How does NEC 120% Rule make sense ?

[Signal32]

Hello,

The US National Electrical Code defines some restrictions about feeding an electrical panel from two sources (the utility grid and a local generator, like solar power).
The intent is to prevent the busbar from being overloaded.
The language is: "Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(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. "
In the following instead of "125 percent of the inverter(s) output" I'll just use "Generator breaker" - so that it'll be easier to follow


Let's start with a circuit where there there is no local generator. Here the main breaker will always protect the busbar, so there is no issue:


In the case we have a local generator and the breaker for the generator is located next to the main grid breaker, the restriction is taht grid breaker + generator breaker can't exceed busbar rating. This totally makes sense since you can easily overload the busbar, as follows:


Now, what doesn't make sense is if the generator is at the opposite side of the bus bar. NEC says that generator breaker + main breaker has to be max 120% of bus bar rating. I don't know how this makes sense or where the 120% comes from. To me seems obvious that it's OK to have both the main breaker and the generator breaker equal to the rating of the bus bar (and sum up to 200% of bus bar rating). Here is the scenario:


In the example above the bus bar is being fed with 200A from both directions. Even if the total load is 270A, no point of the bus bar would be carrying more than 200A at any time since if it did either the main breaker or the generator breaker would trip. The 270A would be split between the upper part and lower part of the bus bar. So, for example, the upper part (and therefore the main grid) would be carrying 170A and the lower part (generator) would be carrying 100A. The bus bar would not be overheating at any point - so what is the problem ?
Can anyone explain why the 120% rule exists and where the 120 number comes from ?

[wizard69]

I can't explain it based on accurate knowledge, but my guess would be that you can't sustain high output from an inverter.   That is no matter what you do a solar panel feed inverter will have an output that varies over the day.

There is likely a very long discussion about how breakers and fuses actually trip in such circuits.   Generally in the NEMA / USA NEC world fuses and breakers are designed to trip at their rated points, not run there.   This is why many systems are designed around 80% of their nominal feeder/circuit capacity.   That is, you would not want to try to run a 15 amp draw on a 15 amp breaker continuously.

You will need to get somebody to respond that is better equipped to answer than me, it has been a very long time since I tracked rule making for the NEC.    There is likely a public record of how the rule was created someplace these days.   I just lean towards the idea of it being a way to deal with breaker behavior in combination with the varying output of solar arrays.

[bdunham7]

The panel is rated for 200A total, period.  It wasn't rated for 200A available on either end of the busbar with the hope that 200A wouldn't be exceeded anywhere in between.  The fact that your drawing shows only 270A of load doesn't stop someone from adding more breakers later--the NEC doesn't review your panel every time an electrician changes a breaker. Or, alternatively, imagine there is an overload of 300A on your 120A circuit, but the 120A breaker fails to trip.  So its a 200A panel, period.  I have way more than 200A worth of breakers in my 200A panel, so something has to limit the max current and that's the main breaker.  And generators are irrelevant as you can't wire them directly into the panel like that under any circumstances. 

The rules were relaxed slightly to allow an additional breaker of up to 20% of the panel (busbar) capacity to be added for solar power hookups.  This allows for solar installations without requiring replacing the main breaker--without this requirement, I would have had to exchange my main 200A breaker for a 150A or 160A version in order to have my solar feed in through a 40A breaker, which it does.  I'm sure the thinking was  something like "the solar loads are not constant, residential breaker panels usually draw nowhere near their rating, 20% is not too much and hey--we can make them put it on the 'other end' so that there won't be 200A of current at any point no matter what!  But the rating of the panel is still 200A, not simply the limit of how much current is flowing at any one point on the busbar.  Breakers and connections generate heat. The 20% rule was a small concession to allow economical installation of grid-tied solar systems.

NEC rules don't work like engineering design.  They are set up to prevent the dumbest electrician around from starting fires, provided they follow the rules.

[ejeffrey]

Yes, you are correct that if you feed 200A from each end, the local current will never exceed 200A.  A concrete example is that a panel with 200 A supply each at the top and bottom and 400 A worth of load in the middle of the bus bar will have the same I2R heating of the busbar as the same panel with a 200 A supply at the top and a 200 A load at the bottom.

I don't know the reasoning behind the 120% rule, but my guess is some combination of the following:

1) There is contact resistance and resistance within the breaker in addition to the bus bar. This gets worse if you try to have multiple power sources, limiting to 120% keeps this from being too bad.
2) Electricians may try to take advantage of the rule when it shouldn't apply.  I have seen systems installed using the 120% rule even with a center-feed main breaker with the inverter input at the top or bottom.  This can technically exceed the busbar rating, but by limiting to 120% instead of 200% it is unlikely to exceed by much
3) The worst case current load at 200% rating may be the same, but the average is not.  Regulations are designed so that under normal circumstances you have a large safety margin in case some things go wrong or were done incorrectly.  Pushing a "normal" operation closer to the absolute limit can cause additional stress and eat into your safety margin.

20% gives a good margin for small to medium solar installations without major overhaul and won't dramatically change the overall system.  For larger systems the cost of redoing the panel to support the current capacity

[bdunham7]

Yes, you are correct that if you feed 200A from each end, the local current will never exceed 200A.  A concrete example is that a panel with 200 A supply each at the top and bottom and 400 A worth of load in the middle of the bus bar will have the same I2R heating of the busbar as the same panel with a 200 A supply at the top and a 200 A load at the bottom.

I²R heating of the busbar is only one factor in determining the panel rating.  Your example pencils out theoretically, but on an actual panel there will be a maximum outgoing circuit breaker size--perhaps 100A for a 200A panel and sometimes less--which means you can't have a single 200A load.  Some panels actually have a copper busbar option, but AFAIK the panel rating stays the same.

2) Electricians may try to take advantage of the rule when it shouldn't apply.  I have seen systems installed using the 120% rule even with a center-feed main breaker with the inverter input at the top or bottom.  This can technically exceed the busbar rating, but by limiting to 120% instead of 200% it is unlikely to exceed by much

Mine is exactly that, but AFAIK it complies with the letter of the rule.  The solar feed-in breaker has to be on one end of the panel and it is.  I worried that the electrician and/or inspector would quibble at this and I had an argument ready, which was that the sum of all the breakers on the top side was less than 200A.  It never came up. 

[Signal32]

Thanks for the answers !

1) There is contact resistance and resistance within the breaker in addition to the bus bar. This gets worse if you try to have multiple power sources, limiting to 120% keeps this from being too bad.

I don't understand how multiple power sources can make breaker contact resistance worse.  Do you mean that each busbar->breaker contact generates heat and at 200% then 2x the heat would be generated and would heat the bus bar more overall ? This would be true, but is heat from breaker contact resistance an actual thing ? In my experience if you're running a 50A circuit at 100% the wire heats up by 10C while you can't even observe any heating of the breaker / contact point.

3) The worst case current load at 200% rating may be the same, but the average is not.  Regulations are designed so that under normal circumstances you have a large safety margin in case some things go wrong or were done incorrectly.  Pushing a "normal" operation closer to the absolute limit can cause additional stress and eat into your safety margin.

But wouldn't this argument apply to basically everything ? What would be the difference between the busbar scenario and a 15A breaker protecting a 15A wire ? Same arguments can be made for busbar and breaker, if one overheats then so will the other.

The panel is rated for 200A total, period.  It wasn't rated for 200A available on either end of the busbar with the hope that 200A wouldn't be exceeded anywhere in between.

Yup, depends what panel ratings are for. Annoying enough my 200A panel has a 400A-capable busbar (going by cooper mm2), - guess because they don't wanna manufacture multiple variants.

The fact that your drawing shows only 270A of load doesn't stop someone from adding more breakers later--the NEC doesn't review your panel every time an electrician changes a breaker.

But it doesn't matter how many breakers you add, you can add 20 more breakers summing up to 1000A, there still wouldn't be an issue. If the loads attempt to draw more than 200A from either side it will trip the breaker protecting that side of the bus bar.

But the rating of the panel is still 200A, not simply the limit of how much current is flowing at any one point on the busbar.  Breakers and connections generate heat. The 20% rule was a small concession to allow economical installation of grid-tied solar systems.

The NEC rule specifically references the bus bar, not the panel rating. Not sure what the panel rating is for, actually. It is up for debate what does a 200A limit on a bus bar means - weather it means that no point can exceed 200A or everything summed up together cannot exceed 200A. The engineering logical version would be the former. I don't understand how breaker connections generating heat has anything to do with this. Unless you're saying that if you end up drawing more than 200A from both sides then there will be more heat at different connection points and therefore more heat in total. But such heating is minimal, no ? I can't imagine that even at full load the busbar heat would be raised more than a few degrees while the wires would be raised tens of degrees.

But the rating of the panel is still 200A, not simply the limit of how much current is flowing at any one point on the busbar.  NEC rules don't work like engineering design.  They are set up to prevent the dumbest electrician around from starting fires, provided they follow the rules.

The only real safety concern I can see is if someone moves the breaker from the opposite side to the main side - which means ignoring the label that NEC requires even with the 120% rule. Following 120% you're probably safe, but with 200% you might blow the bus bar. But what's the difference between this and someone oversizing a regular breaker and melting a circuit that goes throughout the house.