Multi-unit interconnection

[Pelagic13]

Hi folks,
Hoping for some clarity on this one. We have a situation with a 300-amp 3-gang meter bay at a power pole that provides 100-amp service underground to 3 isolated structures. Each meter is breakered.

Is there any issue connecting 3 different 20-amp pv systems to each of the structures with a backfeed at each service panel?

I seem to recall a need for a single point of connection.

Thoughts?

 

[jaggedben]

No issue in the NEC.

How else would you do it? Most states don't have programs that allow offsetting mutliple meters in any other way.

 

[ggunn]

Hi folks,
Hoping for some clarity on this one. We have a situation with a 300-amp 3-gang meter bay at a power pole that provides 100-amp service underground to 3 isolated structures. Each meter is breakered.

Is there any issue connecting 3 different 20-amp pv systems to each of the structures with a backfeed at each service panel?

I seem to recall a need for a single point of connection.

Thoughts?

The final answer is a question for your AHJ, but as jaggedben says the NEC is OK with it as long as the combined output of the PV systems does not exceed the ampacity of the service conductors before the split. Three 20A feeds into three 100A services should be doable with backfed breakers if 125% of each PV system's max inverter output is equal to or less than 20A. If it is more you can consider supply side interconnections.

 

[Pelagic13]

Thanks folks
It also occurred to me that 705.12(b)(2)(3)(c) might come into play here since the 3 meters/ msd's are on the same bus. Assuming it's rated at 300, we'd need to derate each of those down to 80 amps, no?

 

[Pelagic13]

For clarity, this is what we're working with. Each feeder supplies a 100 amp load center with main breaker.

 

[ggunn]

Thanks folks
It also occurred to me that 705.12(b)(2)(3)(c) might come into play here since the 3 meters/ msd's are on the same bus. Assuming it's rated at 300, we'd need to derate each of those down to 80 amps, no?

No. Anything connected on the line side of the service disconnects is governed by 705.11, not 705.12. The individual interconnections, if they are backfed breakers in the service's MDPs are individually governed by 705.12 but there is no issue on the line sides of the disconnects other than that 125% of an individual PV system's maximum output cannot exceed the ampacity of the service conductors between the split and the MDP, and the combined maximum inverter output cannot exceed the ampacity of the service conductors before the split.

 

[Pelagic13]

Interesting,
So theoretically, we could do a load tap of the conductors ahead of the MDP and backfeed 100 amps?

 

[ggunn]

Interesting,
So theoretically, we could do a load tap of the conductors ahead of the MDP and backfeed 100 amps?

If the service conductors are 100A rated, then yes, as long as 125% of the maximum current output of the inverter(s) is equal to or less than 100A.

 

[ggunn]

Interesting,
So theoretically, we could do a load tap of the conductors ahead of the MDP and backfeed 100 amps?

It would be a line (supply) side connection.

 

[jaggedben]

Interesting,
So theoretically, we could do a load tap of the conductors ahead of the MDP and backfeed 100 amps?

Are there breakers in that left hand compartment of the meter bank? Or not?

Either way, yes, theoretically by the NEC you could probably do this. The rules are different depending on the answer to my breakers question. Also the likelihood of the utility accepting ~60kW of backfeed on that service without issue is probably not so good as compared to five times less.

 

[PWDickerson]

Sounds like you are trying to get as much PV at the site as you can, and you are trying to find a way to install more than what is allowed by the 120% rule in 705.12(B)(3)(2). If that is the goal, you are certainly allowed to do that in the NEC, but you will need to check with your utility to see if they have any restrictions on supply-side connections, and you may be limited by the utility transformer size, so make sure and check with them on that. With regard to the NEC, your options are to 1) explore if you can interconnect using 705.12(B)(3)(3) ("sum of the breakers" rule, this option usually doesn't work out), 2) interconnect at a feeder tap if there are breakers in the service gear (705.12(B)(1&2)), or 3) interconnect via a supply-side connection (705.11) if there are no breakers in the service gear. I would be surprised if those three SER runs are service conductors, so I would guess that there are breakers in the serice gear, and if so, it would be easy to tap any/all of those feeders to a PV system disconnect(s) installed nearby. Make sure your downstream panels have a main breaker so that you meet 705.12(B)(1)(b) instead of having to upsize feeder conductors as required by 705.12(B)(1)(a). I am guessing that the tranformer size is going to be your choke point.

 

[ggunn]

Sounds like you are trying to get as much PV at the site as you can, and you are trying to find a way to install more than what is allowed by the 120% rule in 705.12(B)(3)(2). If that is the goal, you are certainly allowed to do that in the NEC, but you will need to check with your utility to see if they have any restrictions on supply-side connections, and you may be limited by the utility transformer size, so make sure and check with them on that. With regard to the NEC, your options are to 1) explore if you can interconnect using 705.12(B)(3)(3) ("sum of the breakers" rule, this option usually doesn't work out), 2) interconnect at a feeder tap if there are breakers in the service gear (705.12(B)(1&2)), or 3) interconnect via a supply-side connection (705.11) if there are no breakers in the service gear. I would be surprised if those three SER runs are service conductors, so I would guess that there are breakers in the serice gear, and if so, it would be easy to tap any/all of those feeders to a PV system disconnect(s) installed nearby. Make sure your downstream panels have a main breaker so that you meet 705.12(B)(1)(b) instead of having to upsize feeder conductors as required by 705.12(B)(1)(a). I am guessing that the tranformer size is going to be your choke point.

"2) interconnect at a feeder tap if there are breakers in the service gear (705.12(B)(1&2))" Be advised that this does not release you from having to comply with 705.12 in the main panel.

"I would be surprised if those three SER runs are service conductors" It seems to me that they are service conductors because there are separate meters for the three buildings after the three way split.

 

[jaggedben]

"2) interconnect at a feeder tap if there are breakers in the service gear (705.12(B)(1&2))" Be advised that this does not release you from having to comply with 705.12 in the main panel.

"I would be surprised if those three SER runs are service conductors" It seems to me that they are service conductors because there are separate meters for the three buildings after the three way split.

It seems to me that there are probably three 100A breakers under the left hand cover that protect the SERs, after the meters. But we can wait for the OP to answer that question definitively.

 

[Pelagic13]

Apologies for the delay getting back to this.
Yes, there are breakers in the service gear. The SER feeders each make their way to 100-amp load centers w/ 100-amp main breakers and 100-amp busbars. One service already has a 3.8kW PV system, 20-amp back-feed. Goal is to add in another 7.6kW, 40-amp system on one of the other services. The hope was to do this at just one of the MDP's and not have to divide the system up between 2 services, which it sounds like we're okay doing. By following 705.12, we're okay to tap the feeder to the 100-amp panel which is protected by the 100-amp main breaker downstream, and since the ampacity of the conductors is greater than 40-amps, we're good to go.
Do I have this right?

 

[PWDickerson]

That sounds right to me.

 

[jaggedben]

Yes sounds right to me. (Except for terminology. You only have one service.)