single phase inverter in a residential 3 phase, 120/240VAC, DELTA, panel

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Zee

Senior Member
Location
CA
Embarrassed to ask...been a while:
Any issue with installing a 2-pole brkr. for a single phase inverter
in a (residential), 3 phase, 120/240VAC, DELTA, panel?


Any two legs of the three OK for PV?
 

Carultch

Senior Member
Location
Massachusetts
Embarrassed to ask...been a while:
Any issue with installing a 2-pole brkr. for a single phase inverter
in a (residential), 3 phase, 120/240VAC, DELTA, panel?


Any two legs of the three OK for PV?

Check the inverter manual, and it will tell you for sure. Look for AC connections and supported grid configurations.

Usually, the inverters actively supply power only to the two live conductors, and simply use the neutral for instrumentation purposes to keep track of phase and grid configurations.


Watch out for "slash rated" breakers, and make sure any two pole breaker connected to the high leg B-phase is straight rated for 240V. As opposed to 120/240V. Slash rated breakers limit you to 120 volts to ground. Straight rated breakers are rated for 240V between any pair of conductors or any conductor and ground.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Assuming you mean a high-leg delta...You should probably just avoid the high leg. Connect to the other two legs.

Some inverters (Solaredge, Fronius, SMA?) can be programmed to operate without the neutral and some (ABB, Enphase) can't.
If you connect to the phase with the neutral then you don't have to ask the question.

I also believe there might be an issue if you connect across an open delta. And the utility would probably prefer that you connect to the phase that's handling the most load, i.e the phase with the neutral.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
:thumbs up:
I am sure POCO would rather not have you pulling full normal load on the 120/240 winding while at the same time feeding back power on another phase. It would just make their imbalance worse. Also the pot or pots for the high leg might not have the current capability for full inverter power if they are scaled only for a fraction of the overall demand.
On the other hand, if the normal service load is mostly three phase, then going with just one single phase inverter may not be acceptable regardless of which phase you put it on.
 

Carultch

Senior Member
Location
Massachusetts
:thumbs up:
I am sure POCO would rather not have you pulling full normal load on the 120/240 winding while at the same time feeding back power on another phase. It would just make their imbalance worse. Also the pot or pots for the high leg might not have the current capability for full inverter power if they are scaled only for a fraction of the overall demand.
On the other hand, if the normal service load is mostly three phase, then going with just one single phase inverter may not be acceptable regardless of which phase you put it on.

POCOs usually require that you configure all inverters in as balanced as possible of a configuration. Some may even reject it if it is imbalanced.

The ideal strategy (if the utilty didn't care) would be to arrange the imbalanced power, so that the power supplied curtails the prevailing imbalance of power consumed. That would be the way to do it for ideal performance, if you know the prevailing imbalance of loads, but it is rare that you are permitted to do that.

If you are allowed to triplex a set of single phase inverters, it is OK if you have dissimilar configurations of modules, orientations, or shade conditions. What matters is that the AC kilowatt rating on each phase is equal. Some utilities don't allow triplexing single phase inverters on a three phase grid.

The high leg delta situation does add a lot of complexity. Dissimilar transformers might be used on each of the phases (big on the A-C phase, smaller on the A-B & B-C). Slash rated 2-pole breakers cannot be connected to the high leg B, which is an easy error to overlook. And you have to make sure you pick an inverter that supports this grid configuration. Additionally, be aware that older buildings may have their high leg on line C, which used to be the standard. You may also find utility meters with their high leg on C, even when the customer's equipment follows the current standard of high leg on B.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I ran into this situation with a small commercial PV system in San Antonio. I just combined the three inverters in a 1P3W configuration and fed the two non high legs. CPS (the POCO in San Antonio) was fine with it.

One thing I saw there that I had never seen anywhere before was that the facility's fused disco had different sized fuses in it; the high leg (B phase, I am pretty sure) had a fuse that was half or less the size of the other two. When I saw that I figured (correctly) that CPS was not going to worry about the imbalance.
 

Smart $

Esteemed Member
Location
Ohio
...One thing I saw there that I had never seen anywhere before was that the facility's fused disco had different sized fuses in it; the high leg (B phase, I am pretty sure) had a fuse that was half or less the size of the other two. When I saw that I figured (correctly) that CPS was not going to worry about the imbalance.
That's exactly how an open delta with a lesser-rated stinger than its complimentary lighter transformer should be configured. There is no phase-to-phase balancing in these cases, only loading issues... but phase-to-neutral balancing is still desired, preferred, recommended, etc.
 

Carultch

Senior Member
Location
Massachusetts
That's exactly how an open delta with a lesser-rated stinger than its complimentary lighter transformer should be configured. There is no phase-to-phase balancing in these cases, only loading issues... but phase-to-neutral balancing is still desired, preferred, recommended, etc.

So assuming the utility has no concern, it better to group every inverter across the A-C phases, and avoid the high leg altogether?

Given loads on the A & C phases dominating the system, and dissimilar fuses/transformers to match, I can certainly see how it is an advantage to build it this way. Plus, any single phase inverter can be connected at that position, and it will look no different to the inverter than a standard residential 1-phase grid.

You certainly don't want phase-to-phase balancing, if the consequence is that you overload the high leg service equipment, that may be ampacity limited.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So assuming the utility has no concern, it better to group every inverter across the A-C phases, and avoid the high leg altogether?
Yes, or at least that's how it worked out for me in the example I posted. Another benefit was that installing the system that way saved money - two pole instead of three pole switch, single phase instead of three phase AC combiner, fewer conductors, smaller conduit, etc.
 

Smart $

Esteemed Member
Location
Ohio
So assuming the utility has no concern, it better to group every inverter across the A-C phases, and avoid the high leg altogether?

Given loads on the A & C phases dominating the system, and dissimilar fuses/transformers to match, I can certainly see how it is an advantage to build it this way. Plus, any single phase inverter can be connected at that position, and it will look no different to the inverter than a standard residential 1-phase grid.

You certainly don't want phase-to-phase balancing, if the consequence is that you overload the high leg service equipment, that may be ampacity limited.
I'd avoid the high leg altogether... and for these open delta systems with small stinger transformer, there's no advantage to going 3 phase with the PV system.
 

Zee

Senior Member
Location
CA
I'd avoid the high leg altogether... and for these open delta systems with small stinger transformer, there's no advantage to going 3 phase with the PV system.

I appreciate the very thoughtful responses.

I will sit down and digest it all with references in hand when the time comes: mainly, a signed contract.

Most of the terms were above my head. My experience with non -resid. wiring being limited to vague recollections of the terms, "Wye", "Delta" and "stinger" (sounded cool) and something about square root of 3 or 2 being a factor in ampacity calculations on certain legs. More or less......:p

Interesting house though...she is an artist with a normal house BUT a HUGE, industrial-sized 400A full closet sized service gear...for glass-melting kilns.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I appreciate the very thoughtful responses.

I will sit down and digest it all with references in hand when the time comes: mainly, a signed contract.

Most of the terms were above my head. My experience with non -resid. wiring being limited to vague recollections of the terms, "Wye", "Delta" and "stinger" (sounded cool) and something about square root of 3 or 2 being a factor in ampacity calculations on certain legs. More or less......:p

Interesting house though...she is an artist with a normal house BUT a HUGE, industrial-sized 400A full closet sized service gear...for glass-melting kilns.
Many POCOs limit the size of residential PV systems according to the usage on their meter, so I'd wager she would qualify for a pretty big system.

The good news is that if you don't connect to the high leg that pesky sqrt3 won't rear its ugly head. :D
 

Zee

Senior Member
Location
CA
ggunn, surprisingly only 14 panels are needed. about 4 kw PV. But if yu saw the loads of cabinets and subpanels, 400 A and 300 rated, it might suggest otherwise!

Birken Vogt, yup, big loads when she does fire up the kilns.
J.ben: kilns are definitely the big loads, although intermittent

I am spec-ing an SMA SB4000TL 240 VAC. She has several subpanels, one has about 20 available brkr spaces (when does that happen?!) :D so i figure i'll find the two of the three stabs that are 240 V to each other and snap on my pv brkr there?
 

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Smart $

Esteemed Member
Location
Ohio
... so i figure i'll find the two of the three stabs that are 240 V to each other and snap on my pv brkr there?
All the stabs should be 240V to each other...

Can you see the transformer or transformer bank. A pad mount is a great indicator of balanced output (coupled with more likely being able to read the nameplate)... or a bank of three pots the same size. A bank of two pots would indicate open delta, and it is only in this case that you should connect A-C*... and those stabs will be 120V to neutral/ground.


*Connecting across the stinger pot is also feasible, depending on rating of pot vs. PV output.
 

Zee

Senior Member
Location
CA
SmartMoney,
I can see two transformers up on the pole.

Guys,
Ok I measured everything.

Main says
"240/120V, 3phase 4 wire Delta"
"208Y/120V, 3ph4wire"
400A rated service.

A to gr 120vac - measured (121)
B to gr. 208v. (211.5)
C to gr. 120v. (121)

AB BC and AC all 240vac

All major loads (30,50,60A) connected across all three phases, except one 2-pole 50 amp breaker across A to C.
( except some 20 amp circuits that are single phase)
I figure I will tie in my 2pole 20 amp breaker similarly A to C.
I also figured that this two pole breaker could connect across any of the three phases.

Am i right?
 
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Zee

Senior Member
Location
CA
My question above has to do with Electrical feasibility. Whether the utility will like it is a different question I guess.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Well, the supply can't be both 240/140V Delta as well as 208/120V Wye, so that is presumably telling you that the equipment is suitable for either. In any case, you measured it so the label is moot. :D

208V is the correct nonimal value for the high leg to ground or neutral, so you have confirmed it's 240/120 Delta, high leg. (Those single phase loads you mentioned, none of them are connected to the B leg, right? ;))

As I said in my first response, connect across A-C to save yourself potential trouble. Depending on the inverter it may or may not work A-B or B-C, or you may have to waste time programming it to make it work. Easy route is A-C if it doesn't require extra breakers or moving around stuff.

I don't believe you told us the inverter model.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
One reason for going A-C is that some 120/240 GTIs require the neutral for voltage imbalance checking (per UL?) even though they do not source current to the neutral.
As jaggedben indicated, some single phase GTIs can work with two hot legs only, but you may have to figure out how to reconfigure that mode to accept 240V rather than 208V.
There may also be a problem handling 208V with respect to ground.
Overall, best to avoid the B phase.
 
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