Service conductors per the 83% rule, and PV interconnections.

[ggunn]

Possibly...when you add the supply side PV connection, the PV is a 2nd "source", so the 2/0 service conductors are no longer "supplying the entire load", and you can't have used 310.15(B)(7) to have sized them.
At night of course the 2/0 SCs are supplying 100% of the load, but they are also supplying zero % during the day when the PV conductors are supplying 100% of the load.

I *think* you could either:
A. Change the 2/0 service wire to 3/0 and go with the 200A PV breaker...

OR

B. Limit the PV to 175A breaker and 2/0 wire.

BUT, if you can't use the 83% rule in the first place with grid tied PV because with PV added an extra source is supplying one load...B above is *not* ok?
--
(1) For a service rated 100 through 400 A, the service conductors supplying the entire load associated with one-family dwelling. or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 per-cent of the service rating.

I don't think that the PV counts against "supplying the entire load". If it did we'd have to upsize the service conductors on every residential system whose service was built under the 83% rule. It wouldn't make any sense to have to do that, anyway, seeing as the contribution from the PV can only reduce the current in the service conductors.

 

[GoldDigger]

I don't think that the PV counts against "supplying the entire load". If it did we'd have to upsize the service conductors on every residential system whose service was built under the 83% rule. It wouldn't make any sense to have to do that, anyway, seeing as the contribution from the PV can only reduce the current in the service conductors.

Speaking of making sense, it does not make sense that if you remove a large load (heat, A/C) from a service conductor or feeder you have to make it larger.
But it is pretty well agreed that the Code requires it nevertheless.

mobile

 

[PVfarmer]

seeing as the contribution from the PV can only reduce the current in the service conductors.

It's an odd situation. Doesn't the PV really only reduce the current in the meter to j-box conductors for the same load, but it can also increase it there, depending on...amount of sun?

Say you have 40A of continuous load for 24 hours, and 72A of PV inverters with a 90A PV breaker.

At noon on a really good day, there will be 40A going to the load and 32A going out to the grid, so that's the same 40A on the "to load" part of the conductors and 8A less on the "to grid" part.

At night, there's just 40A from grid to load.

If there was 100A of PV (output) instead of 72, it would increase the current (with same 40A / 24 hr load) whenever the PV output was over 80A. (85A of PV = 40A to load and 45A to grid).

Is this accurate- you only have to upsize a setup where the 83% was used IF the amp rating of PV (breaker) you want to put in is greater than the amp rating of MDP breaker?

 

[GoldDigger]

It's an odd situation. Doesn't the PV really only reduce the current in the meter to j-box conductors for the same load, but it can also increase it there, depending on...amount of sun?

Say you have 40A of continuous load for 24 hours, and 72A of PV inverters with a 90A PV breaker.

At noon on a really good day, there will be 40A going to the load and 32A going out to the grid, so that's the same 40A on the "to load" part of the conductors and 8A less on the "to grid" part.

At night, there's just 40A from grid to load.

If there was 100A of PV (output) instead of 72, it would increase the current (with same 40A / 24 hr load) whenever the PV output was over 80A. (85A of PV = 40A to load and 45A to grid).

Is this accurate- you only have to upsize a setup where the 83% was used IF the amp rating of PV (breaker) you want to put in is greater than the amp rating of MDP breaker?

You point out an interesting, but unlikely, case, since POCO generally will discourage residential installations that are larger than 10kW or overshoot normal consumption by more than about 10%.

But it does bring to mind that POCO will have sized their conductors for the expected load rather than for the full nominal size of the service the customer has.

 

[electro7]

Service conductors are not always sized with the feeder ampacity calculation to carry the full ampere rating of the service. There is the 83% rule/table for dwellings, and the justification for this inconsistency is usually is based on the diversity of the loads and the low probability that the service will ever see its full rating. To me, it makes no sense, like raising the speed limit 20% and also making the speedometers display a false speed that is 20% higher, but that is a story for another time.

What happens when you interconnect a solar array, and challenge the load diversity assumption?

Can 200A of PV be on a 200A service, with "200A" of service conductors that are really only 175A if it were used in any other application?

Suppose a full service feeder is the same size as the "200A service conductors", and protected by a 200A breaker on both sides. Could you connect via a feeder tap, assuming the main feeder is any other 200A feeder? If a full 200A system were connected this way, it is a weird situation where 3/0 Cu feeder conductors would be tapped onto a 2/0 feeder.

My question is, when will the solar produce 200A when the max output of the inverter would be 160A? And if 2/0 THHN at 75°C is rated at 175A is there really an issue?

By NEC and sizing conductors at 125% for continuous loads I would say you would not be allowed to put 200A of solar OCPD's in your example, only 175A.

I agree with you, that the 83% rule does not make sense to me. I think a conductor ampacity rating should be what it is whether service or branch circuit, continuous or non-continuous load.

 

[PVfarmer]

to be oversized, or not to be....

to be oversized, or not to be....

You point out an interesting, but unlikely, case, since POCO generally will discourage residential installations that are larger than 10kW or overshoot normal consumption by more than about 10%.

But it does bring to mind that POCO will have sized their conductors for the expected load rather than for the full nominal size of the service the customer has.

Thanks!
There can also be an issue of which conductors belong to whom.
In the rough diagram below, adding the j-box changes the ownership somehow, doesn't it?

If the C switch and D panel are called "service disconnects" by the POCO, what conductors are theirs in the diagram?
I know if you are required to put a switch at X, you have to label it NOT SERVICE EQUIPMENT.

My question is, when will the solar produce 200A when the max output of the inverter would be 160A? And if 2/0 THHN at 75°C is rated at 175A is there really an issue?

By NEC and sizing conductors at 125% for continuous loads I would say you would not be allowed to put 200A of solar OCPD's in your example, only 175A.

I agree with you, that the 83% rule does not make sense to me. I think a conductor ampacity rating should be what it is whether service or branch circuit, continuous or non-continuous load.

It'll never produce 200A, and I think really only produce 160A for very few hours per year, *depending* on where it is.
As in, a 10kW AC inverter only makes the full 10kW on say a few hundred hours per year in New England, while New Mexico would be totally different. Or adding tracking in New England would make a big difference.

Anyhow, when this is OK:
90A worth of OCPD associated with the PV. So really 65A operating. It uses #3 Cu conductors, which make a feeder tap from full service feeder conductors that I anticipate are either #2/0Cu or #4/0AL.

You'd think:
1. a 175 A, 2/0 wire (service/for loads) associated with a 200A PV breaker/160A of power (as the OP proposes) would be fine, seeing as...
2. the same 175A, 2/0 is fine when associated with a 90A breaker and 3 AWG.

Something very weird would have to happen for the 2/0 wire to receive 200A (in #1) when the 3 AWG will never get 175A (in #2).

If there was some sort of problem created for the 2/0 wire by a 200A PV breaker in the #1 setup, seems it would also exist for the #3 wire/90A breaker in #2, but the latter is allowed.
If the #3 is exposed to a 175A breaker, why can't the 2/0 be exposed to 200A breaker?

I agree with your bolded comment too, but it's still odd that you "can't" do something which seems perfectly ok.
Also, upgrading from 2/0 to 3/0 service wire so you *could* go with the full 160A of PV could be quite simple...or a huge headache, right?