120 percent busbar NEC rule

[hiltxan]

I had a question regarding the 120% rule and if I have powerwalls does that make a difference? It seems that the gateway system is feeding 125A back into the main service panel, which is also receiving 200A from the main breaker. I've read that you'll need to downsize the main breaker otherwise the busbars can receive 200A+125A = 325A? Is that correct? Or is the backup gateway advanced enough to limit the amperage going to the main service panel?

Also, in the future when I need to add an ADU/extra circuits, can I add a 100A breaker to the main panel? Or does this also exceed the 120% rule? (100A for ADU subpanel + 125A for backup gateway connection = 225A.) I think my busbar is only 200A since I can't find any information on the panel indicating otherwise.

Unless I'm understanding it incorrectly. If I have two inverters and two powerwalls, the most kw I can supply back to the main panel is about 16kw which is 66amps. I can add a 100A breaker to the main panel without ever going over 120%.

Thanks!

 

[hiltxan]

Actually I think I’m misinterpreting it. I’m relocating all the circuits from the main to a separate backup panel.

I should be able to add a 100A breaker to the main panel. It shouldn’t exceed the 120 percent rule since I can’t draw more than 200A from the main panel (even with the 125A to the gateways) without tripping the 200A main breaker.

 

[ScoobyDoo82]

Actually I think I’m misinterpreting it. I’m relocating all the circuits from the main to a separate backup panel.

I should be able to add a 100A breaker to the main panel. It shouldn’t exceed the 120 percent rule since I can’t draw more than 200A from the main panel (even with the 125A to the gateways) without tripping the 200A main breaker.

I was going to chime in, but I see that you found your error and are all good

 

[wwhitney]

In the diagram shown, if the meter/main on the left has a 200A busbar, then adding a 100A breaker to it will be a violation of the NEC 705.12(B) busbar rules (of which the 120% rule is one option). A 70A breaker would comply.

However, it shouldn't be a violation, it's just a limitation due to the rule writers trying to keep things simple rather than cover all the possibilities. With just 3 breakers in the panel (main plus 2 branch), each of which is not more than the busbar rating, there's no way to overload the busbar.

If the backup Gateway is set up to monitor the conductors at (5) in the diagram, and never export more than 32A, and the 125A breaker supplying the Gateway is at one end of the bus in the meter/main, then that complies and further breakers in the meter/main are not limited by 705.12(B).

Cheers, Wayne

 

[hiltxan]

Thanks for the clarifications. Hopefully the inspectors are able to figure it out and approve it rather than take the simplified interpretation and actually think about it so there will be no issue adding the 100A breaker to the main panel.

As far as connection 4 - does that have to be 125A or can it be 200A? I’m guessing it can be only 125A because the main panel breaker (connection 5) is 125A? It seems that my backup panel can only support 125A at once then…instead of 200A originally but Tesla said it was enough.

 

[wwhitney]

As far as connection 4 - does that have to be 125A or can it be 200A? I’m guessing it can be only 125A because the main panel breaker (connection 5) is 125A? It seems that my backup panel can only support 125A at once then…instead of 200A originally but Tesla said it was enough.

What's the make and model number of your meter/main? It presumably can't accept a branch breaker larger than 125A, so that's why a 125A breaker is used there. [Or else Tesla is trying to keep everything cheap and simple and consistent, so they just use 125A without checking whether yours can take a larger one.]

(5) could still be 200A rated, assuming the necessary conductors (2/0 Cu as long as your residential service stays 200A; 3/0 Cu if you upsized it) can fit in the 125A breaker terminals (would need checking). (4) and the new 225A distribution panel breaker could be 200A; this would give you the ability to use 125A from the grid while simultaneously using 75A from the two PW+s.

But Tesla doesn't want to upsize for future flexibility, they just want to get the job done quickly and cheaply. Even if you offered to pay more, I doubt they'd upsize things for you, to avoid having to deal with customization. But maybe they would.

Also, your permit submission should include a load calc from Tesla demonstrating that 125A is sufficient for your dwelling unit load. If that's true then in practice your dwelling load is rarely going to be more than 60A, so in practice 125A is fine. Except for the case that you want to install, say, (3) 40A continuous EVSEs.

Cheers, Wayne

 

[wwhitney]

Hopefully the inspectors are able to figure it out and approve it rather than take the simplified interpretation and actually think about it so there will be no issue adding the 100A breaker to the main panel.

It's not an interpretation issue--the NEC clearly disallows adding a 100A breaker to the meter main in that diagram. It's just that the rule in the NEC is overly conservative, and if it were written more narrowly to achieve the underlying goal would allow it. So you'd have to convince the AHJ to grant permission beyond what the NEC allows. It might be easier to upgrade your meter main.

Cheers, Wayne

 

[hiltxan]

Meter/main picture is below. I don't think I can fit any breaker greater than 125A on there since I looked up and there wasn't any breaker greater than 125A available. That's probably the reason why 125A is the maximum they're willing to put on (5) and (4). Tesla should be fine and I don't think I'll ever use more than 125A at once since I can control when to charge the EV.

I'm adding the 100A to the main panel and PV system within the next 1-2 months. I'm just worried that when inspection comes it may cause confusion and delay approvals or PTO since there are two projects going on simultaneously. It would be very expensive for me to upgrade the main panel: I have underground utilities and the main panel is inside a cabinet right above the gas riser, which Tesla has said they can tap into the panel from the back from the garage side to avoid any conduits and gas riser clearance issues.

 

[CrazyRabbit]

As long as the sum of the breaker in the main panel don’t exceed 200 amp, shouldn’t it be allowed? As long a the panel has a 200 breaker listing.

 

[wwhitney]

As long as the sum of the breaker in the main panel don’t exceed 200 amp, shouldn’t it be allowed? As long a the panel has a 200 breaker listing.

Sure, but 125 + 100 > 200. And that's why I said downsizing the new breaker to 70A from 100A would be one solution.

On reflection, though, this part of my first response would suffice and is probably the best solution:

"If the backup Gateway is set up to monitor the conductors at (5) in the diagram, and never export more than 32A, and the 125A breaker supplying the Gateway is at one end of the bus in the meter/main, then that complies and further breakers in the meter/main are not limited by 705.12(B)."

So the OP would want to get Tesla to install it that way.

Cheers, Wayne

 

[CrazyRabbit]

Sure, but 125 + 100 > 200. And that's why I said downsizing the new breaker to 70A from 100A would be one solution.

On reflection, though, this part of my first response would suffice and is probably the best solution:

"If the backup Gateway is set up to monitor the conductors at (5) in the diagram, and never export more than 32A, and the 125A breaker supplying the Gateway is at one end of the bus in the meter/main, then that complies and further breakers in the meter/main are not limited by 705.12(B)."

So the OP would want to get Tesla to install it that way.

Cheers, Wayne

I didn’t read second paragraph. Limiting an accessory building to 70 amps should not be a problem. If he needs more then upgrade service.

 

[buckets0fun]

If he needs more then upgrade service.

Not really an option unless he wants two gateways and help from @Vines company to make it all work seamlessly on 400A, or the accessory building to not be backed up.

 

[hiltxan]

ADU doesn't need to be backed up. We don't have power outages that often and I can live with ADU without backup power. Plus Tesla is only giving me a 125A breaker for loads to be relocated so I can't even add the 100A ADU to the backup loads.

It seems like the 120% rule is just referring to supply. Doesn't matter about demand or total amperage of circuits so ADU breaker doesn't matter.
“When 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 ampacity of the busbar.”

So if Tesla isn't planning on downgrading my main service panel breaker, then I'm assuming that the gateway is programmed to only be 32A supply back to the main panel.

32A (gateway) * 1.25 = 40A
40A + 200A (main breaker) = 240A = 120% of 200A.

I have a 14kw system with 2 inverters, so the maximum that can be supplied back is 14kw which is 58A? Does the gateway limit the amperage going back to the main panel?

 

[wwhitney]

So if Tesla isn't planning on downgrading my main service panel breaker, then I'm assuming that the gateway is programmed to only be 32A supply back to the main panel.

The 120% rule isn't the only option. You are correct that to use the 120% rule and keep your meter main's main breaker at 200A, you'd need to have the Gateway programmed to 32A maximum export. And if you do that, then you aren't otherwise limited as to what breakers are in the meter main.

One of the other options is that the sum of the breakers in the meter main, excluding the main breaker, is no more than the busbar rating. For that rule, you could do 125A for the Gateway, and 70A for the ADU, but you couldn't do 125A + 100A. Since your diagram doesn't show any ADU breaker, Tesla is probably not planning to program a limit on the Gateway export, since with only a 125A breaker in it, this option is met.

Your 14 kW system, is that DC (sum of panel ratings) or AC (sum of inverter ratings)? Only the AC rating matters for these questions. But if you go for the 120% rule with a 200A main breaker, you'd be limited to 32A * 240V = 7.68 kW export. So if your PV system can generate more than that, and your PW+ batteries were full, the Gateway would have to curtail the PV output.

A 3rd option is to change the meter main's main breaker to 175A (if that part is available) and use the 120% rule. Then you'd an extra 20A of allowable export, and the Gateway can be programmed to allow export up to 52A.

Cheers, Wayne

 

[hiltxan]

14kw panels with probably 2x7.8kw inverters. 22 panels facing SW, 4 panels facing NW and then 11 panels facing NE so most likely peak generation won't be that high. Even with 22 panels facing SW, that's 8.8kw and hopefully Tesla system is advanced enough to program when to charge the battery vs when to send back to grid. I kind of want to limit the 32A since I don't want to change the main breaker (out of caution in case PGE makes me relocate the main panel since it's next to gas riser). Tesla is probably also going to limit it to 32A from the gateway to fulfill the 120% busbar rule.

I'm not understanding the sum of breakers in main panel cannot exclude main breaker. Even before Tesla relocates the circuit breakers, I had 30 some circuits which total well over 400A. Isn't that what a load calculation is for to make sure the 200A is suffice? We did a load calculation for the ADU and main house and it was under 200A.

 

[wwhitney]

Tesla is probably also going to limit it to 32A from the gateway to fulfill the 120% busbar rule.

You can't assume that because of the following:

The "120% rule" is really the "120% option". The rule, for panelboard supplied by multiple sources, provides two other options, one of which is that "the sum of the breakers (other than the main breaker) is no more than the busbar rating." [The other option doesn't apply in your situation.]

So in your diagram from Tesla, since that option is satisfied, the rule is satisfied, and there's no need to consider the "120% option" for your meter/main in the diagram. As such, there's no reason for Tesla to program the GW to limit the exports to 32A (which would be a non-trivial limit given the size of your PV system).

But since you want to add the 100A breaker after Tesla is done, that new configuration no longer satisfies the "sum of the breakers" option. So to comply with the rule, you'll either need to use a 70A breaker instead of a 100A (for the sum of breakers option), or you'll need to get Tesla to program the GW to limit exports to 32A (for the 120% option).

And if your preference between those two is the latter, it would easiest to get Tesla to do that while they are there the first time, rather than calling them back to do it.

Cheers, Wayne

 

[hiltxan]

Thanks for the explanation. I think I get it now that there are 3 options.

It seems from the warning labels that Tesla went with option (b) from the busbar code with the plans they are submitting to the city since the warning label for option B is listed for POI point of interconnection location. I think they already limited it to 32A output to the MSP, which probably is close to what I can max output anyways - 23 panels face SW, 7 face NE and 4 face NW.

I should be ok adding in a 100A breaker later? Do you think I need to confirm with Tesla that option B is the way they're going or just assume based off of the labels? I'm a little concerned that they also put the warning label for option (c) but didn't list a label location, wondering if it's just a generic placeholder from a template.

705.12(B)(2)(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

(a) 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 the ampacity of the busbar.

Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.

(b) 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 ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the power source that displays the following or equivalent wording:

WARNING: POWER SOURCE OUTPUT CONNECTION — DO NOT RELOCATE THIS OVERCURRENT DEVICE.

The warning sign(s) or label(s) shall comply with 110.21(B).

(c) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment displaying the following or equivalent wording:

WARNING: THIS EQUIPMENT FED BY MULTIPLE SOURCES. TOTAL RATING OF ALL OVERCURRENT DEVICES EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE SHALL NOT EXCEED AMPACITY OF BUSBAR.

The warning sign(s) or label(s) shall comply with 110.21(B).