Adding solar to a 3 phase building
- Thread starter matt123
- Start date
- Status
In most cases, you can use either. Whether you should use one versus the other depends on several details. Some out of necessity, some out of preference. Let's start with the building service voltage: 480, 240, or 208 volts? Next, array size?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I would say in general there would be two connections to avoid: one is to the open leg of an open delta, and the other is to the center tap of a high leg delta. I would also check with the utility. There is a good chance they will have requirements and restrictions on this.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
I (mostly) agree with the avoidance of connecting to the high leg (though I have done it when the transformer driving it was of sufficient capacity), but why would you say not to connect to the center tap?
Carultch
Senior Member
- Location
- Massachusetts
A lot of utilities require you to balance your system among all the phases, if you use single phase inverters. Some do not even allow single phase inverters on a three phase grid in the first place.
Phase balancing is a good point.
Have any of you ever run into a phase balance requirement when the delivery is via 'unbalanced' open delta?
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
I haven't; a high leg service is usually inherently unbalanced. When it isn't possible or well advised to interconnect with the high leg, we connect to the A and C phases and the center tapped neutral. I have never encountered a situation where we could not do one or the other.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I meant connecting to the center tap and the high leg. Using the center tap as a neutral/reference is fine.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
Then I agree. With a high leg service you can either connect all three phases ungrounded if your inverter(s) can do it and your high leg transformer will take 1/3 the power output, or you can connect single phase to A, C, and neutral. And of course, the governing AHJ has the final word.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I would think that depends on the relative transformer sizes and whether it is an open or closed delta.
In the case of an open delta, the high leg "stinger" pot would end up handling more than 1/3 of the generated power. Which could be very bad.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
How so?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Both the AB phase current and the BC phase current pass through the stinger pot. In the case of AB, as typically drawn, that phase current also passes through AC.
If the inverter sources equal current to A, B, and C wires, there are four I2R drops, two of which pass through BC.
As for the delivered power, if you have a three phase inverter with balanced power delivery two thirds of the power passes through the stinger pot.
Sent from my XT1585 using Tapatalk
If the inverter sources equal current to A, B, and C wires, there are four I2R drops, two of which pass through BC.
As for the delivered power, if you have a three phase inverter with balanced power delivery two thirds of the power passes through the stinger pot.
Sent from my XT1585 using Tapatalk
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Does the same issue not affect a load? Can the size of the high-leg service conductor not be a clue? Do I have to limit the inverter output per leg to half of the high-leg service conductor ampacity?
FWIW, I've only ever connected single phase to a high-leg.
FWIW, I've only ever connected single phase to a high-leg.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I see two likely cases for high leg:
1. Where most of the load is three phase and the 120/240 is a secondary need. This is likely to be a closed delta, and I would connect three phase inverter or three comparable single phase inverters IF the off center neutral will not be a problem.
2. Where most of the load I'd 120/240 and the three phase load is minor. This will most likely be an open delta with a small stinger. I would use only the AC phase for one or more single phase inverters.
Sent from my XT1585 using Tapatalk
Need to know the service details...
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
That's a pretty small amount of power really, only about 70 amps, so likely no problem. The most important thing is to see what your utility thinks.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Electrical Engineer - Photovoltaic Systems
Probably not. As small as the inverters are, you can most likely connect single phase to the A and C phases no matter what, but I'd advise arranging for a preconstruction meeting with the AHJ just to make sure it's OK with them.
- Status