Supply Side Connection via a PV Dedicated Subpanel

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darunedefig

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Location
HV, New York
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Electrician
Hello,
3 PV inverters with [2] at 16A output and [1] at 28A output are combined in a
125A PV Dedicated Panel via [2] 20A 2pole breakers and [1] 35A breaker.

Building is a office building with 240V and 200A Main Service
The wire I am tapping onto are 90c rated 2/0cu

Using 2008 NEC, NY.

There is a 100A AC Disconnect fused with 80A fuses.

My question is for sizing the wires between the AC Disconnect and the Utility meter.
Should the wire be a #4cu or a #3cu or a #1cu?


110.14(C)(1)(a) if terminal derate and "not a service feeder" then #1?
vs. Table 310.15(B)(6)
I believe it is a #4
as it is a new service and Table 310.15(B)(6).
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It is not a service conductor once it's beyond the disconnect, and would be sized as any other 80A AC circuit....unless you're AHJ decides to be irrational and make you protect it from both sources at once according to a certain interpretation of Article 705 that some have encountered. If your AHJ goes down that rabbit-hole, then you'll need your ampacity to be (80+75)/1.2=129A.

For 80A #4 is probably fine unless the ambient temperature will be very high or you have temperature adders (e.g. on a roof). If your sub is not on the roof, submit plans with #4 and see what the plan checkers say.

BTW technically your 125A panel needs the same calc as above and is actually not okay, you'd need a 150A panel. It'd be reasonable to ask the AHJ for leniency on that one though, especially if you add signage prohibiting adding loads to the panel. (Cost of signage vs. 150A panel may be a wash. You decide.)
 

darunedefig

Member
Location
HV, New York
Occupation
Electrician
It is not a service conductor once it's beyond the disconnect, and would be sized as any other 80A AC circuit....unless you're AHJ decides to be irrational and make you protect it from both sources at once according to a certain interpretation of Article 705 that some have encountered. If your AHJ goes down that rabbit-hole, then you'll need your ampacity to be (80+75)/1.2=129A.
I am assuming "beyond the disconnect" you are referring to the wire between the fused AC switch blade disconnect and the PV dedicated Panel.
For this situation it goes: PV->Inverter->PV Dedicated Panel->Fused AC Disconnect->Supply side connection onto the Service Conductors of the existing 200A MSP. -> Utility Meter -> The Grid
I used to size the wires by the 120% rule, but company policy changed. We now put warning labels that is it a PV dedicated Disconnect.

I am wondering about the wire between the disconnect and the supply side connection "line side tap"??

John Wiles responded a year ago to me:
NEC is not specific in this area. Talk with the AHJ. I usually don't recommend any conductor smaller than the size of the existing service entrance conductors.

Were about to go to 2014 code. Has there been more clarification since 2008?

I would like to use #4 here instead of 2/0 cu(existing 200A Service Conductors)
Common sense tells me #4 is fine if the PV dedicated panel is properly labeled. In this instance only 60A of maximum PV power is produced.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I am assuming "beyond the disconnect" you are referring to the wire between the fused AC switch blade disconnect and the PV dedicated Panel.

Correct.

For this situation it goes: PV->Inverter->PV Dedicated Panel->Fused AC Disconnect->Supply side connection onto the Service Conductors of the existing 200A MSP. -> Utility Meter -> The Grid
I am wondering about the wire between the disconnect and the supply side connection "line side tap"??

John Wiles responded a year ago to me:

Service conductors can't be smaller than #6, but aside from that you size them the same as on the other side of the disco. So #4 in your case.

There has been some intense debate in this forum as to whether the code firmly establishes the categorization of conductors for PV supply side connections as being service conductors. But in my opinion, if they are on the utility side of the disconnecting means, then they are service conductors, plain and simple. See 230.40 Exception 5.

Matching your taps to the existing service conductor size is silly and unnecessary.

I would like to use #4 here instead of 2/0 cu(existing 200A Service Conductors)
Common sense tells me #4 is fine if the PV dedicated panel is properly labeled. In this instance only 60A of maximum PV power is produced.

#4 would be absolutely fine in my opinion.

Were about to go to 2014 code. Has there been more clarification since 2008?

I would say no, still clear as mud.

Correction: This thread topic should be called Supply Side Connection via a PV Dedicated Panel.... not sub.

Strictly speaking you had it right the first time. Your service equipment is the disconnect and your panel is a sub. It matters mainly in terms of where you bond the neutral to ground.
 

darunedefig

Member
Location
HV, New York
Occupation
Electrician
Service conductors can't be smaller than #6, but aside from that you size them the same as on the other side of the disco. So #4 in your case.

There has been some intense debate in this forum as to whether the code firmly establishes the categorization of conductors for PV supply side connections as being service conductors. But in my opinion, if they are on the utility side of the disconnecting means, then they are service conductors, plain and simple. See 230.40 Exception 5.

If the inspector asks I want to have my case down:
NEC 230.42(B) means that you can't use fuse amp rating, but must use rating of equipment
(B) Specific Installations. In addition to the requirements
of 230.42(A), the minimum ampacity for ungrounded conductors
for specific installations shall not be less than the
rating of the service disconnecting means
So 100A Disconnect with 80A fuses must use 100A for wire calculation.

So I can use Table 310.15(B)(6) 2008 NEC {310.15(B)(7) 2011 NEC} ??
The language there makes me think I have to use Table 310.16
310.15(B)(6) says:
"Dwelling", but my case it is an office (120/240V 3 wire, but not a dwelling)

"that supplies... all loads that are part or associated with the dwelling unit", I guess this applies to the solar backfeeding and possibly supplying the loads in the existing main service panel.. (or maybe your neighbor's loads as the solar wouldn't be on the bottom of that buss).

If we don't/can't use Table 310.15(B)(6)
then we use Table 310.16 NEC2008{310.15(B)(16)in 2011NEC} ...?...
[I am at 30C high 2% avg temp, so looking at table as is with no correction factors]


#4 in the 90c column is showing 95A (not 100A)
and this would have to be if all the terminals, equipment and wire is 90c rated??

#3 in the 75c column is showing 100A
I am using a Square D 100A Fusible Disconnect which says "30?100 A switches suitable for 60?C or 75?C conductors"
So it should be a #3 right?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You have just sparked a frightening realization. If a house used 310.15(B)(6) to size a feeder from the service point, then if you add PV to the system downstream of that feeder you have to resize it because the feeder is no longer supplying the entire dwelling load all of the time. :(
Comments?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
You have just sparked a frightening realization. If a house used 310.15(B)(6) to size a feeder from the service point, then if you add PV to the system downstream of that feeder you have to resize it because the feeder is no longer supplying the entire dwelling load all of the time. :(
Comments?

Oh please no.

The code says nothing about 'all the time', and the feeder still serves 'all the load' in the sense that the load factor can be reduced.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
If the inspector asks I want to have my case down:
NEC 230.42(B) means that you can't use fuse amp rating, but must use rating of equipment

So 100A Disconnect with 80A fuses must use 100A for wire calculation.

...

#3 in the 75c column is showing 100A
I am using a Square D 100A Fusible Disconnect which says "30?100 A switches suitable for 60?C or 75?C conductors"
So it should be a #3 right?

#3 it is.

I was not aware of 230.42(B). Thanks for pointing it out.
 

Smart $

Esteemed Member
Location
Ohio
If the inspector asks I want to have my case down:
NEC 230.42(B) means that you can't use fuse amp rating, but must use rating of equipment

So 100A Disconnect with 80A fuses must use 100A for wire calculation.

...
No, that's not what 230.42(b) is saying. In your quote of the 230.42(B) you left off the end, "specified in230.79(A) through (D)", which has significant impact on the meaning. It means the you cannot have conductor ampacity less than the rating specified in the subsection concerning your installation. In your case, the ampacity cannot be less than 60A [230.79(D)].
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Oh please no.

The code says nothing about 'all the time', and the feeder still serves 'all the load' in the sense that the load factor can be reduced.
It makes just as much sense to me as requiring the size to be increased when a specific load, however small, is removed from that feeder.
I am not that either makes sense, just that the words of the code can be interpreted that way. :)
 

Smart $

Esteemed Member
Location
Ohio
You have just sparked a frightening realization. If a house used 310.15(B)(6) to size a feeder from the service point, then if you add PV to the system downstream of that feeder you have to resize it because the feeder is no longer supplying the entire dwelling load all of the time. :(
Comments?

It makes just as much sense to me as requiring the size to be increased when a specific load, however small, is removed from that feeder.
I am not that either makes sense, just that the words of the code can be interpreted that way. :)
The PV System is not a load (assuming non-hybrid).
 

GoldDigger

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Staff member
Location
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Occupation
Retired PV System Designer
The PV System is not a load (assuming non-hybrid).
No, but it is also supplying some or all loads instead of the feeder providing that current. How is that different from another wire supplying those loads? At least in the exact wording of the code section?
I suppose you can make the argument that if at any specific point in time the feeder supplies all loads the section applies.
Otherwise you would also have problems if an alternate power source and transfer switch is involved.
 

Smart $

Esteemed Member
Location
Ohio
No, but it is also supplying some or all loads instead of the feeder providing that current. How is that different from another wire supplying those loads? At least in the exact wording of the code section?
I suppose you can make the argument that if at any specific point in time the feeder supplies all loads the section applies.
Otherwise you would also have problems if an alternate power source and transfer switch is involved.
It's not like we're reducing the size of the service or feeder conductors because we have a PV System tapped in.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
No, but it is like you have already reduced the size of the conductors and then you have removed some load and the exception at the same time.
The comparable non-PV problem is when you calculate full load size and reduce the wire size on that basis and then remove a load such that the unreduced size for the smaller load is still larger than the reduced size for the original load.
Neither case makes sense, but there is a general agreement among electricians and inspectors that the second case is what the NEC language requires.
 

Smart $

Esteemed Member
Location
Ohio
I understand what you are saying... at least in concept... but I'm not seeing how it is actually occurring.

Consider a PV System t-tap to the SE conductors. The conductors are sized per 310.15(B)(6)... or (B)(7) in 2011+. How does tying in the PV System affect the maximum load on the SE conductors both line and load sides of the t-tap? Consider only non-hybrid PV Systems.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I can understand your concern, since it is a rather limited case.
First, I am not talking about service conductors of any kind, since the NEC does not apply there.
And for a supply side tap at the service point it would not make any difference.
So consider this situation:
The service point is a breaker only.
From there a feeder goes to an inside panel where the PV connects, either line or load side of that panel's main. Without the PV, the feeder from service point to panel can be sized for less than the full load amps as long as the feeder supplies the full dwelling load.
If you put a breaker for the new hot tub at the service point the feeder is now technically too small, even though the load in it has not changed.
All I am saying is that one can argue that adding PV to the inside panel also causes the loss of the exception since the feeder (alone) is no longer carrying the entire dwelling load.
I am not arguing that that is reasonable, just that some inspector somewhere may make that argument.
 

darunedefig

Member
Location
HV, New York
Occupation
Electrician
No, that's not what 230.42(b) is saying. In your quote of the 230.42(B) you left off the end, "specified in230.79(A) through (D)", which has significant impact on the meaning. It means the you cannot have conductor ampacity less than the rating specified in the subsection concerning your installation. In your case, the ampacity cannot be less than 60A [230.79(D)].

I left "means specified in 230.79(A) through (D)." off because I thought that 230.79D didn't apply.
230.79(D) 2008NEC For all other installations, the service disconnecting means shall have a rating of not less than 60 amperes.
So yes PV must have a minimum 60A disconnect when doing a supply Side Connection "line side tap",
but my case is I already have a 100A Service Disconnect (with 80A fuses [next standard size up from 35+20+20=75A]).
3 PV inverters with [2] at 16A output and [1] at 28A output are combined in a
125A PV Dedicated Panel via [2] 20A 2pole breakers and [1] 35A breaker.

Let me see if I can combine 230.42B and 230.79D into one sentence.
The minimum ampacity for ungrounded conductors shall not be less than the rating of the service disconnecting means and that disconnect should not have a rating less than 60A.

And unfortunately this inspector agrees
We are approving the design review for this project "as noted" with the following comment(s) -
When doing a supply side connection the conductors between the disconnect and the point of interconnection must be rated to match the disconnect (even when it contains a smaller fuse or breaker) per NEC.230.42(B). (The conductors between the AC Disconnect and the tap-off point are not labeled on the submitted Three-Line, please ensure during installation that they are rated for at least 100A.)
Yeah I didn't put a callout for those feeders... my bad, but still the above applies to this thread.

I would love to go with your interpretation Smart $ and size the wire as a #4 for the 80A fuses (75c column=85A)
How would you combine 230.42B and 230.79D into one sentence?
Or who ever want to chime in...

I really want to understand the correct interpretation.
 

Smart $

Esteemed Member
Location
Ohio
I left "means specified in 230.79(A) through (D)." off because I thought that 230.79D didn't apply.

So yes PV must have a minimum 60A disconnect when doing a supply Side Connection "line side tap",
but my case is I already have a 100A Service Disconnect (with 80A fuses [next standard size up from 35+20+20=75A]).


Let me see if I can combine 230.42B and 230.79D into one sentence.
The minimum ampacity for ungrounded conductors shall not be less than the rating of the service disconnecting means and that disconnect should not have a rating less than 60A.

And unfortunately this inspector agrees

Yeah I didn't put a callout for those feeders... my bad, but still the above applies to this thread.

I would love to go with your interpretation Smart $ and size the wire as a #4 for the 80A fuses (75c column=85A)
How would you combine 230.42B and 230.79D into one sentence?
Or who ever want to chime in...

I really want to understand the correct interpretation.
I still say you (and the plans examiner) are reading 230.42(B) wrong. It simply means that for installtions that do not fall under 230.79(A) through (C), the minimum ungrounded conductor ampacity is 60A. 230.79 says the minimu rating of the disconnect must be 60A, regardless of what it is fused. But that does not apply to SE conductors or disconnecting means connected thereto that are rated greater than 60A. If this were the case, there's be no point in having 230.90(A) Exception No. 2.
 

Smart $

Esteemed Member
Location
Ohio
I can understand your concern, since it is a rather limited case.
First, I am not talking about service conductors of any kind, since the NEC does not apply there.
And for a supply side tap at the service point it would not make any difference.
So consider this situation:
The service point is a breaker only.
From there a feeder goes to an inside panel where the PV connects, either line or load side of that panel's main. Without the PV, the feeder from service point to panel can be sized for less than the full load amps as long as the feeder supplies the full dwelling load.
If you put a breaker for the new hot tub at the service point the feeder is now technically too small, even though the load in it has not changed.
All I am saying is that one can argue that adding PV to the inside panel also causes the loss of the exception since the feeder (alone) is no longer carrying the entire dwelling load.
I am not arguing that that is reasonable, just that some inspector somewhere may make that argument.
But the feeder is carrying the entire load when the PV System output is zero. And at other times, the feeder is carrying something less than that, so no harm, no foul.

If someone installs a hot tub and powers it from the service then you may have a non-compliant install... but only if the hot tub is inside the dwelling... many are not.
 
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