480v 3ph with 480v 1ph transformers

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BackCountry

BackCountry

Electrician
Location
Southern California
Occupation
Licensed Electrician and General Contractor
I’m bidding a large ground mount solar job with a standby generator, and the existing service is throwing me for a loop. I’m used to either 120/240 service or 480/277 3ph service.

The existing meter from the power company is a 480/277 3ph meter, fed into a fused disconnect, where to 3/0 conductors and a ground exit and travel approximately 800ft to a gutter with three disconnects, all ultimately feeding three 50kva single phase 480v primary to 120/240 secondary dry transformers, placing the entire load on two phases. For load balancing, I thought that was a no go?

I was planning on using an SMA 50kw TriPower inverter, which is 480v three phase, and we could run that back to the meter to grab the missing phase conductor; however the load imbalance would result in exporting on the unused phase 100% of the time, and importing via the other two phases to make up the difference. To keep it balanced, the other option would be to back feed at 120/240 and use a single phase transformer and tap the existing feeders and keep it all on the existing phase. More complicated and can’t use the built in three phase voltage configuration of a tripower.

The same challenge exists with the generator transfer switch, and third unused phase. If the generator were after the existing transformers, that wouldn’t be that big of a deal, but they’re asking to cover the entire property, so I need to be on the 480v side — which would again place all of the load on two of the three phases. I haven’t seen a 480v single phase option from Generac.

My business is nearly entirely residential solar and electric, I rarely deal with 3ph so pardon my lack of intelligence, and thank you for your wisdom and I appreciate your help.
 
Sure. Three phase ends at the disconnect after the meter on a panel board approximately 800 feet from the building. There are two legs coming out from the disconnect (3/0), and a ground. That travels to a gutter where it distributes in the building to three disconnects serving each transformer.

The array is calc'd right now at 48kw DC. I had planned to use a SMA TriPower 50kw. I am thinking my only option now is going to be outputting at 240v with a different inverter configuration, or micro's, and then stepping the combiner panel output to 480v single phase tapped into the existing feeders. Same with the generator, although I would put that after the solar tap so the rapid shutdown and transfer switch will work together properly.

I could pull a third phase up to the array from the meter, but I would constantly be backfeeding one phase while importing/exporting depending on active loads from the other two. It's a smart meter, so it wouldn't be any different than heavy loads on one phase in a single phase service.

And then the final option would be to pull a third phase all the way to the building and balance the transformers. The value proposition being is that expense and reconfiguration more beneficial than using a transformer at the array (which is about half way up that 800 foot run).
 
BackCountry said:
Three phase ends at the disconnect after the meter on a panel board approximately 800 feet from the building. There are two legs coming out from the disconnect (3/0), and a ground.
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Is it possible you actually have two lines and a grounded line, a.k.a. a corner-grounded delta
 
Kinda sounds like somebody screwed up somewhere. Was this an engineered job? Or did the customer say they wanted this, and left it up to the electrician to make it work? Makes no sense at all to have a 480 three phase service, yet only use single phase. If that was the intention, the poco could have just dropped a 480 volt single phase service, saved a ton of money on transformers (unless it’s pad mount, which they could do, but would need a spare in the yard.).
 
The fused disconnect has three phase legs coming in, and only two outbound legs, the third leg is fused and connected from the line side, and nothing connected to the load side.
 
hillbilly1 said:
Kinda sounds like somebody screwed up somewhere. Was this an engineered job? Or did the customer say they wanted this, and left it up to the electrician to make it work? Makes no sense at all to have a 480 three phase service, yet only use single phase. If that was the intention, the poco could have just dropped a 480 volt single phase service, saved a ton of money on transformers (unless it’s pad mount, which they could do, but would need a spare in the yard.).
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Just speculation, but I have found many pocos won't do "odd ball" configurations like single phase 480,so maybe that was the only way to get 480. Sounds like 480 was desired because of the long wire run.
 
I would think pulling in the third phase would be the ticket. I would think that would beat transformer cost and losses forever.

I don't see your concern of balancing/unbalancing as valid. It's already unbalanced, and if you add a three phase inverter, that is balanced so what's the worry? If the POCO isn't yelling at you then let it ride.
 
What is ahead of the 3 phase disconnect ? ie: a bank of transformers ? individual 3 phase transformer ? etc.
 
electrofelon said:
Just speculation, but I have found many pocos won't do "odd ball" configurations like single phase 480,so maybe that was the only way to get 480. Sounds like 480 was desired because of the long wire run.
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A lot of roadway lighting uses 480 single phase, I figure the poco would have some laying around, though probably not a 150 KVA.
 
I think you're right regarding the phase balancing. It's already way off. Ahead of the three phase disconnect is the meter, and then the power company's transformer supplying 3ph 480/277.

The only challenge I forsee is putting a generator into this mix. Whatever the 3 phase generator's rated capacity is would obviously be wasted. The real solution, to me, is to bring three phase all the way up to the building, and balance the existing transformers which also fixes the generator problem.
 
Had a similar situation here. Original service was 240/120 3 wire. Long run and the load increased. Solution was to change service to 480 and install transformers at load end using existing conductors. POCO would only supply 480 at three phase so contractor used just the two phases and a grounding conductor. (Brought all 3 phases to disconnect to hid unbalance from POCO)
Could you imstall a 240/120 generator and transfer after the transformers ?
 
I think what you’re describing is exactly what occurred here. The transformer has to cover the entire 200A load (a few different buildings all with their own transformers) so I would need to install it before the step down transformers.

I hate to say it, but I think my only solution here is to pull an additional conductor 800 feet through the existing conduit, and balance the transformers to all three phases, and fix it. It’s going to be difficult — but it’ll make it right. To do anything on top of what’s there would only band aid the problem.
 
Yes, you already have 3 phase service, and already have 3 single phase transformers. The only thing missing is the 3rd phase conductor.

If your generator and selected inverters could handle a corner grounded system, then you wouldn't need an additional phase conductor, but IMHO the wye configuration is cleaner/preferred.

Jon
 
BackCountry said:
I think what you’re describing is exactly what occurred here. The transformer has to cover the entire 200A load (a few different buildings all with their own transformers) so I would need to install it before the step down transformers.

I hate to say it, but I think my only solution here is to pull an additional conductor 800 feet through the existing conduit, and balance the transformers to all three phases, and fix it. It’s going to be difficult — but it’ll make it right. To do anything on top of what’s there would only band aid the problem.
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Sounds like somebody didn't want to pay for that third ungrounded conductor at initial installation and load apparently was low enough it didn't create any problems.
 
electrofelon said:
Just speculation, but I have found many pocos won't do "odd ball" configurations like single phase 480,so maybe that was the only way to get 480. Sounds like 480 was desired because of the long wire run.
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I found the same thing. I wanted 480 single phase, but the PoCo said no because it was a non stock transformer and they would not get into this for one customer.
 
oldsparky52 said:
I found the same thing. I wanted 480 single phase, but the PoCo said no because it was a non stock transformer and they would not get into this for one customer.
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Non stock for that particular POCO. We see many of them around here. Mostly 25 kVA and under, some get used to make open delta banks.
 
winnie said:
Yes, you already have 3 phase service, and already have 3 single phase transformers. The only thing missing is the 3rd phase conductor.

If your generator and selected inverters could handle a corner grounded system, then you wouldn't need an additional phase conductor, but IMHO the wye configuration is cleaner/preferred.

Jon
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Thats a clever idea Jon. Just to be clear, are you saying make the three phase service corner grounded, use the EGC as the grounded phase/conductor, and use 250.30(A)(1) exception 2?
oldsparky52 said:
I found the same thing. I wanted 480 single phase, but the PoCo said no because it was a non stock transformer and they would not get into this for one customer.
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Yup I tried the exact same thing once. They said a residential service could only be 120/240. There were only single phase lines, but if there were three phase, I don't know if they would give me a three phase 120/208 or 277/480 or not.
 
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