Being told to upsize service wire due to supply-side connection

[Cprisbrey]

I have an inspector telling me that I either need to provide load calcs or upsize the current 4/0 AL service wire, since we added the solar supply side connection, He said since we are tapping the derate does not apply to the wire anymore. I don't believe this is needed since we are not a tap, since we are supply side. Am I missing something?

 

[wwhitney]

I don't see anything in the language in (2020) 310.12(A) that would be affected by the addition of the PV interconnection. Maybe ask the inspector to explain what language in that section would exclude your new configuration?

Cheers, Wayne

 

[Carultch]

How large is the PV system? I can understand the interpretation of why you would need to upsize the service conductors, if we are talking a 200A system. It logically follows that if you need 250 kcmil AL for the PV system, then you would need at least 250 kcmil AL for what it is tapped onto. If it were my choice, I would treat the ampacity of the existing service conductors, outside the context of 310.15(B)(7), as the limit to 1.25*max total inverter current.

By contrast, if the interconnected system uses 175A worth of OCPD or less (i.e. the ampacity of the #4/0AL conductors), I see no reason to increase the service conductors. Simply follow Kirchhoff's current law, and determine whether you add a possibility for overload in the blindspot of the OCPD's. The combination of source and load currents will be subtractive utility-side of the tap point. And for customer-side of the tap point, only the mix of loads on the service panel governs the load on the section of conductor. It complies prior to the PV, and it would comply after it as well.

 

[jaggedben]

I have an inspector telling me that I either need to provide load calcs or upsize the current 4/0 AL service wire, since we added the solar supply side connection, He said since we are tapping the derate does not apply to the wire anymore. I don't believe this is needed since we are not a tap, since we are supply side. Am I missing something?

I'm speculating somewhat here without clearer info, but it seems like the inspector may think that with the solar present the service conductors no longer 'supplies the entire load' as referred to on 310.12. This is an unreasonable interpretation because:
-calculated load is a measure of power, not energy, and the peak load on the service may not be coincident with solar production and therefore does not change whether the service supplies the entire load as meant in 310.12
- solar can only reduce the load on the service conductors, so requiring a larger conductor for that is illogical

Good luck.

 

[ggunn]

I'm speculating somewhat here without clearer info, but it seems like the inspector may think that with the solar present the service conductors no longer 'supplies the entire load' as referred to on 310.12. This is an unreasonable interpretation because:
-calculated load is a measure of power, not energy, and the peak load on the service may not be coincident with solar production and therefore does not change whether the service supplies the entire load as meant in 310.12
- solar can only reduce the load on the service conductors, so requiring a larger conductor for that is illogical

Good luck.

Or it could be that the inspector does not understand how electricity works and thinks that current from the PV system will add to the current in the service conductors rather than reducing it, i.e., that current will flow both ways simultaneously and additively. Believe it or not, those guys exist.

 

[ggunn]

How large is the PV system? I can understand the interpretation of why you would need to upsize the service conductors, if we are talking a 200A system. It logically follows that if you need 250 kcmil AL for the PV system, then you would need at least 250 kcmil AL for what it is tapped onto. If it were my choice, I would treat the ampacity of the existing service conductors, outside the context of 310.15(B)(7), as the limit to 1.25*max total inverter current.

Correct and compliant; it is now clarified in the 2020 NEC by 705.11(A). It just used to say that the limiting factor on a supply side interconnected PV system was "the size of the service", whatever that meant.

 

[RumRunner]

I'm speculating somewhat here without clearer info, but it seems like the inspector may think that with the solar present the service conductors no longer 'supplies the entire load' as referred to on 310.12. This is an unreasonable interpretation because:
-calculated load is a measure of power, not energy, and the peak load on the service may not be coincident with solar production and therefore does not change whether the service supplies the entire load as meant in 310.12

. . . . . . - solar can only reduce the load on the service conductors, so requiring a larger conductor for that is illogical
Everything was going well until you came up with the comparison between POWER and ENERGY.
It is true that we are analyzing the power that worked itself into the discussion--however, when energy was thrown into the mix, you are legitimizing the inspector's purported "unreasonable demand".

Notwithstanding what NEC says, the inspector's train of thought was thrown into disarray because of the potential increase that would be available due to the added solar energy source.

The difference between POWER and ENERGY is:

Energy CAN be stored whereas power is NOT STORABLE and for the simple reason that power is an instantaneous quantity.

With enhanced (increased) stored solar energy, it can be drawn in large quantity that appurtenance may not be big enough to support it--thus the inspector's requirement to increase the wire size and other supporting infrastructure.

 

[jaggedben]

That makes no sense, and I'm sorry you're dealing with an inspector who doesn't understand the most basic aspects of how electricity works. The point was that the peak load demand (VA, power) that needs to be carried by the service stays the same (or goes down) regardless of how many kWh (energy) are delivered by the solar. If that's too consfusing I don't know how to set it straight.

 

[jaggedben]

If you have an opportunity for another go, remind the inspector that the previously existing overcurrent device protecting the loads (the existing service disconnect) still limits how much the load can draw. And ask him to look up Kirchoff's law.

 

[PWDickerson]

I think the inspector might be considering this from a different angle, and if so, I think he is right. Consider a 50 kW grid-tied PV system line-side connected to a 200A service. The design utilizes five 7.6 kW inverters each landing on a 40A breaker in an AC combiner panel. The inverters have a combined max output current of 160A x 1.25 for continuous current requires 200A OCPD and conductor ampacity. The subject home is a vacation home and only used on the weekends. There will be essentially no load during the summer weekday days, and when the sun is shining, the 1.3 DC:AC ratio will ensure that the system will be exporting 160 amps to the grid for more than 3 hours a day. Seems like the service conductors should be fully rated.

 

[ggunn]

If the inverters' total maximum rated output current is 160A the PV AC conductors should be #3/0 copper or 250kcmil aluminum. Continuous use is in there and local load is not relevant. Of course, the service conductors must be that large or larger.

 

[wwhitney]

the 1.3 DC:AC ratio will ensure that the system will be exporting 160 amps to the grid for more than 3 hours a day.

Which is not a problem for 4/0 Al service conductors, as they have an ampacity of 180A, and so are rated to carry 180A continuously.

Cheers, Wayne

 

[wwhitney]

If the inverters' total maximum rated output current is 160A the PV AC conductors should be #3/0 copper or 250kcmil aluminum.

No need for that, 160A ampacity would suffice for the service conductors.

Cheers, Wayne

 

[ggunn]

No need for that, 160A ampacity would suffice for the service conductors.

Cheers, Wayne

So, you would tap 250kcmil aluminum PV AC conductors onto 4/0 aluminum service conductors? I would not do that.

 

[wwhitney]

So, you would tap 250kcmil aluminum PV AC conductors onto 4/0 aluminum service conductors?

Yes, that's what 2020 NEC 705.11 tells us to do:

705.11(A) says the service conductors have to have an ampacity of 160A. 4/0 Al has an ampacity of 180A. Check.
705.11(B) tell us to size the line side PV conductors per 705.28 which includes a 125% factor. So now we need a 200A ampacity, and I agree 4/0 Al is too small for those.

Cheers, Wayne

 

[jaggedben]

I think the inspector might be considering this from a different angle, and if so, I think he is right. Consider a 50 kW grid-tied PV system line-side connected to a 200A service. The design utilizes five 7.6 kW inverters each landing on a 40A breaker in an AC combiner panel. The inverters have a combined max output current of 160A x 1.25 for continuous current requires 200A OCPD and conductor ampacity. The subject home is a vacation home and only used on the weekends. There will be essentially no load during the summer weekday days, and when the sun is shining, the 1.3 DC:AC ratio will ensure that the system will be exporting 160 amps to the grid for more than 3 hours a day. Seems like the service conductors should be fully rated.

The OP's description "He said since we are tapping the derate does not apply to the wire anymore" does not sound like that. Also a 50kW system on a 200A service is something I've never heard of. Of course unless the OP comes back and clarifies it's hard to say for sure, but I suspect the inspector is not that smart and this deviation into discussing a very unlikely case will not help the OP.

 

[PWDickerson]

You are probably right, but it is hard to tell without more information from the OP about the system size.

 

[ggunn]

Yes, that's what 2020 NEC 705.11 tells us to do:

705.11(A) says the service conductors have to have an ampacity of 160A. 4/0 Al has an ampacity of 180A. Check.
705.11(B) tell us to size the line side PV conductors per 705.28 which includes a 125% factor. So now we need a 200A ampacity, and I agree 4/0 Al is too small for those.

Cheers, Wayne

Well, just because the NEC allows something by interpretation, that doesn't necessarily mean that it's a good idea. I still wouldn't do it. YMMV.

 

[wwhitney]

Well, just because the NEC allows something by interpretation, that doesn't necessarily mean that it's a good idea. I still wouldn't do it. YMMV.

Why do you see it as a problem? The only problem I see is the requirement to run 250 kcmil Al from the 4/0 Al service conductors to the 200A OCPD. 4/0 Al would be sufficient for those.

[This gets back to my pet peeve that the NEC requires a factor of 125% for both the OCPD and the ampacity of conductors for continuous loads. It would be enough to require that the OCPD be sized at 125%, and that the conductors have sufficient ampacity (i.e. 100%, not 125%) and be large enough to be properly protected by the OCPD.

E.g. under 240.4(B), 4/0 Al is properly protected by 200A OCPD even though its ampacity is 180A. I don't see how that changes between a 180A non-continuous load and a 160A continuous load.]

Cheers, Wayne

 

[ggunn]

I agree that the rules should be consistent. If 4/0 aluminum wiring is sufficient for the service conductors it should be for the PV AC conductors as well. With all the loads off they see the same current; what's good or bad for one should be for the other. I would not do it, if for no other reason I would expect some inspectors to see it the same way as the OP's and it would be hard to change things to satisfy them.