Neutral on a 120?240V High Leg Delta

Ctay005

Member
Location
Orem, Utah, USA
This seems like it should be obvious but I wanted to run this by you guys.

We have a 3 phase high leg delta that were installing 6 single phase inverters on (balanced across the phases). So on these inverters we will be hooking L1 and L2 to 2 of the phases and I wanted to make sure that the neutral hooks up as normal? I was confused on this since hooking L1/L2 across a phase and neutral will give 120 or 208V but hooking the inverter N out to the N on the building is the same as a residential split phase correct? I guess im trying to say, will the neutral on these inverters hook up like normal?

Thanks for the help!
 

SolarPro

Senior Member
Location
Austin, TX
It's been a while since I've done this. But I seem to recall that we had to set the inverters up to run without a neutral. At the time, that just meant moving around some jumpers within the inverters. You may find that only a subset of inverters can accommodate your proposed interconnection approach.
 

GoldDigger

Moderator
Staff member
An inverter that is configured for 120/240 should work fine on A-C with the N connected.
Inverters on A-B and B-C will have to be configured to connect to a 240 delta without a neutral.
Do not try to connect a 208V single phase inverter from B to neutral, as the transformer does not expect significant power on that path.
 

Carultch

Senior Member
Location
Massachusetts
This seems like it should be obvious but I wanted to run this by you guys.

We have a 3 phase high leg delta that were installing 6 single phase inverters on (balanced across the phases). So on these inverters we will be hooking L1 and L2 to 2 of the phases and I wanted to make sure that the neutral hooks up as normal? I was confused on this since hooking L1/L2 across a phase and neutral will give 120 or 208V but hooking the inverter N out to the N on the building is the same as a residential split phase correct? I guess im trying to say, will the neutral on these inverters hook up like normal?

Thanks for the help!
Moving the jumpers around was the way you'd do this with the former transformer-in-inverter topology. The market has shifted to primarily transformerless inverters.

In general, inverters that function at either 120/208V or 120/240V single phase, will feed their current onto the line connections. Little (if any) current would be flowing on the neutral. It is mostly used as a voltage reference point, and occasionally the inverters use the neutral to operate internal power supplies. If the inverter does use the neutral, and requires 120V phase-to-neutral, then any inverter connected to the B-phase could be problematic on high leg delta systems, as it would receive 208V across said connection. I'm assuming the phase convention meets the modern standard of a B-phase high leg.

If you connect exclusively to the A and C phase, then it will function just like a single phase residential system. But for obvious reasons, you probably don't want to do that. A balanced system has less overall current, which means smaller equipment. Also it may be required by the utility to balance as much as possible.

So the inverter manual is what you really need to check. If it explicitly disallows a high leg system, then find another solution. If it explicitly allows it, you are good to go. If it doesn't specify, you may need to discuss with the manufacturer.
 

pv_n00b

Senior Member
Location
CA, USA
Another thing to keep in mind is that utilities have a habit of supplying these 240/120V 3 phase services through an unbalanced open delta transformer with the larger transformer on the split phase leg and a smaller transformer for the other two legs. This can limit the amount of power you can export compared to a fully rated balanced 3 phase service.
 

jaggedben

Senior Member
Most important advice: confirm with the inverter manufacturer that this will work before you buy the equipment. Hope we're not too late.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This seems like it should be obvious but I wanted to run this by you guys.

We have a 3 phase high leg delta that were installing 6 single phase inverters on (balanced across the phases). So on these inverters we will be hooking L1 and L2 to 2 of the phases and I wanted to make sure that the neutral hooks up as normal? I was confused on this since hooking L1/L2 across a phase and neutral will give 120 or 208V but hooking the inverter N out to the N on the building is the same as a residential split phase correct? I guess im trying to say, will the neutral on these inverters hook up like normal?

Thanks for the help!
In most cases I have encountered the way to deal with a 240V high leg service is to connect to the A and C phases and the neutral as if it were a split/single phase service, ignoring the B (high leg) phase. Most utilities do not care about the imbalance, as the high leg may not even be used at all.
 

SolarPro

Senior Member
Location
Austin, TX
You know, that makes sense. These services are built the way they are because of a presumed load imbalance. (The original utility engineers assumed a relatively large single-phase load and a small three-phase load.)
 

Smart $

Esteemed Member
Location
Ohio
You know, that makes sense. These services are built the way they are because of a presumed load imbalance. (The original utility engineers assumed a relatively large single-phase load and a small three-phase load.)
It can go the other way, too.

Best to verify what's what with the POCO.
 

Carultch

Senior Member
Location
Massachusetts
It can go the other way, too.

Best to verify what's what with the POCO.
I thought the precise purpose of a high leg system, is when you have primarily 3-phase loads and thus the 240V phase-to-phase is 15% better than the 208V counterpart. And you'd still need a small set of single phase 120V loads on the premises.
 

GoldDigger

Moderator
Staff member
I thought the precise purpose of a high leg system, is when you have primarily 3-phase loads and thus the 240V phase-to-phase is 15% better than the 208V counterpart. And you'd still need a small set of single phase 120V loads on the premises.
Actually, I think that both needs led to similar but differently proportioned high leg systems.
The 120/240 dominant found in residential and the 3-phase dominant found in commercial. But with some crossover, so, OP, find out what you actually have.
 

Smart $

Esteemed Member
Location
Ohio
...The 120/240 dominant found in residential...
Apt to be a two-pot open-delta configuration. Allows pots to be sized proportional to imbalance.

....and the 3-phase dominant found in commercial....
Light commercial and light industrial may also be open delta.

Full-delta configuration where there are substantial 3Ø loads, and the facilities would no longer be considered "light" (pun intended). :D
 

Ctay005

Member
Location
Orem, Utah, USA
So after a visit to the building to day I found that this building has a 3 phase high leg delta and 2 single phase services that all splice together coming out of the transformers. So I was thinking that a supply side connection on this 3 phase disco would be a valid interconnection method (given that the POCO approves) and that the power could be distributed throughout the entire building through the splices outside. Ill call to try and talk to somebody at Rocky Mtn Power today to see what they think.

I had another question about voltage drop. This connection is going to be putting out around 300A and were running wires 300' indoors through conduit from the aggregation panel. Is the voltage drop going to be affected by running the conduit and lines indoors instead of outdoors? Currently the voltage drop calculators are saying that we need #1000 Al which seems a little overkill to me but i may be mistaken.
 

Attachments

GoldDigger

Moderator
Staff member
It is hard to follow the wires completely, but I would identify that as one large 120/240 single phase pot and two 240V single phase pots forming the other two sides of the closed delta. Not a three phase service and two single phase services.
 

Carultch

Senior Member
Location
Massachusetts
So after a visit to the building to day I found that this building has a 3 phase high leg delta and 2 single phase services that all splice together coming out of the transformers. So I was thinking that a supply side connection on this 3 phase disco would be a valid interconnection method (given that the POCO approves) and that the power could be distributed throughout the entire building through the splices outside. Ill call to try and talk to somebody at Rocky Mtn Power today to see what they think.

I had another question about voltage drop. This connection is going to be putting out around 300A and were running wires 300' indoors through conduit from the aggregation panel. Is the voltage drop going to be affected by running the conduit and lines indoors instead of outdoors? Currently the voltage drop calculators are saying that we need #1000 Al which seems a little overkill to me but i may be mistaken.
That color scheme looks like a 120/208 WYE system. You should use your voltmeter to confirm the actual situation. The high leg is supposed to be marked orange, and now is required by the NEC to mark it orange per 110.15. I suggest re-marking it orange, to distinguish it. Or marking it orange in addition to red, if somehow red is a local requirement. Anything you can do to make the high leg stand out is encouraged.

As for voltage drop, indoors vs outdoors makes no difference. What matters is distance, wire metal material, wires per phase, effective operating temperature, and to a slight extent, the fact that it is AC wire in metal conduit.

If it is my choice, I would parallel, when the calculations prescribe sizes that large (greater than 500 kcmil). Generally all KCMILs are close to equal, when curtailing voltage drop. Although smaller wires have slightly less effective resistance. Ampacity is a significantly different story, as smaller wires have disproportionately more ampacity per KCMIL.
 
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jaggedben

Senior Member
I agree with Carultch, I would be pretty miffed if I put my voltmeter on that and it wasn't 280/120 wye. You owe it to yourself and everyone to double check what's what.
 

Ctay005

Member
Location
Orem, Utah, USA
We checked the voltages and got 122V from the outside legs to the neutral, 245V across any 2 legs and 211V from the middle leg to the neutral.

In the transformer picture you can see that the largest of the 3 pipes going into the building has 4 wires and those are the 4 wires shown coming into the disconnect that I attached.

Seems like parallel conductors would be a better option for this job due to the long distance were running.
 

Ctay005

Member
Location
Orem, Utah, USA
Now im just confused because the colors just dont match up...:? I know that the voltages I measured with our electrician are correct but the wires and colors just arent matching up... This building has 3 main disconnects on the inside. 2 1950's square D 600A and 400A single phase discos and this 400A 3 phase disco all going out into the bundle of wires shown into the pictures.

Can anyone make any sense of this?
 
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