120% Rule

[wwhitney]

unlike (B).

meant to say unlike (D).

Cheers, Wayne

 

[jbeecroft]

Your reasoning through Kirchhoff's current law (KCL) is correct, but that isn't all there is to what governs the allowable utilization of the panelboard. For a busbar in general, perhaps it is acceptable to reason through KCL alone to justify that you won't overload the busbar, but not for a busbar that is part of a manufactured product like a panelboard or a switchboard. The additional factor that comes in to play is the mutual heating among the breakers. It is an industry compromise to set the limit at 120%, when fed from opposite ends, even though KCL would seemingly justify a 200% rule.

I agree that group heating would obviate a 200% usage maybe. This issue is encountered with multi-pole connectors where the rating of a single mated pair of contacts is greater than that of the same pair in a group of contacts. e.g. Molex connectors de-rate a group of mated connectors about 20% from the rating of a single contact. A group of breakers in a panel will get warmer but the worst case is 2 100% load breakers placed next to each other. Since PV feed breakers are placed at the opposite end of the busbar anyway it would allow for increasing the 120% rule. Maybe not to 200% but 150% would not be unreasonable or unsafe. I find it astonishing that an installation with a 100 A service is limited to having a 20 A PV feed limit when in fact that could be raised to 40 A or even 60 A without ever presenting a hazard, PROVIDED THE BREAKER IS PLACED AT THE OPPOSITE END OF THE BUSBAR from the main feed. I get that the 120% rule must be in place assuming some less well informed installers place the PV feed breaker next to the main feed breaker. In which case even 120% is pushing it .
If the panel manufacturers made panels with this in mind, it would make sense to provide a purpose built panel with two feed breakers installed at opposite ends of the busbar, and clearly marked.

 

[wwhitney]

but 150% would not be unreasonable or unsafe.

Per the previous simplistic analysis, 150% would worsen the worst case heating by a 25% increase over the single supply baseline. That's a level that's arguably too much without any specific testing.

Cheers, Wayne

 

[jaggedben]

I doubt the code rule is going to be able to change based on a theoretical analysis (much as I appreciate the educational value of Wayne's approach here). I think that the best hope is a new or expanded UL standard, with testing done for backfeed scenarios. Consider, for example, that an 8-space panel filled with source and load top and bottom on both sides is going to have a lower ability to dissipate heat than a 20 space panel with the same sources and loads top and bottom. I mean, I agree that most likely "150% would not be unreasonable or unsafe" but until there is an incentive for someone to put it in a lab and prove it, I don't think we know for sure. There's also the very large issue of old panels. Heck, I've seen the 120% rule invoked on FPEs and Zinscos. It's probably better if 120% stays the rule for all the existing panels installed out there, but new panels could be tested and labeled to a revised standard.

 

[Carultch]

I get that the 120% rule must be in place assuming some less well informed installers place the PV feed breaker next to the main feed breaker. In which case even 120% is pushing it .
If the panel manufacturers made panels with this in mind, it would make sense to provide a purpose built panel with two feed breakers installed at opposite ends of the busbar, and clearly marked.

If both sources are at the same end of the busbar, 120% gets replaced with 100%, as you can no longer take credit for KCL working in your favor. Both the main supply breaker + 125% of the inverter source current need not exceed the rating of the busbar, if you put them on the same side of the bus and feed all the loads in the same direction. This one is more intuitive.

The 120% rule is an industry compromise for panels not specifically listed otherwise for the purpose of combining sources, based on the industry norm that panels are routinely populated with breakers that add up to far greater than the main supply, and on the issue of mutual heating of the breakers. The main supply is sized such that it is unlikely that the sum total of the branch breakers never draw their full load all at once, and it should be a statistically rare event that the breakers ever do draw more than the main supply from both sources. But this is the rule we have. The closest we have for a way around this rule is 705.13 in the 2020 NEC, that allows power control systems to enable dynamic monitoring of the sources and control of the inverters to prevent a panel overload from happening.

 

[jbeecroft]

I doubt the code rule is going to be able to change based on a theoretical analysis (much as I appreciate the educational value of Wayne's approach here). I think that the best hope is a new or expanded UL standard, with testing done for backfeed scenarios. Consider, for example, that an 8-space panel filled with source and load top and bottom on both sides is going to have a lower ability to dissipate heat than a 20 space panel with the same sources and loads top and bottom. I mean, I agree that most likely "150% would not be unreasonable or unsafe" but until there is an incentive for someone to put it in a lab and prove it, I don't think we know for sure. There's also the very large issue of old panels. Heck, I've seen the 120% rule invoked on FPEs and Zinscos. It's probably better if 120% stays the rule for all the existing panels installed out there, but new panels could be tested and labeled to a revised standard.

Yes, I agree with your approach. My suggestion would be for the panel manufacturer to modify say a 200 A panel, but only fit it with 100 A breakers at each end of the busbar and rate it at '100 A dual feed' now the busbar would be fine and the heating of breakers would be accounted for with the baseline product design built to handle it. This would still violate the 120% rule but be fine in practice.

 

[wwhitney]

Yes, I agree with your approach. My suggestion would be for the panel manufacturer to modify say a 200 A panel, but only fit it with 100 A breakers at each end of the busbar and rate it at '100 A dual feed' now the busbar would be fine and the heating of breakers would be accounted for with the baseline product design built to handle it. This would still violate the 120% rule but be fine in practice.

You can do that yourself right now. And it would not violate the 120% rule, as 100A + 100A < 1.2 * 200A. It would in fact qualify for the 100% rule, so there would be no need for the two 100A supplies to be at opposite ends of the 200A busbar.

Cheers, Wayne

 

[jbeecroft]

You can do that yourself right now. And it would not violate the 120% rule, as 100A + 100A < 1.2 * 200A. It would in fact qualify for the 100% rule, so there would be no need for the two 100A supplies to be at opposite ends of the 200A busbar.

Cheers, Wayne

With a regular 200 A panel, You can only place a 40 A max breaker for the solar PV feed, even if you change the main feed breaker to less than 200 A. I have run into this with several inspectors and even a couple of district engineers. Their concern is that in the future someone would come and replace the main feed breaker with a 200 A part again and theoretically overload the panel. If a purpose built dual feed panel was available it would remove the possibility of ill informed installers (I won't call them electricians) from creating a problem.

 

[wwhitney]

With a regular 200 A panel, You can only place a 40 A max breaker for the solar PV feed, even if you change the main feed breaker to less than 200 A. I have run into this with several inspectors and even a couple of district engineers. Their concern is that in the future someone would come and replace the main feed breaker with a 200 A part again and theoretically overload the panel.

This is a clear overreach or misreading of the relevant section of the NEC (CEC). 2017 NEC 705.12(B)(2)(3)(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. . ."

What matters is what is installed at the time of inspection, not what mistakes could occur in the future.

Cheers, Wayne

 

[jbeecroft]

This is a clear overreach or misreading of the relevant section of the NEC (CEC). 2017 NEC 705.12(B)(2)(3)(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. . ."

What matters is what is installed at the time of inspection, not what mistakes could occur in the future.

Cheers, Wayne

Thanks for the reply Wayne. Yes, overreach is quite common I have found. I enjoy getting the insight from experienced electricians. I have been a 'lurker' for a long time. The NEC is an almost uniquely strange publication compared to what is done elsewhere. In Europe and in the UK particularly the wiring regulations are constructed by the IET a professional engineering organization where the regulations are governed by PE's (they are called 'Chartered Engineers or C.Eng) in consultation with other interested parties. One of my colleagues served on an NEC committee and he was quite surprised by the absence of a PE on his committee. He witnessed pressure being brought to bear to block some regulations, and tighten others, in order to protect the Unions or the firefighters. UL have people on many committees and they have blocked proposals from people who work for organizations that don't use UL for their certifications. There is way too much politics involved with NEC arrangement. But I digress! Cheers!
John

 

[Dennis Alwon]

With a regular 200 A panel, You can only place a 40 A max breaker for the solar PV feed, even if you change the main feed breaker to less than 200 A. I have run into this with several inspectors and even a couple of district engineers. Their concern is that in the future someone would come and replace the main feed breaker with a 200 A part again and theoretically overload the panel. If a purpose built dual feed panel was available it would remove the possibility of ill informed installers (I won't call them electricians) from creating a problem

It is unfortunate that there are inspectors out there without any training or common sense. What if doesn't make it into the NEC-- what if after inspections people add more circuits to a panel and overload it. Do we make everyone install panels so no overcurrent protective device can be added? It is too bad-- some are on power trips but I bet most just haven't been told or taught not to do that. I have found most inspectors to be pretty reasonable even if they don't agree with me or I them. Clear cut rules I go over there heads if they aren't willing to check it out on their own (most are willing) and if I am wrong or the code is unclear on my interpretation then I will make the necessary change to satisfy the inspector.

 

[jaggedben]

With a regular 200 A panel, You can only place a 40 A max breaker for the solar PV feed, even if you change the main feed breaker to less than 200 A. I have run into this with several inspectors and even a couple of district engineers. Their concern is that in the future someone would come and replace the main feed breaker with a 200 A part again and theoretically overload the panel. If a purpose built dual feed panel was available it would remove the possibility of ill informed installers (I won't call them electricians) from creating a problem.

I've had an AHJ or two ask for a placard that says something like: "Main breaker size xxxA. Do not upsize." I'm fine with that. I've never had to deal with the kind of overreach you're talking about it and I frankly wouldn't tolerate it. Elected officials would be hearing from my customers. Beyond requiring signage, the 'what if stupid or unscrupulous people come afterward' argument is essentially fallacious since that can happen to any kind of installation, anywhere, anytime. Ridding the world of stupid and unscrupulous people is not within the Code's scope.

 

[Carultch]

I've had an AHJ or two ask for a placard that says something like: "Main breaker size xxxA. Do not upsize." I'm fine with that.

It happened for me when the switchboard didn't get built in a manner that followed the opposite end of the busbar rule. It was specified the correct way on the electrical plan set, but the physical layout of the breakers got lost in translation when it came time to order it, and wasn't in my scope to review that submittal. To salvage it, I asked for the original load calculation from the general electrical team, to justify that the main breaker could be reduced for satisfying the 100% counterpart rule to the 120% rule. It was a simple matter of adjusting the dials on the main breaker to correct the issue. But I could see how someone might move the dials not knowing what they do, so I saw the value with having a placard spelling out the correct setting so future users would understand.

Swapping a breaker is something that is clearly intentional. However, turning a trip dial on a breaker could happen by accident if someone in the future doesn't realize the dial's purpose. You might quickly realize your mistake and want to remediate the issue, but not remember the correct setting.

 

[ggunn]

When you feed from opposite ends of the busbar you can feed 100% from each end. Kirchoff's law applies and the current in any part of the busbar will never exceed 100%. If there is no load and 100% is fed from a solar PV source back to the utility feed, the busbar current never exceeds 100%.
If you have 2 loads of 100% each then the utility feed will supply 100% to the nearest load breaker and the PV source will feed another 100% to the breaker nearest to it. Thus 200% load is supplied but the busbar current never exceeds 100%

To state the obvious: Even if you are correct we still have to comply with the NEC.

 

[SteveO NE]

It is not allowed to use a 40A breaker on the solar. 705 is clear on this, overcurrent devices must be rated for not less than 125% of inverter output. Part of the reason for this is that if you run 35A through a 40A breaker for 4 hours straight it may trip due to the heat buildup.

Also you must use 43.2A in your busbar calculations regardless of the breaker size.

Unfortunately the code does not allow us to consider east-west orientation or anything like that. Also if the roof slope is shallow enough and the panels are oversized enough compared to the micro-inverters then it may make no difference, they may still output 34.5A.

Your subpanel is fine. A 45A or a 50A breaker for the solar would both be fine, both protect the #6 wire from the utility side, codewise it makes no difference which you use. Easy to fix.

So your biggest problem is your main panel is not compliant with the 120% rule at 200/200. You can downsize the main breaker if the load calc allows, or maybe it qualifies under the next rule down (sum of all breakers). Hopefully one of those solutions works, because other solutions (downsizing the system or doing a supply side tap) are usually are less likely to be satisfy all the people and rules.

Glad you mentioned the next rule down...(c) most people forget about that one and though not practical on a system this size, one could then have potential gains on 100% rated gear and breaker to get the smaller breaker but as you point out no dice just because its an easy west system.

Out of curiosity though, do you routinely get kickback on supply side taps (that's what I infer from your parenthesized comment)? In our area that is the go-to for any self contained service - completely avoid the customer panel especially because small services like that tend to have unqualified hands in them too frequently. Seeing as we need a utility visible break disconnect anyway, its pretty marginal in extra cost for fuses vs a backfed breaker anyway.

 

[GoldDigger]

Some jurisdictions and some POCOs simply do not allow supply side taps.

 

[SteveO NE]

To state the obvious: Even if you are correct we still have to comply with the NEC.

Also, his comment has 2 oversights.
1) Overall heating effects of the bus - sure each inch of bus is not overloaded and overheating but the overall bus has 200A of current on it and may not be able to dissipate heat properly and will become heat soaked sooner. Its the same reason once you go to 100% rated gear everything upstream needs to be 100% rated, because even a 100% rated 100A breaker in a 400A panel could have higher spot temperatures than a 400A 80% breaker in the same panel.
2) The second oversight would be that the scenario is pretty unlikely and further unlikely that it would stay that way, we have to consider stupidity of future meddlers in our work, not to extremes but we have to consider simple things...is there spare space for a breaker in that panel that someone can easily load in and mess up the simple 1 for 1 logic and not consider that its a problem, maybe someone makes one breaker smaller and 1 larger, this is the reason, for example, you need to count full breaker Ir when dialing back a breaker unless the dials are locked and inaccessible to unqualified persons.

 

[SteveO NE]

Some jurisdictions and some POCOs simply do not allow supply side taps.

Ah ok so I'll consider myself lucky in the northeast! Wouldn't the simplest solution still just be a modified version of a supply side and add a new main disconnect ahead of the panel and tap in between that (avoid moving loads to a new panel)? Do they still balk at that? Not sure they could at that point though its not like there is anyone to call when they don't accept it and fighting them is like pushing a rope!

 

[jaggedben]

Glad you mentioned the next rule down...(c) most people forget about that one and though not practical on a system this size, one could then have potential gains on 100% rated gear and breaker to get the smaller breaker but as you point out no dice just because its an easy west system.

Out of curiosity though, do you routinely get kickback on supply side taps (that's what I infer from your parenthesized comment)? In our area that is the go-to for any self contained service - completely avoid the customer panel especially because small services like that tend to have unqualified hands in them too frequently. Seeing as we need a utility visible break disconnect anyway, its pretty marginal in extra cost for fuses vs a backfed breaker anyway.

Around here there's so many meter-main combos that the opportunity for true supply side taps is rare. Out of hundreds of installs I've participated in I can probably count the opportunities for compliant supply side taps with both hands. I've actually done far more supply side connections in MLO meter mains.