70amp solar on a 200 amp main

[ggunn]

I had no idea the inverters were the limiting factor. I never looked much at solar installations. Probably because all of the solar installers I've met around here are not doing your industry's reputation any favors. Little to no knowledge of electrical theory, and giving customers the impression that one kilowatt of panel is going to be producing one kilowatt from dawn until dusk.

That certainly sucks, but as one instructor in PV I know is fond of saying, it's the wild west out there for PV. For sure there are charlatans and ignoramuses "working" in the industry and taking advantage of people, but I have had success in aligning my efforts with colleagues who take pride in their work and hold themselves accountable.

 

[kingpb]

Does this 120% rule, only apply to residential panels? What about switchboards and switchgear?
What is the actual code reference?

 

[jaggedben]

Does this 120% rule, only apply to residential panels? What about switchboards and switchgear?
What is the actual code reference?

It applies to everything now. The busbar rules were updated from just covering panelboards at some point, I think in 2017. They now cover everything. With one exception (centerfed panel in a dwelling) all the rules apply to all occupancies.

705.12

 

[SKSolar]

Assuming the busbar is rated 200A (not 225A)...

(200×1.2)-175=65.
The rule states that 125% of inverter output cannot exceed that number, i.e. 65 in this case.

So if your inverter output is 52A or less (52×1.25=65) you can put it on a 70A solar breaker and it's okay with the 175A main.
If your inverter output is between 52 and 56A, the 70A solar breaker is okay but you need to downsize the main to 150A, or change something else (like, downsize the system.)

what I have learnt is: 200x1.2 = 240A Max. 240A -70A = 170A. So your main breakers on a 200A Bus shouldn't be more than 170A.

 

[jaggedben]

what I have learnt is: 200x1.2 = 240A Max. 240A -70A = 170A. So your main breakers on a 200A Bus shouldn't be more than 170A.

But the rule actually goes by 125% of inverter output, not breaker size. If you have a solar system that could go on a 65A breaker, then 240-65=175. Since they don't make 65A breakers, you must install a 70A breaker. But you still get to use 65 in the calculation and your main breaker can be 175A, which happens to be a standard available size. If your solar breaker is actually required to be greater that 65A, this doesn't work.

Devil in the details.

 

[kingpb]

Is there really a reason they have to make this so difficult? lol. Can we just go back to the 80's. Smoked a lot a weed and didn't have to think so much.

 

[Bdwalster]

Very confused on installing solar. If I have a 200 amp main is there a way to technically put a 70amp solar system to feed this system?
I have been told putting a 175amp main will allow me to install a 70amp system nit sure how that actually changes anything.

You'll probably need a panel swap to a solar friendly panel with a 225 bus.

 

[solarken]

The BUS is in RED.
Where is the amperage on the bus over 200 amps? Where is this I2R that exceeds ratings at??

View attachment 2565923

Replace the 240V 270A load in your picture with 40 x 15A loads distributed along the bus length, and you can find many places that could exceed 200A.

 

[Joethemechanic]

Replace the 240V 270A load in your picture with 40 x 15A loads distributed along the bus length, and you can find many places that could exceed 200A.

Not seeing it. And I've worked on many industrial buses fed from both ends. Including ones where the source was multiple generators.

 

[jaggedben]

Replace the 240V 270A load in your picture with 40 x 15A loads distributed along the bus length, and you can find many places that could exceed 200A.

Yeah, this is not correct.

 

[scrubbin]

Not to beat a dead horse but if I would pull out a 100 amp panel and replace it with a 200 amp panel with 200 a buss. I change the main breaker to 100A so no need to upgrade the wires feeding the panel. Now using the 200 x 1.2 =240A -100A max solar inverter output(not that it exists) so use a 125A breaker for solar. So 240 - 125 = 115A so the 100 amp main breaker is fine?

 

[jaggedben]

Not to beat a dead horse but if I would pull out a 100 amp panel and replace it with a 200 amp panel with 200 a buss. I change the main breaker to 100A so no need to upgrade the wires feeding the panel. Now using the 200 x 1.2 =240A -100A max solar inverter output(not that it exists) so use a 125A breaker for solar. So 240 - 125 = 115A so the 100 amp main breaker is fine?

This doesn't actually work because if there is no load and the full solar output went to the grid it could trip the 100A main breaker. But if you don't make the solar breaker larger than 100A then the rest of the logic is fine.

As far as applying code to that, I've always maintained that the supply side rules (705.12(A) earlier, 705.11 recently) apply to any load side connection in series. That's reading between the lines a bit but it makes sense.

 

[OldBroadcastTech]

It's about heating from the busbar connections. With a 200% rule, you potentially have double that heating. With a 120% rule, the worst case is only 104% of the previous worst case, given the square in I²*R.

Cheers, Wayne

"With a 120% rule, the worst case is only 104% of the previous worst case, given the square in I2*R."

If 120 % gives you 104 % of worst case, wouldn't 200 % give you 141.4 % of worst case ?

 

[wwhitney]

"With a 120% rule, the worst case is only 104% of the previous worst case, given the square in I2*R."

If 120 % gives you 104 % of worst case, wouldn't 200 % give you 141.4 % of worst case ?

(100% * x)² + (20% * x)² = 104% * x².

(100% * x)² + (100% * x)² = 200% * x²

Cheers, Wayne

 

[Carultch]

(100% * x)² + (20% * x)² = 104% * x².

(100% * x)² + (100% * x)² = 200% * x²

Cheers, Wayne

What is the rationale behind using (100%*x)^2 + (20%*x)^2, rather than (100%*x + 120%*x)^2?

The current also is not additive on the busbar, when fed from the opposite direction. It's the heat among the breakers that led to the industry compromise to allow a 20% partial credit for Kirchhoff's current law, rather than a 200% rule as KCL alone would imply.

 

[wwhitney]

What is the rationale behind using (100%*x)^2 + (20%*x)^2, rather than (100%*x + 120%*x)^2?

With the two sources at opposite ends of the bus, even with a 200% rule, the current at any cross-section of the busbar never exceeds the busbar rating. So the heating from current in the busbar is never more than the worst case of the busbar carrying its full rated current along its entire length, e.g. via a single source at one end and a single load at the other end.

The additional heating from a 120% rule or a 200% rule comes from the additional connections to the busbar, and the resistance of those connections (and if you like the heating from within the breakers at those connections). Take the simple model that every connection to the busbar has the same fixed resistance R (and if you like R can include the resistive heating of the breakers). Let's do the accounting only for the sources, since the worst case for sources and loads will be the same, so the overall worst case connection heating is just double the worst case source connection heating.

With a 120% rule the worst case for connection heating is one connection at 100% of the busbar rating, and one connection at 20% of the busbar rating. While the worst case for a 200% rule would be two connections, each at 100% of the busbar rating.

Hence the comparison I made, where x is the busbar rating, and we are looking at I²R heating, omitting the factor of R.

Cheers, Wayne