120% Rule

[KDough]

I sent this to my colleagues to see what kind of answers I would get. Now I am putting it out to all of you. These are not trick questions and I am sure that each one has a correct answer but I’ll say right up front that I am not sure what the correct answers are in each case.

Ok, I am going to lay out a few scenarios. I want to know which are code compliant if any.

#1) you have a 100a service with 100a hot buss and a 100a breaker feeding a 125a lug only sub-panel. Then you put a 30a solar back feed breaker in the 125a sub panel.

#2) You have a 100a service with a 100a main breaker feeding a 100a buss that has another100a breaker feeding a 125a lug only sub-panel. Then you put a 30a solar back feed breaker in the 125a sub panel.

Are both installations code compliant?

Is one installation compliant and one not?

Are both not code compliant?

#3) you have a 200a service with 200a main breaker feeding a 200a buss. There is a 100a breaker on that 200a buss feeding a 125a lug only sub-panel. You put a 50a solar back feed breaker in the 125a sub-panel.

Is that code compliant?

#4) take the same 200a service in #3 above with the 100a breaker feeding a 125a lug only sub-panel but you change out the 200a main breaker to a 175a main. You put a 50a solar back feed breaker in the 125a sub-panel.

Is this installation code compliant?

In my opinion I really don’t know. At first I wanted to say that they are all code compliant and are keeping in line with the 120% rule. But if you think about what the 120% rule is trying to accomplish, and that is to protect the buss from over heating, then it may be that only one or none are code compliant.

I would like to get your comments and opinions on this.

 

[wwhitney]

Your description does not mention what other breakers if any are on the upstream most bus. So I will answer as regards to 705.12 for both cases. I also assume the only production sources are the ones described.

1) Compliant. The upstream 100A bus are service conductors, so you only have to qualify the downstream bus. You have a 100a main breaker protecting a 125A MLO panel with at most 24A of continuous inverter output current. 100 + 1.25*24 < 1.2 * 125, so it complies with the 120% rule..

2) The downstream panel is compliant under the 120% rule. The upstream panel is only compliant under the 120% rule if that 30A solar back feed breaker has only 16A or less of continuous inverter output current on it. The upstream panel is compliant under the 100% rule if it has no other breakers in it. Otherwise, not compliant.

3) The downstream panel is compliant under the 120% rule. The upstream panel is only compliant under the 120% rule if that 50A solar back feed breaker has only 32A or less of continuous inverter output current on it. The upstream panel is compliant under the 100% rule if the other breakers in it add up to only 100A or less (for each leg separately). Otherwise, not compliant.

4) The downstream panel is compliant under the 120% rule. The upstream panel is now compliant under the 120% rule, as it has at most 40A of continuous inverter output current on it, and 175 + 1.25*40 < 1.2 * 200.

Cheers, Wayne

 

[KDough]

Thanks Wayne, that kinda the direction I was going. It will be fun to see what the other inspectors have to say at our morning meeting tomorrow .

 

[jaggedben]

It seems like what you are trying to get at is whether panel boards in series must all comply under 705.12 or if only the first panel where the solar interconnects has to comply. The answer is that they all have to comply. Note, however, that the 120% rule is no longer the only way that a panel can comply. The backfeed can violate the 120% if it complies with the following section, what I call the 'sum of breakers' rule.

 

[ggunn]

I sent this to my colleagues to see what kind of answers I would get. Now I am putting it out to all of you. These are not trick questions and I am sure that each one has a correct answer but I’ll say right up front that I am not sure what the correct answers are in each case.

Ok, I am going to lay out a few scenarios. I want to know which are code compliant if any.

#1) you have a 100a service with 100a hot buss and a 100a breaker feeding a 125a lug only sub-panel. Then you put a 30a solar back feed breaker in the 125a sub panel.

#2) You have a 100a service with a 100a main breaker feeding a 100a buss that has another100a breaker feeding a 125a lug only sub-panel. Then you put a 30a solar back feed breaker in the 125a sub panel.

Are both installations code compliant?

Is one installation compliant and one not?

Are both not code compliant?

#3) you have a 200a service with 200a main breaker feeding a 200a buss. There is a 100a breaker on that 200a buss feeding a 125a lug only sub-panel. You put a 50a solar back feed breaker in the 125a sub-panel.

Is that code compliant?

#4) take the same 200a service in #3 above with the 100a breaker feeding a 125a lug only sub-panel but you change out the 200a main breaker to a 175a main. You put a 50a solar back feed breaker in the 125a sub-panel.

Is this installation code compliant?

In my opinion I really don’t know. At first I wanted to say that they are all code compliant and are keeping in line with the 120% rule. But if you think about what the 120% rule is trying to accomplish, and that is to protect the buss from over heating, then it may be that only one or none are code compliant.

I would like to get your comments and opinions on this.

None of these are compliant under the 120% rule unless the main panel is MLO; 705.12(B)(3) applies to all panels between the interconnection and the service conductors, although the 120% rule is only one of three ways to qualify busbars, and you may apply one rule to one panel and another to another panel.

Also, under the 120% rule the ratings of the backfed breakers are irrelevant; 125% of the inverter Imax is the number that applies to all the panels.

 

[wwhitney]

None of these are compliant under the 120% rule unless the main panel is MLO;

The description specifies that #1 is MLO, while #2, #3 and #4 are not. So we agree that on #1 the 120% rule doesn't apply to the MLO service panel bus. But #4 MB main panel is compliant with the 120% rule, as the main breaker has been downsized to 175A. And as you state:

Also, under the 120% rule the ratings of the backfed breakers are irrelevant; 125% of the inverter Imax is the number that applies to all the panels.

So #2 or #3 may or may not be compliant under the 120% rule, as it is 125% of the inverter continuous output current that matter, not the breaker size. If we assume the two are equal, they are not compliant under the 120% rule.

Cheers, Wayne

 

[ggunn]

The description specifies that #1 is MLO, while #2, #3 and #4 are not. So we agree that on #1 the 120% rule doesn't apply to the MLO service panel bus. But #4 MB main panel is compliant with the 120% rule, as the main breaker has been downsized to 175A. And as you state:


So #2 or #3 may or may not be compliant under the 120% rule, as it is 125% of the inverter continuous output current that matter, not the breaker size. If we assume the two are equal, they are not compliant under the 120% rule.

Cheers, Wayne

A "hot bus" means MLO? Makes sense, but I hadn't heard that term for it.

 

[wwhitney]

A "hot bus" means MLO? Makes sense, but I hadn't heard that term for it.

Me neither, but the other examples specified a "service with main breaker," so I infer that in #1 there is no main breaker in the service panel.

Cheers, Wayne

 

[ESolar]

I have a similar question.
o Combined (max) amperage of (Enphase) solar micro inverters: 34.56
o 125% of (max) amperage: 43.2
o Combined feeder cable: #6 copper
o Lands in a 125 amp subpanel with 70 amp subpanel breaker located in main panel
o Solar breaker in subpanel: 40 amps (yes - less than the potential maximum amperage, that will almost certainly never bee produced)
o 200 amp main panel with 200 amp breaker (200 x 20% = 40 amps available)

Assumptions: The solar is never likely to kick out more than 40 amps in part because it is 1/2 East and 1/2 West. However, if it does, then the 40 amp breaker in the subpanel would protect the bus bars in the main panel. Subpanel protected by 70 amp breaker (20% x 125 + (125 - 70) = 80 amps; so anything less than or equal to 70 amps appears good). #6 wires also protected by 40 amp breaker.

Is the subpanel code compliant?
Is use of the 40 amp solar breaker to protect the bus bar code compliant?

 

[ESolar]

FYI: I also have 45 amp breaker for the solar and a 175amp Main breaker for derating the main if necessary. However, I assum that that is not necessary.

 

[jaggedben]

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.

 

[Carultch]

A "hot bus" means MLO? Makes sense, but I hadn't heard that term for it.

MLO means main lug only, in contrast from MCB for main breaker.

"Hot bus" is a special case of MLO, where the panel of the service disconnects is an MLO panelboard, and cannot be de-energized without pulling the meter globe or requesting a utility-controlled shutdown.

 

[ESolar]

Looks like I will sub in those 45 and 175 amp breakers. The load calcs work. Although I have to revisit those.

 

[jbeecroft]

What is the basic purpose of the '120% rule'? My understanding is that it serves to ensure that a panel busbar current does not exceed it's rating by more than 20%. If an alternate feed is added to a panel (e.g. a solar PV feed) then the breaker for that feed cannot exceed 20% of the rating.

I see this as a logical rule if the alternate feed breaker is placed next to the main feed breaker. If, however, the alternate feed breaker is placed at the opposite end of the busbar to the main feed breaker, then there is no reason why the alternate feed cannot be run up to the busbar rating, as such a configuration would never exceed the busbar rating no matter what combination of load breakers were place between the two feed breakers.

 

[wwhitney]

Your analysis would be plausible if the busbars and breakers were superconducting and the connections had no resistance. However, all of those have ohmic I²R heating losses, generating heat in the panel.

So the purpose of the 120% rule is to limit the heat generation in the panel to no more than 120% of the original design. [Actually, as the heat generation varies as I², it would be 104% of the original design, if the currents were to two load breakers, one of 100% of the busbar rating and one of 20% of the busbar rating. I believe that's the worst case.] And the reason for the breakers to be at opposite ends of the bus is to ensure that the current at any cross section does not exceed 100% of the busbar rating.

Having said all that, 120% is a somewhat arbitrary threshold that was adopted.

Cheers, Wayne

 

[Carultch]

Your analysis would be plausible if the busbars and breakers were superconducting and the connections had no resistance. However, all of those have ohmic I²R heating losses, generating heat in the panel.

So the purpose of the 120% rule is to limit the heat generation in the panel to no more than 120% of the original design. [Actually, as the heat generation varies as I², it would be 104% of the original design.

I think you mean heat generation would be 144%.

 

[wwhitney]

I think you mean heat generation would be 144%.

1.0^2 + 0.2^2 = 1.04.

Cheers, Wayne

 

[jbeecroft]

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%

 

[Carultch]

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%

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.

1.0^2 + 0.2^2 = 1.04.

Ok, so you are squaring 100% and 20% individually and adding them, rather than squaring 120% itself. Is there any physical basis for your particular calculation?

Bear in mind that the 120% rule doesn't exclusively require that you feed 100% on one side, and 20% on the other side, as is most commonly the case. You could feed 60% on both sides and it would still comply. For instance, a 100A panelboard fed with a 60A main on top and a 60A interconnection on the bottom, would also comply.

 

[wwhitney]

Tell me if I get the physics wrong, but the basis is that the heating effects will all be proportional to I². I'm unclear on the relative magnitude of the heating effects from the connections, vs the busbar itself, vs the breakers. In terms of the total rate of heating in the panel, the busbar heating will also depend on the length of the busbar between nodes.

For simplicity consider the model where the heating is dominated by the connections and the breakers, with the busbar itself a lesser contribution (this may be inaccurate). Then the heating is approximately proportional to the sum of squares of currents through each breaker on the bus. Here are some examples of this model with a 100A busbar, a 100A main, and various case of other breakers, assuming all breakers are loaded to rating:

A) (5) 20A loads 100² + 5 * 20² = 12,000
B) (1) 100A load 20,000.
C) (6) 20A loads, (1) 20A source 12,800
D) (1) 100A load, (1) 20A source, (1) 20A load 20,800
E) (10) 20A loads, (1) 100A source 24,000
F) (2) 100A loads, (1) 100A source 40,000

From this I draw a few conclusions: (B) is the worst case for a single supply to the panel. (D) is the worst cast for the 120% rule, and the heating effect is 104% of (B). And (F) is the worst case for the proposed "200%" rule, the heating effect is twice that of (B). Far too great an increase without additional testing, unlike (B).

Cheers, Wayne