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

[Carultch]

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.

 

[GoldDigger]

An excellent question. The practical answer is that the service wires would now be undersized. Especially if the PV is continuous.
But I am not convinced the current code language rules against it.

mobile

 

[texie]

Interesting observation/question by the OP. One argument would be that the service conductors would not see the full 200 amps of PV output because the house would be using at least a portion of the output. It would seem though that if, say, all the branch circuits were turned off there would be no load from the house and then the service conductors would see the full 200 amps, perhaps continuously.
Not being a PV expert, I would wonder though if any POCO would allow that much PV capacity in relation to the service size. Don't most POCOs have limits as a percentage of service size?

 

[Carultch]

Interesting observation/question by the OP. One argument would be that the service conductors would not see the full 200 amps of PV output because the house would be using at least a portion of the output. It would seem though that if, say, all the branch circuits were turned off there would be no load from the house and then the service conductors would see the full 200 amps, perhaps continuously.
Not being a PV expert, I would wonder though if any POCO would allow that much PV capacity in relation to the service size. Don't most POCOs have limits as a percentage of service size?

I've never encountered such a POCO, but I've also never designed this ambitiously to the service size, except for a dedicated stand-alone service.

The system in question is only 90A on a 200A service, and I'm looking for complications that might arise by using feeder tap rules as if it were any ordinary feeder. Rather than one that only has 175A of wire on 200A breakers.

 

[jaggedben]

I definitely think it is a gray area and your AHJ's opinion is the only one that will ultimately matter. Here's my opinion for what it's worth...

1) The diversity of loads logic does not apply to the continuous output of a solar system on a good sunny day. Maybe you can argue that there will always be some load consumed in the house, but that's a bit weak. Thus it seems to me that, conservatively, you need to asses the conductors ampacity as an inverter output circuit separately from the allowed 83% for feeders and service conductors. That's especially true for a feeder.

2) On the other hand, 705.12(A) refers to the 'rating of the service', and if the service was approved with, say, 2/0 Cu for a 200A main breaker... whose to say that 200A isn't the 'rating of the service'? And moreover, inverter output conductors are defined as between the inverter and the service equipment. Thus one could argue that service conductors (on the other side of the service equipment) do not have to be sized per 705.60.

3) For the purposes of feeder tap rules you would conservatively have to use the table 301.15(B)(16) rating. So for example to comply with the 705.12(D)(2)(1)(b) requirement for a downstream overcurrent device, that might have to be lower than existing supply side overcurrent device.

 

[PVfarmer]

howdy folks!

howdy folks!

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?

Hey there y'all.
Here's a diagram.
https://app.box.com/s/weujjwjxh55boul2dllbn3oa1gof647g

Of course I could be wrong, but it seems like what is pictured would have to be 200A sized conductors (3/0 AWG) as opposed to 2/0 for 175A with the 83%?
And if the PV was 175A, 2/0 AWG would be ok? I'm using this: http://www.electrician2.com/calculators/wireocpd_ver_1.html

I'm a little unclear- you mean 200A of PV breaker, not PV output, correct?
Thanks.

 

[shortcircuit2]

In looking at it from a different angle...that 83% service conductor is doing more than just supplying the load of the dwelling. It is now also providing a path for the Grid Tied PV System (or any other interconnected power production source) and therefore violates the 83% rule.

Full size conductors are needed.

 

[Carultch]

In looking at it from a different angle...that 83% service conductor is doing more than just supplying the load of the dwelling. It is now also providing a path for the Grid Tied PV System (or any other interconnected power production source) and therefore violates the 83% rule.

Full size conductors are needed.

Right. Worst case scenario, assuming all loads shut off and the house exclusively exports. Probably a common situation if the residents regularly leave the house during the day.

 

[Carultch]

I'm a little unclear- you mean 200A of PV breaker, not PV output, correct?
Thanks.

Thanks for the diagram.

Yeah, I meant a 200A PV breaker. So that would be a max 160A nominal current system. 38 kW.

 

[PVfarmer]

Thanks for the diagram.

Yeah, I meant a 200A PV breaker. So that would be a max 160A nominal current system. 38 kW.

No prob, here's another one!
I'm thinking- conductors should be sized to the largest breaker (or fuses) from these 3 possible options- PV output, main load, or main service.
So in this pic...it's 96.4A of PV output with a 125A breaker, and the conductors would be sized to 125A, even for the 60A load panel.

Then I guess if you had a 200A PV breaker in this pic, it would have to be 200A main fuses and 200A (3/0 AWG) conductors, even to a 50 or 60A load panel?

What about this- you have "200A service", but 125A of PV breaker and 125A main load breaker instead of the 60A pictured.
You use 125A main fuses- but do you size the conductors to 200A...just in case of future PV or load expansion? I would.

 

[pv_n00b]

For supply side interconnections here it is text from 705.12(A):

The sum of the ratings of all overcurrent devices connected to power
production sources shall not exceed the rating of the service.
​

The key here is what does it mean by “service”? The NEC does not define the term “service”. That means it’s interpretation time. Here’s mine.

It does not say service conductors so to me the question of the current rating of the service conductors is moot. The rating of the service to me means the rating of the service on the utility's ledger for that service point. So if this is a 200A residential service according to the utility then it can take 200A of PV back feed according to the code.

Now on the design side I would double check the conductors to make sure the utility had properly provisioned the service as rated. I remember one job where there was an old 1000A Main SWBD but the utility had decided when it under-grounded the distribution in the area that the building only needed 400A of service so that’s all they provisioned for. It turned into an ugly lawsuit.

 

[jaggedben]

The NEC absolutely defines the term 'Service', but I'll grant your point that it doesn't define or explain how to determine a service's rating.

 

[PVfarmer]

1.
For supply side interconnections here it is text from 705.12(A):

The sum of the ratings of all overcurrent devices connected to power
production sources shall not exceed the rating of the service.
​

2.
The key here is what does it mean by “service”? // The NEC does not define the term “service”.

3.
It does not say service conductors so to me the question of the current rating of the service conductors is moot.
The rating of the service to me means the rating of the service on the utility's ledger for that service point.
So if this is a 200A residential service according to the utility then it can take 200A of PV back feed according to the code.

4.
Now on the design side I would double check the conductors to make sure the utility had properly provisioned the service as rated.

1.
Isn't the meaning of "rating of service" the same as "amperage of MDP panel main breaker OR PV breaker, whichever is greater" in this case?
So 200A PV breaker is max for a 200A service?
And if the PV breaker was larger than MDP breaker, the "rating" would be based the PV breaker (so 125A of PV and a 100A MDP on a 150A or 200A utility "service" *could* be "rated" 125A?)

2.
But it does define the service breaker or breakers and conductors and how to size them.
I mean, there is no "calculated load" for PV, PV output is the opposite of load, but you are supposed to use it the same way as load here?

230.79 Rating of Service Disconnecting Means. The service disconnecting means shall have a rating not less than the calculated load to be carried, determined in accordance with Part Ill, IV, or V of Article 220, as applicable. In no case shall the rating be lower than specified in 230.79(A),(B), (C), or (D).

(A) One-Circuit Installations. For installations to supply
only limited loads of a single branch circuit, the service
disconnecting, means shall have a rating of not less than
15 amperes.
(B) Two-Circuit Installations. For installations consisting
of not more than two 2-wire branch circuits, the service
disconnecting means shall have a rating of not less than
30 amperes.
(C) One-Family Dwellings. For a one-family dwelling,
the service disconnecting means shall have a rating of not
less than 100 amperes, 3-wire.
CD) All Others. For all other installations, the service dis-
connecting means shall have a rating of not less than
60 amperes.

3.
The rating of the service to me means the rating of the service on the utility's ledger for that service point.
So if this is a 200A residential service according to the utility then it can take 200A of PV back feed according to the code.

I see it as- the service conductor size CAN BE based on the calculated load(s), or the PV output if it is larger than loads.
I'm not sure what is based on the utility saying "200A service" other than the kVA of the transformer they provide, therefore the "maximum load".

That said, I think there's something to be said for using conductors sized for 200A feeding just a 125A MDP breaker- then you wouldn't have to touch them when adding any amount of PV breaker up to 200A.


230.23 Size and Rating.
(A) General. Conductors shall have sufficient ampacity to
carry the current for the load as calculated in accordance
with Article 220 and shall have adequate mechanical
strength

230.31 Size and Rating.
(A) General. Underground service conductors shall have
sufficient ampacity to carry the current for the load as cal-
culated in accordance with Article 220 and shall have ad-
equate mechanical strength.

4.
Aren't there some service conductors that are the customer's responsibility?

 

[PVfarmer]

1.
I've never encountered such a POCO, but I've also never designed this ambitiously to the service size, except for a dedicated stand-alone service.

2.
The system in question is only 90A on a 200A service, and I'm looking for complications that might arise by using feeder tap rules as if it were any ordinary feeder. Rather than one that only has 175A of wire on 200A breakers.

1.
There is one inverter tech doc that says the inverter(s) "apparent power" in total can only be 90% of the "step-up" transformer's power rating...so I'd assume that applies to the POCO's xfmr, but not necessarily all inverters? But does seem to make sense in general.
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf

2.
So that 90A / 21,600kW of PV is..."combined into" a 125A breaker?
What I'm wondering (if you don't mind) is what's the AWG now for the 90A of PV output relative to the AWG from the meter to the 200A MDP breaker. All...the same AWG?

So to this:
"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?"

Do you mean 200A of "actual" loads, times 83% = 166A, so 175A rated conductors...again, what's the MDP breaker?

I'd think that even with a 200A PV breaker (meaning 160A PV output) and a 175A MDP breaker, you'd want 200A rated wire all around. I'm not sure how to express that in actual code however.
By "all around" I mean to the meter and from meter to the MDP and also from the PV breaker to the supply side connection (between meter and MDP).

 

[Carultch]

1.
There is one inverter tech doc that says the inverter(s) "apparent power" in total can only be 90% of the "step-up" transformer's power rating...so I'd assume that applies to the POCO's xfmr, but not necessarily all inverters? But does seem to make sense in general.
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf

2.
So that 90A / 21,600kW of PV is..."combined into" a 125A breaker?
What I'm wondering (if you don't mind) is what's the AWG now for the 90A of PV output relative to the AWG from the meter to the 200A MDP breaker. All...the same AWG?

So to this:
"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?"

Do you mean 200A of "actual" loads, times 83% = 166A, so 175A rated conductors...again, what's the MDP breaker?

I'd think that even with a 200A PV breaker (meaning 160A PV output) and a 175A MDP breaker, you'd want 200A rated wire all around. I'm not sure how to express that in actual code however.
By "all around" I mean to the meter and from meter to the MDP and also from the PV breaker to the supply side connection (between meter and MDP).

1. SMA has issued several white papers on that issue, some of which require the 90% limit, some of which don't. I also haven't seen any rule that applies it to all inverter/transformer combinations. Also, it is common for utility-owned transformers to have significantly less KVA than a customer-owned transformer, for an otherwise identical service. But in this particular question, I'm talking about service amps in general, regardless of whether it comes as the sole service from the transformer, or one service from a neighborhood transformer.

2. The example that prompted me to think about this, has 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.

 

[Carultch]

2.
Do you mean 200A of "actual" loads, times 83% = 166A, so 175A rated conductors...again, what's the MDP breaker?

I'd think that even with a 200A PV breaker (meaning 160A PV output) and a 175A MDP breaker, you'd want 200A rated wire all around. I'm not sure how to express that in actual code however.
By "all around" I mean to the meter and from meter to the MDP and also from the PV breaker to the supply side connection (between meter and MDP).

What I meant was, given a 200A service (fully utilized or not) with a 200A main breaker and #2/0 Cu service wire, can you make a supply side interconnection or feeder tap to connect 160A continuous [200A breaker] worth of solar onto the existing #2/0Cu service conductors? Ironically, the PV tap conductors would have to be #3/0 Cu, and to make intuitive sense, one would think that the service conductors would also have to be #3/0 Cu.

 

[PVfarmer]

What I meant was, given a 200A service (fully utilized or not) with a 200A main breaker and #2/0 Cu service wire, can you make a supply side interconnection or feeder tap to connect 160A continuous [200A breaker] worth of solar onto the existing #2/0Cu service conductors? Ironically, the PV tap conductors would have to be #3/0 Cu, and to make intuitive sense, one would think that the service conductors would also have to be #3/0 Cu.

Is it because the main loads are both continuous and non-continuous while the the PV "load" is continuous only?
And I agree, when you add in 3/0 AWG for PV, all conductors between PV breaker and any other breakers/fuses should be 3/0.

But I don't get how there can be 2/0 Cu wire (which if USE-2 is rated for 195A?) with a 200A breaker. You don't have to go to a 175A breaker?

I can't seem to find the 2/0 / 200A combo here.

http://www.electrician2.com/calculators/wireocpd_ver_1.html

 

[Carultch]

Is it because the main loads are both continuous and non-continuous while the the PV "load" is continuous only?
And I agree, when you add in 3/0 AWG for PV, all conductors between PV breaker and any other breakers/fuses should be 3/0.

But I don't get how there can be 2/0 Cu wire (which if USE-2 is rated for 195A?) with a 200A breaker. You don't have to go to a 175A breaker?

I can't seem to find the 2/0 / 200A combo here.

http://www.electrician2.com/calculators/wireocpd_ver_1.html

The mix of continuous & non-continuous is already accounted for by the load calculation including/not including the 125% factor when adding up the loads. 125% of continuous + 100% of non-continuous.

Service conductors, per 310.15(B)(7), are allowed to have only 83% of the service rating, as their ampacity. After the service rating has already been calculated per the diversity factors and the mix of continuous and non-continuous loads. That link you gave does not account for applications where 310.15(B)(7) comes in to play, but rather feeders in general.

 

[PVfarmer]

maybe...?

maybe...?

Well. I turn around for 30 seconds and my kid turns my typed comment into a screenshot!
No idea how.
Here it is. :huh:

In short, go by 705.12(A), a 200A PV breaker would be greater than the "rating" of any "service" using less than 241A of load for the the 83% rule for that service rating calc. (241A *83% = 200.03A)
So you can't have the 200A PV breaker with the 200A MDP breaker WHEN the defined "rating" (having used 83%) is under 200A.

705.12(A) prevents the "3/0 with 2/0 wire problem" by not allowing a 200A PV breaker to be connected to 2/0 AWG?
It doesn't say it outright, but it does by...process?

How about this-- 238A of load * 83% =197.54A.
Then you'd have 3/0 AWG without PV.

Would adding a 200A PV breaker be OK?
The 3/0 wire @225A is fine, but 200A is greater than 197.54A. That seems...odd too?


And re the pic here... 235A * 83% = 195.05A.

 

[PVfarmer]

310.15(B)(7)- supplying the ENTIRE load means...?

310.15(B)(7)- supplying the ENTIRE load means...?

What I meant was, given a 200A service (fully utilized or not) with a 200A main breaker and #2/0 Cu service wire, can you make a supply side interconnection or feeder tap to connect 160A continuous [200A breaker] worth of solar onto the existing #2/0Cu service conductors? Ironically, the PV tap conductors would have to be #3/0 Cu, and to make intuitive sense, one would think that the service conductors would also have to be #3/0 Cu.

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.