120% rule

[SolarPro]

Load tap ampacity... so the conductor (feeder tap) to the right of the blue line? What about the inverter "tap" ampacity... or is that what you are attempting to depict?

I suppose I should read the linked article. Maybe later...

Please do. The author is on the Code-Making Panel and heads a codes and standards working group for SEIA, a solar industry trade association. The article goes through multiple interconnection scenarios. I may have grabbed a formula and figure from different places in the text, but the basic point is how you apply the tap rule without winding up with an oversized conductor.

 

[ggunn]

But that isn't it- there's a 200A breaker on one end of the feeder, and either the 100A subpanel breaker or the 80A PV breaker at the other end of the feeder.
I'd say it's the 80A at the end of the feeder. The 100 and 80A breakers are used for the 120% *for that subpanel* and bus, not the conductors between the 100A and 200A breakers.

If the 80A breaker is for the PV, its rating is not part of the 120% rule calculation. 125% of the inverter's max current is the PV contribution to the 120% rule in every panel between the PV and the service. Only the panel main breaker is part of the calculation, and it only counts toward the 120% rule in the panel it is the main breaker for.

 

[jaggedben]

If the 80A breaker is for the PV, its rating is not part of the 120% rule calculation. 125% of the inverter's max current is the PV contribution to the 120% rule in every panel between the PV and the service. Only the panel main breaker is part of the calculation, and it only counts toward the 120% rule in the panel it is the main breaker for.

Actually, IIRC, the OP may be operating under the 2011 code...

Just to confuse things.

 

[ggunn]

Actually, IIRC, the OP may be operating under the 2011 code...

Just to confuse things.

Thanks for that.

 

[PVfarmer]

There's a pic down below- does it somewhat represent what we are talking about?

125% of the inverter's max current is the PV contribution to the 120% rule in every panel between the PV and the service. Only the panel main breaker is part of the calculation, and it only counts toward the 120% rule in the panel it is the main breaker for.

Well yes ok, you are right, so 62.5 * 125% = 78.125A.

And with the 100A main breaker in the barn, that's 178A for the 120% rule.
A 150A rated bus (with 100A main breaker) would even be ok (150 *120% = 180A).
A 200A main barn breaker would not be ok for either a 200A *or* 225A rated bus, because you only get 225 * 120% = 270A with a 225A bus.
A 150A breaker would be ok with 200A bus (240A allowed) because 150 + 78 = 228A.
A 175A breaker? Plus 78A of PV = 253A, you'd need a 225A bus for that.

However, none of that affects the size of the conductors between the 100/150/175 or 200A breaker and the 200A main service breaker, as far as I can tell.

I still think if you have 2 100A breakers, 1 in the barn and one in the house, and they are connected to the same 200A breaker lugs, and one of them is over 25 feet, you're looking at 3/0 (or larger) wire.
Look at it this way- forget the PV, all it will actually do is reduce the current going to the barn during the day.

So there is a 100A breaker for the barn and a 100A breaker for the house. If each was fed by a separate 100A breaker, #2 would be fine.

However. there is a 200A main breaker feeding them both. If you were to tap the barn conductors into the house conductors, or the other way 'round, you would NOT use #2, because there would be a section (between 200A breaker and tap) that could potentially have to carry 200A (100 for barn, 100 for house).
When the barn and house conductors are connected to the same lugs of a 200A breaker, that section might only be a few millimeters, but it still exists.

If eds' meter had 2 100A breakers instead of 1 200A, he'd be all set.
However, a 100A fused switch at under $300 (not a $3000 stainless...) is a lot less than the difference between 200 feet of 2 AWG vs. 200' of 3/0.

Whoa! I just looked real quick- for 1000', the 3/0 is 10x as much as the 100A switch+2 AWG! (copper)

Actually, IIRC, the OP may be operating under the 2011 code...

Just to confuse things.

Hee hee. Well... can't the AHJ say "well yes the 2014/2017 makes a lot more sense for this part, so that bit of the set up is fine"...?
It's possible the OP would be under 2014 by July 1 2017? (or Dec. 31)
http://www.electricalcodecoalition.org/state-adoptions.aspx

 

[Meko]

Question
We have a stand alone system we are installing, the Solar is tied into the gear direct to the utility pole along with CT cab and meter. With this being a stand alone system are we required to implement the 120% rule since there will be no more loads added to the gear/panel.