Inverter PV Size?

[Cmy]

Looking at ways to pass the 120% rule. Need to get to 40 amps or less and the Growatt 9k-18kPV does that with 38amps AC out. Larger two are 42 and 48 amps.

Is an 18kPV inverter enough for 15-17kPV of panel's any over production? Will it just clip with planned east west split helping? Worst case is a string of 8 Adani 535's = 4.28kW or 4 strings =17.1kW. Could get by with 28 535s (15kW), but a pallet is 31 panels and 28 panels cost the same as a pallet. Who am to turn down free panels run a minisplit and cool the inverter? Could drop down to more 400+W panels but the higher wattage is tempting.

Tesla online showed 12.4kW PV needed using monthly average billing and local installer similar after reviewing year's kW usage. I came up with 14-15kW to cover possible future EV or PHEV, and warmer summers. Monthly net zero and not worried about excess as long as most of summer AC offset.

A 200a disconnect before the 200a main panel requires a utility decision on which to apply the rule. If disconnect, then I am limited to 40a unless the disconnect changing to blade/knife first is acceptable. If main panel rules, then lower main breaker or tap to a new panel. So the Growatt 9k-18kPV saves money.

 

[Ampster]

A 200a disconnect before the 200a main panel requires a utility decision on which to apply the rule.

The rule is a building code issue and in mosr cases, the utility does not care about how you load your bus bar. They do care about whether their transformer can handle your export. There are several workarounds including a line side tap.

Is an 18kPV inverter enough for 15-17kPV of panel's any over production?

DC to AC ratios of about 1.30 to 1 are common. If you have East and West panel orientations you could push that number or base it on individual MPPT inputs using PV Watts to find optimum annual production.

 

[Hedges]

40A and 120% rule sounds like 200A busbar x 120% = 240A, 240A - 200A main breaker = 40A PV breaker allowed. 80% continuous loading = 32A from inverter. 32A x 240V = 7680W max.

Your photo shows a 200A breaker with cables in and out.
What is downstream? A main-lug breaker panel? Can you put a main breaker in that panel? If so, you can do a "load side tap", install Polaris or similar in the cables coming out of the breaker in your photo, branch off to a PV combiner panel.

That is what I've done, have a 100A disconnect switch for PV.

 

[Cmy]

No city inspectors only utility. Nothing has raised a question about size until you get to megawatt production. Utility currently feeding my meter enough for three 200a main panels, should be able to send a few watts back.

Didn't know about the 120% rule until joining here. The 200a picture is in a disconnect panel next to the meter panel. 50' downstream from the disconnect is a 200a main panel, so meter>disconnect>200a main. Prior discussion questioned which or both breakers does the 120% apply. Have not found an answer yet, if utility doesn't care then I don't and will put in a 175 amp main in the 200a panel.

With the downstream panel having a 200a QOM2 bolt on main, I can tap its feed from the 200a disconnect and before the main panel. Is it load after the disconnect or after the main panel's breaker?

I could I hire an electrician, have utility pull meter, and we replace CB disconnect with a levered knife switch. Does replacing of the disconnect cb make it line until the 200a main panel? If so tap before the main panel for the inverter fed

Square D 100a disconnect arrived to day for inverter out. Your 80% continuous seems to negate the 38 amps out though, depending sun, is variable not continuous.

 

[Ampster]

No city inspectors only utility.

How about County or State? Are you saying that there is NO building code and someome can build whatever they want however they want?

 

[hwy17]

Prior discussion questioned which or both breakers does the 120% apply. Have not found an answer yet, if utility doesn't care then I don't and will put in a 175 amp main in the 200a panel.

The 120% rule does not apply to the pictured disconnect. It's a pass through breaker and cannot become overloaded in either direction.

120% rule applies to bus bars with multiple power sources and loads on them.

 

[zanydroid]

Utility currently feeding my meter enough for three 200a main panels, should be able to send a few watts back.

This may trigger utility side equivalent of feeder tap rules since backfeeding solar into one of the 3x 200A will increase the amount of current that can traverse those conductors.

If the utility doesn’t care then you don’t care.

You can try to guesstimate the situation using the feeder tap rules for customer side of the system

 

[Hedges]

If you have three 200A panels, complex enough you should sketch out a diagram for us, showing at least what boxes and what breakers located where (not branch circuit breakers unless feeding another panel, but all main breakers and pass-through breakers.)

Since you'll have both utility and PV feeding some wires, need to think of them like busbars, also any main lug only panels.

 

[glandpuck]

Of course you could change the main panel to one with a 125 amp buss bar instead of 100 amps or you could get a new panel with the design that doesn't input the PV where the buss bar for the breakers are located. This would solve your issues at some cost.

 

[Cmy]

When we did the addition for a mother in law apartment the County asked for a septic engineer's report that the current system could handle an additional bathroom. It could but, before we drain line covered, a County septic person came out, checked the drain connection for proper slope, and said OK to back fill. Next County person was from the Appraisal office with tape measure and camera, Don't know how county folk survive without a big city's building department protecting us . Electric was handled by licensed subs, 200a wire run,the coop utility did a transformer disconnect, and the meter's third line was changed from the old shop's 75a disconnect to the addition's 200a disconnect. Didn't see much inspection by the utility.

Square D's Q22200NRB product page

https://tinyurl.com/yfpfukwd

shows: "Circuitbreaker enclosure, PowerPacTQ, steel, 70A to 225A, 2pole, NEMA 3R". My label says 200A. Asked SQ D tech support if I could change to a 225A breaker and was referred to the faqs, but the short answer was no. Might a local electrician decide differently?

With hwy 17's clarification, the 120% rule applies to the connected 200a main panel. Changing to a 175a main breaker helpful? Do I use 200x1.2=240-175=65 or does it become 175x1.2=210-175=35?

Tapping before the 200a main panel breaker appears to allow a new main panel fed from the tap. Can I put a 225a main on this tap extension? Don't need that much for an inverter out but may help with the amp limits and deductions.

I have a one line somewhere, will find or redo.

Thanks for all the help.

 

[Hedges]

Do I use 200x1.2=240-175=65

Yes.

Tapping before the 200a main panel breaker appears to allow a new main panel fed from the tap. Can I put a 225a main on this tap extension? Don't need that much for an inverter out but may help with the amp limits and deductions.

What is between main panel breaker and meter?

If no other breaker, then you may need utility's permission to have 200A & 225A panels both fed from single meter and utility connection.

If you have grid --> meter --> 200A main breaker only
(which is what I have)
Then you can hang anything downstream of that without overloading the utility.

Yes, a 225A panel with 200A main can have 70A PV breaker, or larger with smaller main breaker.

If more than 3 things tapped off that node (more than 200A main breaker only, 200A main breaker panel, 200A added panel), then you have to consider whether utility plus backfeed into that node can overload wires. Just like busbars, which the 120% rule is to protect.

That's where a sketch will help us.

 

[Cmy]

Cut and paste dropped lines. Replacing 12 breaker 100a shop panel with larger, need more breakers and amps as need for inverter. 225a on order,

 

[hwy17]

Changing to a 175a main breaker helpful? Do I use 200x1.2=240-175=65

Yes that one, 65 amps of solar breaker allowed. Probably 60 amp breaker and 48 true amps of solar is the most practical.

 

[Hedges]

Two breaker panels in the path, "Shop" 100A and "Addition" 200A.

200A "Addition" with 175A main breaker (likely located in Q22200NRB) allows up to 65A backfed breaker supplying shop panel.

100A "Shop" upgraded to 225A, fed with up to 65A from "Addition" means Shop can have lots of PV. But can't backfeed more than 80% of breaker feeding it (about 48A as Hwy17 says.)

You can adjust those numbers if more PV desired. i.e. smaller than 175A breaker, or put the 225A panel where "Addition" is.

 

[zanydroid]

Cut and paste dropped lines. Replacing 12 breaker 100a shop panel with larger, need more breakers and amps as need for inverter. 225a on order,


View attachment 210905

Thanks. How many of these are actually mains? (In the sense of having a N-G bond and 3 wire on line side / 4 wire on load side). Trying to understand where you end and the POCO begins.

 

[hwy17]

200A "Addition" with 175A main breaker (likely located in Q22200NRB) allows up to 65A backfed breaker supplying shop panel.

I thought you could leave a 100A (or 200A, whatever is appropriate for the wiring) subpanel breaker on the Addition panel, and rely on the fact that there is only a 65A solar breaker on the Shop panel, to make your 65A backfeed calculation on the Addition panel.

But this is near the edge of my 120% understanding, and also into the grey area where inspectors might have their own "spirit of the law" interpretation.

Maybe you're saying the same thing though, and you don't mean it has to be a 65A subpanel breaker on the Addition panel.

 

[zanydroid]

Yes that one, 65 amps of solar breaker allowed. Probably 60 amp breaker and 48 true amps of solar is the most practical.

I believe the text of most sections in 705.12 is actually on output current. So 52A solar output nameplate would pass plan review with even a 70A backfeed breaker (which you may need to round up to if 65A is not available for the panel brand). This does potentially get into "math too hard" territory.

With the exception of sum of breakers (load + backfeed breakers but not main breakers), pretty sure that is breaker rating and not inverter output nameplate.

 

[hwy17]

I believe the text of most sections in 705.12 is actually on output current. So 52A solar output nameplate would pass plan review. This does potentially get into "math too hard" territory.

The text of this is certainly into the "breaking my brain" territory lol, but I read this as an either/or, either 1.25x nameplate OR breaker size sum cannot exceed 120. It's the "and" in the sentence that really gets me though.

Edit: I see it now, you're right it's based on nameplate but it's still nameplate x 1.25 which ends up being your breaker size anyway.

705.12 (B) (2) Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the busbar ampere rating.

 

[hwy17]

Oh and @zanydroid 's clarification provides the textual basis for my above claim that you still get to use a 100A/200A subpanel breaker on the Addition panel going to the shop, because you get to calculate your 120% there based on nameplate x 1.25 not breaker sum.

 

[zanydroid]

Oh and @zanydroid 's clarification provides the textual basis for my above claim that you still get to use a 100A subpanel breaker on the Addition panel going to the shop, because you get to calculate your 120% there based on nameplate x 1.25 not breaker sum.

Yes. I was about to post a message regarding this.

(There's plenty of confusion on this in otherwise pretty reputable blog posts. That's why I think this is tickling the math dragon a bit. But you have no choice if you are backfeeding into an existing subpanel. For instance I backfeed my solar into a subpanel that is fed with 100A from my main)