Splitting one inverter output to two separate services

[david67]

What would be the effect of splitting out a single split phase 240V inverter to two separate services? For example, if you took one hot leg from the inverter to a 120 volt breaker on one meter and took the other hot leg to another 120 volt breaker on a separate meter of a residential duplex (2-position) meter socket, what would the issue(s) be?

Thanks.

 

[ggunn]

What would be the effect of splitting out a single split phase 240V inverter to two separate services? For example, if you took one hot leg from the inverter to a 120 volt breaker on one meter and took the other hot leg to another 120 volt breaker on a separate meter of a residential duplex (2-position) meter socket, what would the issue(s) be?

Thanks.

Don't even think about it. At the very least your AHJ will laugh you out of their office for suggesting something like this.

 

[iwire]

Don't even think about it. At the very least your AHJ will laugh you out of their office for suggesting something like this.

I agree on both points.


I think it could work electrically, I just see a lot of code violations along the way.

 

[GoldDigger]

Don't even think about it. At the very least your AHJ will laugh you out of their office for suggesting something like this.

'

Two paths, one leading to laughter, one to scratching head at technical absurdity:

1. With a connection to two different services/meters the interconnect agreement with POCO and the NEC safety provisions become totally unmanageable. Hence the laughter.
2. Most if not all GTI inverters which have 240 volt output (unlike off-grid inverters with similar outputs) do not provide independent 120 volt outputs with phase coordination but rather source and sink current from line to line only, not line to neutral. The newer inverters also check for balance of the two line to neutral voltages, but still do not actually run current through the neutral. Having the two output legs connected to different services can cause all sorts of technical problems.

Bottom line: Forget it twice!
(I can see a motivation for wanting to do something like this where a 240 volt POCO connection for some reason supplies two separate 120 volt-only services on opposite phases. But it is not going to be practical. 120 volt GTIs are hard to find, and if they exist two of them could not be supplied from the same panel array anyway. I believe that you could legally connect a 240 volt GTI through a step down (step up?) transformer to just one 120 volt leg, but you would have to deal with twice the current.)

 

[jaggedben]

If you actually brought the solar output directly to a duplex meter socket, and terminated them with listed equipment, there might not be any ill effects as far as saftey. OTOH, if you split the two legs into different raceways at any point you'd get inductive heating, potentially serious levels.

I'm not totally sure how it would affect the meters, but I believe it would reduce the kWh count equally between the two.

As far as actually benefiting from this, even if the AHJ didn't balk and prevent you from doing it (which they probably would) the utility would throw out your interconnection application, and you'd never get any credit for exported energy. Worst case, they could probably shut down your power or penalize you financially.

 

[weressl]

What would be the effect of splitting out a single split phase 240V inverter to two separate services? For example, if you took one hot leg from the inverter to a 120 volt breaker on one meter and took the other hot leg to another 120 volt breaker on a separate meter of a residential duplex (2-position) meter socket, what would the issue(s) be?

Thanks.

I think you should be able to do that by feeding a transformer and two parallel services on the secondary.

 

[ggunn]

I think you should be able to do that by feeding a transformer and two parallel services on the secondary.

Even if that would work, the AHJ would never approve the interconnection, IMO.

 

[GoldDigger]

I think you should be able to do that by feeding a transformer and two parallel services on the secondary.

Think carefully about that and then consider what would happen if you pulled the meter on service A.
Both A and B and the inverter would then be fed through meter B.

(Actually I am not quite sure what you mean by "two parallel services on the secondary", but I can't think of anything that would work.)

 

[weressl]

Think carefully about that and then consider what would happen if you pulled the meter on service A.
Both A and B and the inverter would then be fed through meter B.

(Actually I am not quite sure what you mean by "two parallel services on the secondary", but I can't think of anything that would work.)

Inverter-> primary transformer protection/disconnect-> transformer-> parallel fused isolating switch for service A and service B -> separate meters connected to each isolating/secondary protection switch to serve each customer.

 

[GoldDigger]

Inverter-> primary transformer protection/disconnect-> transformer-> parallel fused isolating switch for service A and service B -> separate meters connected to each isolating/secondary protection switch to serve each customer.

If both meters are connected to the same transformer winding, or to two separate transformer secondaries, I do not see any type of isolating switches that when closed will prevent meter A's output from serving load B and vice-versa. If the connections are made to the line side of the meters, then the PV sent back to the grid is not being metered.

An isolator which only allows power to pass in one direction is pretty sophisticated, and requires active circuitry. But even with such a device, you cannot allow the PV power to go back to the grid through the meters and still prevent cross-feeding from the two meter outputs.

If both meters bill the same party, then this is not necessarily a problem, but would violate the one-service rule for both sets of loads.

 

[Smart $]

Inverter-> primary transformer protection/disconnect-> transformer-> parallel fused isolating switch for service A and service B -> separate meters connected to each isolating/secondary protection switch to serve each customer.

Assuming a GTI, there is no reason to split into two 120V feeds (or use a transformer). The only issue is if one service is disconnected (situation is rendered moot if both are disconnected).

The way I see it (at present ), will need one main PV disconnect/ocp feeding two service disconnects/ocps of equal rating to the PV disconnect, each tied in after the meters on the line side of the individual dwelling service disconnects.

The other option is to use a three meter stack and tie the PV system directly to the service through a single disconnect/ocp. This option renders both dwelling services being disconnected moot.

 

[GoldDigger]

Assuming a GTI, there is no reason to split into two 120V feeds (or use a transformer). The only issue is if one service is disconnected (situation is rendered moot if both are disconnected).

Regulatory considerations aside, that looks like it would be OK. From the original premise, the inverter will deliver equal current through both phase lines, forcing an equal split of the PV power output between the A side and the B side. Depending on A and B loads, there may or may not be sell-back to POCO on each service independently but each will benefit from half the PV output. As long as you do not install a transformer, there will be no current-carrying neutral attached from the GTI to either service.

With GTIs that meet existing regulations, the GTI will shut down if either line-to-neutral voltage goes out of range compared to the other. That should take care of most problems resulting from disconnecting only one service.

 

[Smart $]

Regulatory considerations aside, that looks like it would be OK. From the original premise, the inverter will deliver equal current through both phase lines, forcing an equal split of the PV power output between the A side and the B side. Depending on A and B loads, there may or may not be sell-back to POCO on each service independently but each will benefit from half the PV output. As long as you do not install a transformer, there will be no current-carrying neutral attached from the GTI to either service.

With GTIs that meet existing regulations, the GTI will shut down if either line-to-neutral voltage goes out of range compared to the other. That should take care of most problems resulting from disconnecting only one service.

Last sentence, yes take care of problems, but then the potential PV power isn't beneficial to anyone.

As stated, regulatory issues aside... installing a split-phase secondary transformer would make the PV system beneficial to the remaining dwelling, as the PV power wouldn't be dependent on connection through the meters. The three meter, direct-connect option would benefit whomever is the POCO's customer for the meter, regardless of dwelling services connection.

 

[ggunn]

Regulatory considerations aside, that looks like it would be OK. From the original premise, the inverter will deliver equal current through both phase lines, forcing an equal split of the PV power output between the A side and the B side. Depending on A and B loads, there may or may not be sell-back to POCO on each service independently but each will benefit from half the PV output. As long as you do not install a transformer, there will be no current-carrying neutral attached from the GTI to either service.

With GTIs that meet existing regulations, the GTI will shut down if either line-to-neutral voltage goes out of range compared to the other. That should take care of most problems resulting from disconnecting only one service.

But wouldn't this connect the two services together so that any load in either residence would pull equally through both meters?

 

[GoldDigger]

But wouldn't this connect the two services together so that any load in either residence would pull equally through both meters?

Not if the only connection to the GTI is to opposite phases of the split-phase service. That is, connects to black on A and to red on B. It cannot transfer power between the two sides. I think... Let me keep thinking about it for awhile.
Yes, as long as there are no 240 volt loads on either A or B, there can be no power transfer between A and B. Maybe not even then, but I am not convinced yet.
(OK, I am convinced now. The only current flow that the GTI will allow is that which it is generating by itself. No problem.) This also assumes that the A and B meters are in fact connected to the same transformer secondary and so are matched in phase and amplitude.

 

[Smart $]

Not if the only connection to the GTI is to opposite phases of the split-phase service. That is, connects to black on A and to red on B. It cannot transfer power between the two sides. I think... Let me keep thinking about it for awhile.
Yes, as long as there are no 240 volt loads on either A or B, there can be no power transfer between A and B. Maybe not even then, but I am not convinced yet.
(OK, I am convinced now. The only current flow that the GTI will allow is that which it is generating by itself. No problem.) This also assumes that the A and B meters are in fact connected to the same transformer secondary and so are matched in phase and amplitude.

I'm wondering whether this affects the usage reported by the meters. Looking at one dwelling having only 240V loads in excess of the PV-supplied power and the other dwelling none, the PV current would go through the load then back out on the not-connected line, through the meter (net current geater than connected leg), to the service bus, then back through the other dwelling meter on the connected leg (net current geater than not-connected leg). In effect, both customers end up paying the POCO for PV-supplied power!!!

 

[jaggedben]

This also assumes that the A and B meters are in fact connected to the same transformer secondary and so are matched in phase and amplitude.

The OP kinda confused the question actually, by putting "two separate services" in the thread title but then referring to a residential duplex meter socket, which to my mind is one service with two meters.

If the latter, I actually kind of like the idea: install one system on a duplex building and have both customers share it equally. Crazy, but it could be much easier than installing two systems the same size, or installing a third meter for a generating account. Granted, it's just too loony for the utilities to ever accept, I'm sure of that, even if all the starts aligned for the AHJ with code and meter-socket listings, which would be between rare and non-existent anyway. Probably violates some part of Art 230 no matter what.

 

[GoldDigger]

I'm wondering whether this affects the usage reported by the meters. Looking at one dwelling having only 240V loads in excess of the PV-supplied power and the other dwelling none, the PV current would go through the load then back out on the not-connected line, through the meter (net current geater than connected leg), to the service bus, then back through the other dwelling meter on the connected leg (net current geater than not-connected leg). In effect, both customers end up paying the POCO for PV-supplied power!!!

Nope. As long as the meters are accurate for unbalanced loads, the accounting will be correct. Remember that if the PV output on the one line of service A exceeds the total 120/240 load of that service, the meter will register power going out to POCO. Whether that power goes out to POCO and vanishes or gets turned around immediately and sent to B does not matter. A will get credit for it.
The return current from the PV at B will still be credited to B, whether it goes to loads at B or is just the other side of the PV delivered to A.

Both A and B will get meter credit (or reduced total load "credit") for 1/2 of the 240 volt GTI output power, since each meter will see the PV as a 120 volt supply.
If there are no loads at all at either A or B, the PV current will go "out" through the meter at A and be matched by an exactly equal current back through the neutral at A. Also neutral current in the same amount at B.
The PV current delivered to B will be matched by an equal return current on the neutral at B and at A. Net result: the two neutral currents cancel out, leaving zero current in the neutrals and identical magnitude currents in the single phase lines at A and B.
Normally you would expect that an overall current balance would be present at the A and B meters, but because the PV constitutes a cross connection between the services the currents will NOT balance. Now if that causes problems for the way the meter was designed (and it will for some types of split phase meters that are not theoretically correct because they took a design shortcut!) then there may be odd problems.

A general inability for POCO and most others to understand what is really going on is one clear reason that the connection would not be allowed.

 

[GoldDigger]

I'm wondering whether this affects the usage reported by the meters. Looking at one dwelling having only 240V loads in excess of the PV-supplied power and the other dwelling none, the PV current would go through the load then back out on the not-connected line, through the meter (net current geater than connected leg), to the service bus, then back through the other dwelling meter on the connected leg (net current geater than not-connected leg). In effect, both customers end up paying the POCO for PV-supplied power!!!

Nope. As long as the meters are accurate for unbalanced loads, the accounting will be correct. Remember that if the PV output on the one line of service A exceeds the total 120/240 load of that service, the meter will register power going out to POCO. Whether that power goes out to POCO and vanishes or gets turned around immediately and sent to B does not matter. A will get credit for it.
The return current from the PV at B will still be credited to B, whether it goes to loads at B or is just the other side of the PV delivered to A.

Both A and B will get meter credit (or reduced total load "credit") for 1/2 of the 240 volt GTI output power, since each meter will see the PV as a 120 volt supply.
If there are no loads at all at either A or B, the PV current will go "out" through the meter at A and be matched by an exactly equal current back through the neutral at A. Also neutral current in the same amount at B.
The PV current delivered to B will be matched by an equal return current on the neutral at B and at A. Net result: the two neutral currents cancel out, leaving zero current in the neutrals and identical magnitude currents in the single phase lines at A and B.
Normally you would expect that an overall current balance would be present at the A and B meters, but because the PV constitutes a cross connection between the services the currents will NOT balance. Now if that causes problems for the way the meter was designed (and it will for some types of split phase meters that are not theoretically correct because they took a design shortcut!) then there may be odd problems.

If instead of PV you connected a 240 volt load in the same way, you would have the same 50/50 billing except that the identical problem with the meter design may raise its ugly head.

 

[Smart $]

Nope. As long as the meters are accurate for unbalanced loads, the accounting will be correct. Remember that if the PV output on the one line of service A exceeds the total 120/240 load of that service, the meter will register power going out to POCO. Whether that power goes out to POCO and vanishes or gets turned around immediately and sent to B does not matter. A will get credit for it.
The return current from the PV at B will still be credited to B, whether it goes to loads at B or is just the other side of the PV delivered to A.

Both A and B will get meter credit (or reduced total load "credit") for 1/2 of the 240 volt GTI output power, since each meter will see the PV as a 120 volt supply.
If there are no loads at all at either A or B, the PV current will go "out" through the meter at A and be matched by an exactly equal current back through the neutral at A. Also neutral current in the same amount at B.
The PV current delivered to B will be matched by an equal return current on the neutral at B and at A. Net result: the two neutral currents cancel out, leaving zero current in the neutrals and identical magnitude currents in the single phase lines at A and B.
Normally you would expect that an overall current balance would be present at the A and B meters, but because the PV constitutes a cross connection between the services the currents will NOT balance. Now if that causes problems for the way the meter was designed (and it will for some types of split phase meters that are not theoretically correct because they took a design shortcut!) then there may be odd problems.

A general inability for POCO and most others to understand what is really going on is one clear reason that the connection would not be allowed.

Rethought this, and I was wrong regarding one customer... the one without the load gets POCO credit... and the one with the load pays extra.

For the sake of discussing this aspect, let's assume the meters are accurate.

With a normal 240V PV connection, power going out to the POCO results from current which is, for the most part, 180? out-of-phase with the voltage. Hopefully you know this and we don't have to get into the theory behind it.

However, in the case I brought up, the 240V load is using all of the PV system's output. Current is in phase with voltage on the meter for the customer with the load...