120% RULE

[mnlara]

i have a predicament in designing. I have have a residential house with a 400 amp main bus bar rating and a 400 Amp Main Service panel. Per the 120% rule = 400A x 120% - 400A = 80A allowed total PV System breaker.

The existing setup is that 400A main service is broken into (2) subpanel - 200A for the Main house and 200A for the Garage (subpanel). is it 400A x 120% - 200A = 280A? I'm installing a separate PV system for the Main House and the Garage (both are single detached structure). How does the 120% rule apply to this setup?

 

[ggunn]

i have a predicament in designing. I have have a residential house with a 400 amp main bus bar rating and a 400 Amp Main Service panel. Per the 120% rule = 400A x 120% - 400A = 80A allowed total PV System breaker.

The existing setup is that 400A main service is broken into (2) subpanel - 200A for the Main house and 200A for the Garage (subpanel). is it 400A x 120% - 200A = 280A? I'm installing a separate PV system for the Main House and the Garage (both are single detached structure). How does the 120% rule apply to this setup?

The rating of the first breaker that the inverter is connected to is the number to be used in the 120% rule(s) all the way back to the service. It sounds like you will be having a couple of inverters. You can place up to 40A of backfed breaker in each subpanel, and if you do that they contribute 80A to the main panel bus irrespective of the ratings of the breakers feeding the subs. You cannot put an 80A backfed breaker into a 200A subpanel; the 120% rule is calculated separately for all the panels which are backfed.

 

[mnlara]

The rating of the first breaker that the inverter is connected to is the number to be used in the 120% rule(s) all the way back to the service. It sounds like you will be having a couple of inverters. You can place up to 40A of backfed breaker in each subpanel, and if you do that they contribute 80A to the main panel bus. You cannot put an 80A backfed breaker into a 200A subpanel; the 120% rule is calculated separately for all the panels which are backfed.

On the Garage, I'm installing a 10 kw PV System - SB10000Tl-US (240v) SMA America inverter with a 55A breaker on the subpanel. Then on the Main house I'm installing a 7 KW PV Sysytem - SB7000TL-US (240v) SMA America Inverter with a 40A breaker on the Main House subpanel.

Do you mean that on the garage I cannot install no more than 40A for PV system? That system needs a 55A breaker and If I can't do that, can I change the 200A Garage subpanel to 175A will that work?

 

[SparkyHC]

On the Garage, I'm installing a 10 kw PV System - SB10000Tl-US (240v) SMA America inverter with a 55A breaker on the subpanel. Then on the Main house I'm installing a 7 KW PV Sysytem - SB7000TL-US (240v) SMA America Inverter with a 40A breaker on the Main House subpanel.

Do you mean that on the garage I cannot install no more than 40A for PV system? That system needs a 55A breaker and If I can't do that, can I change the 200A Garage subpanel to 175A will that work?

Read NEC 705.12 (B)(2)...you are limited by the rating of the bus bars in the panel and the conductors connected to it. Therefore to connect a 55A breaker from the solar to the 200 ampere garage service you will need to have a panel with a bus rating of 200 + 55 / 1.2 = 212.5 amperes. You could replace the panel with a 225 ampere bus panel that complies but you also might have to increase the size of the conductor supplying this panel as well.
What size conductors supply the existing garage panel?

 

[ggunn]

On the Garage, I'm installing a 10 kw PV System - SB10000Tl-US (240v) SMA America inverter with a 55A breaker on the subpanel. Then on the Main house I'm installing a 7 KW PV Sysytem - SB7000TL-US (240v) SMA America Inverter with a 40A breaker on the Main House subpanel.

Do you mean that on the garage I cannot install no more than 40A for PV system? That system needs a 55A breaker and If I can't do that, can I change the 200A Garage subpanel to 175A will that work?

Yes, that is what I mean, and no, that would make it worse; a 175A subpanel can only accept a 35A backfed breaker.

If you changed the main breaker on the sub to 175A (leaving the busbars at 200A), then 1.2 X 200A = 240A, 240A -175A = 65A; that would work as far as the subpanel itself is concerned. The conductors between the main and the sub, however, are also subject to the 120% rule and have to be rated for (200 + 55)/1.2 = 212.5A minimum (assuming a 200A breaker in the main feeding the sub) after derating for conditions of use. You could, of course, then change the breaker in the main feeding the sub to a 175A breaker (no reason not to) and bring that requirement down a bit: (175 + 55)/1.2 = 191.7A.

 

[mnlara]

Yes, that is what I mean, and no, that would make it worse; a 175A subpanel can only accept a 35A backfed breaker.

If you changed the main breaker on the sub to 175A (leaving the busbars at 200A), then 1.2 X 200A = 240A, 240A -175A = 65A; that would work as far as the subpanel itself is concerned. The conductors between the main and the sub, however, are also subject to the 120% rule and have to be rated for (200 + 55)/1.2 = 212.5A minimum (assuming a 200A breaker in the main feeding the sub) after derating for conditions of use. You could, of course, then change the breaker in the main feeding the sub to a 175A breaker (no reason not to) and bring that requirement down a bit: (175 + 55)/1.2 = 191.7A.

As per site visit the Bus Bar rating on the main service panel is 425A and a 400A main service breaker then it's split into to 2 subpanels of 200A each. If i change one of the Subpanel 200a breaker to 175A, It can accommodate the 55A PV System breaker.? if the bus bar at the main service 425A x 120% rule - 400A Main breaker size = 110A, then it can accomoodate both system with 55A and 40A installed on the subpanels?

 

[ggunn]

As per site visit the Bus Bar rating on the main service panel is 425A and a 400A main service breaker then it's split into to 2 subpanels of 200A each. If i change one of the Subpanel 200a breaker to 175A, It can accommodate the 55A PV System breaker.?

Yes.

You are feeding the main with 95A of backfed breakers, which would be too much for a 400A main with 400A busbars, but your main busbars are 425A. 1.2 X 425A = 510A and 510A - 400A = 110A, so you are OK.

 

[Sierrasparky]

First of all you can back feed each of the 200 a panels with 40amps and no more than 80 amps at the main.
Second , if you derate the sub feed then you would be subject to upsize of the ground wire as a technicality.

 

[Zee]

You got a lot of excellent advice.

Another approach to consider:
The sma 8000 only needs a 40A breaker. (current limited at 32A) (32 A @ 125% = 40 A)
This is the biggest inverter you can get on a 40 A brkr. In many climates you can safely design pv watts at 120% of inverter watts.
That could get you 9.6 kw of pv power on the 8000 inverter.

 

[mnlara]

You got a lot of excellent advice.

Another approach to consider:
The sma 8000 only needs a 40A breaker. (current limited at 32A) (32 A @ 125% = 40 A)
This is the biggest inverter you can get on a 40 A brkr. In many climates you can safely design pv watts at 120% of inverter watts.
That could get you 9.6 kw of pv power on the 8000 inverter.

Thanks guys for the heads up. I'm sending out our site evaluator to double check if it's indeed a 425A versus a 400A Bus Bar Rating at the main - and the main breaker is 400A. If it turn out that its only 400A Bus Bar Rating, Therefore 400A Bus Bar X 120% = 480A - 400A Main breaker = 80A or less for PV. As I said before the main is broken down into 2 200A subpanels. One 200A for the garage and one 200A for the main house. I'm designing to install two PV System. The Garage a 10kw system with SB10000TL-US (240v) 55A breaker and the Main house a 7.8 kw system with SB7000TL-US (240v) Max. output Current at 29.2A x 1.25=36.5A next breaker size is 40A. I couldn't use the SB8000TL-US (240v) Max. output Current at 33.4A x 1.25=41.75A next breaker size is 45A.

As for the garage, I was thinking of downsizing the subpanel breaker from 200A to 175A, thus giving me room to install 55A breaker for the PV. 400A bus bar x 120% rule = 480A minus (175A garage + 200A main house= 375A) = 105A. The main house is just fine since i'm installing a 40A PV breaker that leaves me 65A or less for PV at the garage therefore is 55A is perfect. Em I right?

 

[GoldDigger]

Thanks guys for the heads up. I'm sending out our site evaluator to double check if it's indeed a 425A versus a 400A Bus Bar Rating at the main - and the main breaker is 400A. If it turn out that its only 400A Bus Bar Rating, Therefore 400A Bus Bar X 120% = 480A - 400A Main breaker = 80A or less for PV. As I said before the main is broken down into 2 200A subpanels. One 200A for the garage and one 200A for the main house. I'm designing to install two PV System. The Garage a 10kw system with SB10000TL-US (240v) 55A breaker and the Main house a 7.8 kw system with SB7000TL-US (240v) Max. output Current at 29.2A x 1.25=36.5A next breaker size is 40A. I couldn't use the SB8000TL-US (240v) Max. output Current at 33.4A x 1.25=41.75A next breaker size is 45A.

As for the garage, I was thinking of downsizing the subpanel breaker from 200A to 175A, thus giving me room to install 55A breaker for the PV. 400A bus bar x 120% rule = 480A minus (175A garage + 200A main house= 375A) = 105A. The main house is just fine since i'm installing a 40A PV breaker that leaves me 65A or less for PV at the garage therefore is 55A is perfect. Em I right?

If the main panel has a 400A bus, a 400A main breaker, and two 200A feeder breakers, one going to each sub-panel, you will still be limited to a backfeed of 80A total even if it is divided over the two 200A subpanels.
Just changing the subpanel main to allow for a larger backfeed at that point does not exempt you from the 120% rule at the main panel. Now you could make the argument that no loads are fed from the main panel directly and therefore you could not have simultaneous backfeed from both panels and a high bus loading in the main panel. And that would be a valid argument based on technical principles. But that does not mean that logic overrides the actual language of the Code.

Or, to paraphrase that:
1. You could not connect a separate breaker or breakers for backfeed of more than 80A in the main panel.
2. Nor could you connect more than 80A total backfeed to the two subpanels since they would each backfeed through the 200A breakers into the main panel.

 

[ggunn]

As for the garage, I was thinking of downsizing the subpanel breaker from 200A to 175A, thus giving me room to install 55A breaker for the PV. 400A bus bar x 120% rule = 480A minus (175A garage + 200A main house= 375A) = 105A. The main house is just fine since i'm installing a 40A PV breaker that leaves me 65A or less for PV at the garage therefore is 55A is perfect. Em I right?

Unfortunately, no. The calculation for the 120% rule in the main is based on the first breakers attached to the inverters. If one inverter breaker is 40A and the other is 55A, then you must use 95A in the calculation of the 120% rule in the main. The ratings of the breaker in the main feeding the sub and the main breaker in the sub are irrelevant to that calculation. If your busbars in the main are 400A, the total of the inverter breakers must be 80A or less. The other breakers between the inverters and the main bus don't figure into the calculation at all.

EDIT: What GoldDigger said.

 

[ggunn]

You got a lot of excellent advice.

Another approach to consider:
The sma 8000 only needs a 40A breaker. (current limited at 32A) (32 A @ 125% = 40 A)
This is the biggest inverter you can get on a 40 A brkr. In many climates you can safely design pv watts at 120% of inverter watts.
That could get you 9.6 kw of pv power on the 8000 inverter.

True, as long as you realize that the 8000W inverter will still only put out a maximum of 8000W. Overloading inverters is common practice; I have overloaded them by 35% or more in some situations (high latitude and/or low tilt angle, for example). Some power clipping mid day is acceptable when the lost power is more than made up for on the shoulders of the power curve.

 

[Zee]

I couldn't use the SB8000TL-US (240v) Max. output Current at 33.4A x 1.25=41.75A next breaker size is 45A.

Exactly my point: you CAN use the 8000US (not the TL)
It is deliberately rated at 32 A (even tho it should be 33.4).
Find a way, spacewise on the roof, to add a couple panels to the garage system ......and use a 8000 there too and you are set.
And, yes as GGUNN said, you will lose no power. <1%, probably way less.

 

[ggunn]

Exactly my point: you CAN use the 8000US (not the TL)
It is deliberately rated at 32 A (even tho it should be 33.4).
Find a way, spacewise on the roof, to add a couple panels to the garage system ......and use a 8000 there too and you are set.
And, yes as GGUNN said, you will lose no power. <1%, probably way less.

Well, that's not exactly what I said. Most systems are designed with more STC wattage in modules than in inverter capacity in part because STC seldom exists in the real world. How much overloading would be best is a judgement call. What is appropriate for an array on a 20 degree tilted roof facing due south in the mountains at 20 degree latitude will be different from that for a 10 degree tilted array on a flat roof facing east somewhere in Canada.

My point was that no matter how much you overload an 8000W inverter, it can never put out more than 8000W. Also, even if an array delivers more than 8000W in cool clear conditions and that excess power is lost, it will deliver more power at less ideal conditions as well and will max out the inverter more of the time (the "shoulders" of the power curve). The total energy delivered over the course of the day is the thing to be balanced against the cost of extra modules. You cannot get this information from PVWatts but more sophisticated software like PVsyst can help. That is expensive software, though, and probably not worth getting if you are not designing a lot of systems. Most folks just take a guess. The good news is that modules are cheap and you cannot damage your inverter by overestimating the amount of PV you need.

 

[ggunn]

...you cannot damage your inverter by overestimating the amount of PV you need.

I hasten to add that that assumes one has paid attention to the highest possible Voc of an array and does not make strings so long that at the coldest possible temperature Voc can exceed the maximum voltage of the inverter. Exceeding that voltage WILL damage the inverter.

 

[Smart $]

Unfortunately, no. The calculation for the 120% rule in the main is based on the first breakers attached to the inverters. If one inverter breaker is 40A and the other is 55A, then you must use 95A in the calculation of the 120% rule in the main. The ratings of the breaker in the main feeding the sub and the main breaker in the sub are irrelevant to that calculation. If your busbars in the main are 400A, the total of the inverter breakers must be 80A or less. The other breakers between the inverters and the main bus don't figure into the calculation at all.

EDIT: What GoldDigger said.

That's the generally accepted "theory". However, it is debatable in this case, where the 400A main distribution panel only feeds the two 200A supanels.


Before jumping directly to 705.12(D)(2), read (with a clear mind) 705.12(D) general statement. It states, "Where distribution equipment including switchboards and panelboards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7)." I've underlined the pertinent sections that make this debatable.


The reasoning being is that the 400A main and both 200A feeds are not capable of simultaneously supplying multiple branch circuits or feeders. In essence, all three cannot be a source of power simultaneously to the mains busbar in question. At most, only two of the three sources can supply power simultaneously as the third "connection" has to be the load (KCL).


As such, if the 400A service panel has 400A-rated busbars, either feeder can be up to an 80A source (400A x 120% - 400A = 80A).

 

[GoldDigger]

Before jumping directly to 705.12(D)(2), read (with a clear mind) 705.12(D) general statement. It states, "Where distribution equipment including switchboards and panelboards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7)." I've underlined the pertinent sections that make this debatable.


The reasoning being is that the 400A main and both 200A feeds are not capable of simultaneously supplying multiple branch circuits or feeders. In essence, all three cannot be a source of power simultaneously to the mains busbar in question. At most, only two of the three sources can supply power simultaneously as the third "connection" has to be the load (KCL).

Good reasoning, and why I mentioned that there is no practical as opposed to regulatory wording reason for applying the 120% rule for this panel.
The only weakness in your argument is that it seems to apply only if the panel does not have any slots in which additional breakers could be installed.
If there is room for even one more breaker, the panel "...is capable of supplying multiple branch circuits or feeders or both...." regardless of whether those feeders or branches are currently in existence. The "is capable of" part seems clearly intended to take it beyond the counting of existing loads only.
An inspector could even make the argument that the two 200A feeder breakers alone satisfy the wording to make the 120% rule applicable even though they are each also the backfed breakers.

 

[Smart $]

Good reasoning, and why I mentioned that there is no practical as opposed to regulatory wording reason for applying the 120% rule for this panel.
The only weakness in your argument is that it seems to apply only if the panel does not have any slots in which additional breakers could be installed.
If there is room for even one more breaker, the panel "...is capable of supplying multiple branch circuits or feeders or both...." regardless of whether those feeders or branches are currently in existence. The "is capable of" part seems clearly intended to take it beyond the counting of existing loads only.
An inspector could even make the argument that the two 200A feeder breakers alone satisfy the wording to make the 120% rule applicable even though they are each also the backfed breakers.

As I said, debatable... as in not without question (or weakness). The "is capable" can also be interpreted as meaning how currently configured if the interpreter so chooses. Almost all electrical distribution equipment fits into this "is capable" description, whether utilized as such or not at present. So why does the section even state it to begin with, if it does not mean as currently configured?