loads.
Hi Mike, thanks for chiming in. I have seen that number in some charts before. I just did a search and came up with 90.3 amps at 240V at 80% power factor. See inclosed chart. If correct would that be 90.3 amps both 3phase and single phase? What about per leg to neutral?
 

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If it were me, I'd wire it up to simply directly charge the batteries, let the inverters the be ones powering the loads from the batteries. That way, when the batteries are getting low and there's not enough solar, you fire up the genny, you've got it set to the most efficient load for the generator (don't max it out, usually something around 70-80% is highest efficiency for a lot of generators), and then you can shut the thing back down and keep going on batteries.

Less fuel usage and less generator run-time so less noise too.
But that's 26KW, (or is it 17KW? I have no idea if/how you derate a generator based on its wiring configuration, and it may be different for different generators), which is more than one CV or equivalent.

Yeah, it's already been converted to single phase, so converting it back to make more efficient use of three phases seems like the wrong answer... Proper loading for three phase would be like 6 CVs, assuming he has enough batteries to absorb the 600A of charge current.
 
Needs to be measured L1 to L2 to verify it is actually converted. Just sticking a neutral between two of the three would net you 120V L1 to N and L2 to N, but unless they did more than that, L1 to L2 may still only be 208V not 240V.
 
This is a generator that cam out of an air force base already converted to 240 with a neutral. It runs produces 2 120V lines with no problem. I want to protect the generator from being over loaded which is why I asked the question. What is the current limit of this generator?

I understand it produces 2, 120V lines.
But does it produce 240V between those two lines?

I thought a 3-phase generator was wound with three poles each 120 degrees out of phase. A split-phase generator is wound with two poles 180 degrees out of phase.
(That's for 3600 RPM generator; for 1800 RPM twice as many poles either 60 or 90 degrees apart.)

Oh, there are ways to change them. Described here.

 
Needs to be measured L1 to L2 to verify it is actually converted. Just sticking a neutral between two of the three would net you 120V L1 to N and L2 to N, but unless they did more than that, L1 to L2 may still only be 208V not 240V.
L1 and L2 are 240 and 120 per leg
 
I understand it produces 2, 120V lines.
But does it produce 240V between those two lines?

I thought a 3-phase generator was wound with three poles each 120 degrees out of phase. A split-phase generator is wound with two poles 180 degrees out of phase.
(That's for 3600 RPM generator; for 1800 RPM twice as many poles either 60 or 90 degrees apart.)

Oh, there are ways to change them. Described here.

It does produce 240V L1 to L2
 
If it were me, I'd wire it up to simply directly charge the batteries, let the inverters the be ones powering the loads from the batteries. That way, when the batteries are getting low and there's not enough solar, you fire up the genny, you've got it set to the most efficient load for the generator (don't max it out, usually something around 70-80% is highest efficiency for a lot of generators), and then you can shut the thing back down and keep going on batteries.

Less fuel usage and less generator run-time so less noise too.
This makes sense to me, a starting point. I can load it, set gen charge parameter to like 20 amps per the 4 6000xp inverters, monitor it and see how the gen responds. That brings up another question. A charge limit setting is for each inverter or is it a total combined? I'm running 4 EG4 6000xp inverters and have 12 EG4 100AH batteries all in Com with SOC.

Thanks
 
This makes sense to me, a starting point. I can load it, set gen charge parameter to like 20 amps per the 4 6000xp inverters, monitor it and see how the gen responds. That brings up another question. A charge limit setting is for each inverter or is it a total combined? I'm running 4 EG4 6000xp inverters and have 12 EG4 100AH batteries all in Com with SOC.

Thanks
That I'm not sure. @EG4TechSolutionsTeam can you answer that?
 
Given the 6000xps can draw up to 37.5 amps each that could be 150 amps @240. What you haven't stated is how much battery do you have? Unless you know the generator puts out < 3% THD it may be better to use two Chargeverters. They would draw around 46 amps at 240 and make 200 amps of battery charge current.
 

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