Electrical Theory of 120% rule

SunFish

Member
Can someone please explain the electrical theory behind the 120% rule? I understand how to calculate breaker sizes to meet the 120% rule but how does locating the solar breaker at the opposite end of the busbar protect the panel vss having it at the top of the busbar next to the main service breaker?
 

GoldDigger

Moderator
Staff member
If the main and PV sources are at the same end of the bus, the combination of all of the load breakers combined can pull a load greater than the carrying capacity of the bus.
There is, fortunately, no NEC limit on the sum of the branch and feeder breakers in the panel. That means that the bus protection has to on the supply side.
(Note that this does not apply when you have an MLO main panel with up to six service disconnect breakers.)
The real question I never see answered is why they chose 120%, since in theory you could have each end supplied up to the full bus capacity without ever having an overload at any single point on the bus.
I suppose that you would never be allowed to install a load breaker whose capacity is greater than that of the bus, but if you did, then the current limit for a single stab could become a problem.
 

ggunn

PE (Electrical), NABCEP certified
Can someone please explain the electrical theory behind the 120% rule? I understand how to calculate breaker sizes to meet the 120% rule but how does locating the solar breaker at the opposite end of the busbar protect the panel vss having it at the top of the busbar next to the main service breaker?
If the main breaker rating is the same as that of the busbar:

If the solar breaker is right up under the main breaker and the total loads in the rest of the breakers combine to demand more than can be serviced by the main breaker alone but less than what the main and the solar breaker together can supply, neither breaker will trip and the part of the busbar right under the solar breaker will have the sum of the two currents going through it, which will overload the bus. If the main and solar breaker are at opposite ends of the bus, that won't happen.

Why 120%? I dunno; I guess it was the number that the largest number of code writers could agree on.
 

kwired

Electron manager
What if you have bus with rated higher current carrying capacity then the mains supplying it?

Maybe 200 amp bus, 100 amp main utility breaker, 40 amp PV breaker?

Not familiar with the PV sections, maybe answer is there, but asking anyway.
 

electrofelon

Senior Member
What if you have bus with rated higher current carrying capacity then the mains supplying it?

Maybe 200 amp bus, 100 amp main utility breaker, 40 amp PV breaker?

Not familiar with the PV sections, maybe answer is there, but asking anyway.
Right that is a common method, to say swap a 200 amp main for a 175 or 150, or 200 amp MB with a 225 amp bus to get a little more PV backfeed capacity. The rule is the "sum of the OCPD's supplying the bus" (but I think 2014 allows the inverter output current now instead of the breaker rating which gives you a little extra).
 

ggunn

PE (Electrical), NABCEP certified
Right that is a common method, to say swap a 200 amp main for a 175 or 150, or 200 amp MB with a 225 amp bus to get a little more PV backfeed capacity. The rule is the "sum of the OCPD's supplying the bus" (but I think 2014 allows the inverter output current now instead of the breaker rating which gives you a little extra).
Close. 2014 code allows 125% of the inverter maximum output current to be used in the calculation.

But yes, a 200A bus with a 200A breaker gives you only (1.2)(200A) - 200A = 40A electrical room to mount PV, but a 225A bus with a 200A breaker allows (1.2)(225A) - 200A = 70A.
 
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jaggedben

Senior Member
Don't know if I could find the link to the John Wiles article I read a long time ago now, but the 120% number is basically arbitrary. The ideais basically that 'panelboards are not tested by NRTLs for this type of application, so even though electrical theory suggests everything should be fine we are going to go with a really conservative number'. Basic electrical theory tends to suggest that if the interactive source is at the opposite end, you could have a 200% rule. But real-world thermal loading of busbars may not hold up to that, not to mention that power factor might be an issue.

There is some kind of proposal to increase the limit to 150% in the 2017 code. There's a CMP 'taskforce' looking at it, so I read in a public input somewhere. But who knows if that will fly.

Note that the 2014 code allows some other options besides the 120% rule, such as requiring that all the load breakers do not exceed the busbar rating.
 

GoldDigger

Moderator
Staff member
The logical rule, taking bus capacity as affecting only current through each cross section, would not be a 200% rule. It would be an "up to 100% each" rule.
Limiting the source at one end to 50% does not allow you to raise the source at the other end to 150% of bus capacity.
.
 

ggunn

PE (Electrical), NABCEP certified
Note that the 2014 code allows some other options besides the 120% rule, such as requiring that all the load breakers do not exceed the busbar rating.
Actually, that language - 705.12(D)(2)((3)(c), added to the NEC in the 2014 cycle - is what legitimizes the Solar Aggregation Panels, aka AC Combiner Panels, that we have been using for years under a gray area in the code. It says that you can have a panel where the sum of the ratings of all breakers, both load and backfed, and disregarding the breaker which connects it to the service, is less than the rating of the busbars.
 

Carultch

Senior Member
Actually, that language - 705.12(D)(2)((3)(c), added to the NEC in the 2014 cycle - is what legitimizes the Solar Aggregation Panels, aka AC Combiner Panels, that we have been using for years under a gray area in the code. It says that you can have a panel where the sum of the ratings of all breakers, both load and backfed, and disregarding the breaker which connects it to the service, is less than the rating of the busbars.
One issue with that language, is that it indicates you need to accumulate rounding errors that went in to sizing each breaker, when determining busbar ampacity. And this becomes significant when you have numerous small inverters, where 1.25*I may be a couple amps less than a standard breaker size.

For instance, take 17 qty inverters at 18 amps each.
18*1.25 = 22.5, which means 25A breakers needed for each inverter. 17*25 = 425A

However:
17*18*1.25=382.5, which should allow use of a 400A bus with a 400A main.
 

jaggedben

Senior Member
The logical rule, taking bus capacity as affecting only current through each cross section, would not be a 200% rule. It would be an "up to 100% each" rule.
Limiting the source at one end to 50% does not allow you to raise the source at the other end to 150% of bus capacity.
.
True. Although I hope we don't need additional code language to affirm that the primary source OCPD must comply with Article 2140. :slaphead:


Actually, that language - 705.12(D)(2)((3)(c), added to the NEC in the 2014 cycle - is what legitimizes the Solar Aggregation Panels, aka AC Combiner Panels, that we have been using for years under a gray area in the code. It says that you can have a panel where the sum of the ratings of all breakers, both load and backfed, and disregarding the breaker which connects it to the service, is less than the rating of the busbars.
Combiner Panels have been the inspiration for that section, but the way it's written permits other situations. Say I have a small 100A subpanel serving a large detached garage on a residence. It's fed by a 100A feed but it only has 60A of load breakers in it for lighting and outlets. Well, I can put 40A of solar on that garage if the solar panels will fit. I'm not limited to 20A anymore as with the 120% rule.
 

Carultch

Senior Member
The logical rule, taking bus capacity as affecting only current through each cross section, would not be a 200% rule. It would be an "up to 100% each" rule.
Limiting the source at one end to 50% does not allow you to raise the source at the other end to 150% of bus capacity.
.
From a standpoint of keeping track of currents, you are correct. Eventually 100% fed from both ends would diminish to zero within the panelboard, and never exceed 100%. However, the reason for a 120% rule is not based on current only. It is based on the heating of each breaker, and is likely a compromise number without any hard science to prove why it is that and not 125% or 130%
 

ggunn

PE (Electrical), NABCEP certified
Combiner Panels have been the inspiration for that section, but the way it's written permits other situations. Say I have a small 100A subpanel serving a large detached garage on a residence. It's fed by a 100A feed but it only has 60A of load breakers in it for lighting and outlets. Well, I can put 40A of solar on that garage if the solar panels will fit. I'm not limited to 20A anymore as with the 120% rule.
The main difference being that you no longer have to keep loads out of the AC combiner, which was the compromise we used to make due to the semi legitimacy of solar AC combiners in earlier code cycles. Loads like power supplies for monitoring would sometimes complicate the issue.
 

mwm1752

Senior Member
Can someone please explain the electrical theory behind the 120% rule? I understand how to calculate breaker sizes to meet the 120% rule but how does locating the solar breaker at the opposite end of the busbar protect the panel vss having it at the top of the busbar next to the main service breaker?
Just to think outside the box -- a load is a draw of current -- if the pv system input is not being used it is sent to the utility provider system as a draw of current is somewhere on the utility lines -- the pv current is always used before a utility power scource -- installing the the pv input on opposite ends would seem to disperse the current to the most close immediate power draw. Theoretically the main OCPD is able to draw 100% of its rating. If utility & PV feeds were on the same side you could possibly have a section of buss that could draw 100% of both of their OCPD. With both feeds supplying for opposite sides it would be impossible to load the buss past its limits due to branch circuit protection sizing.
 

ggunn

PE (Electrical), NABCEP certified
Just to think outside the box -- a load is a draw of current -- if the pv system input is not being used it is sent to the utility provider system as a draw of current is somewhere on the utility lines -- the pv current is always used before a utility power scource -- installing the the pv input on opposite ends would seem to disperse the current to the most close immediate power draw. Theoretically the main OCPD is able to draw 100% of its rating. If utility & PV feeds were on the same side you could possibly have a section of buss that could draw 100% of both of their OCPD. With both feeds supplying for opposite sides it would be impossible to load the buss past its limits due to branch circuit protection sizing.
It is an inaccurate concept that electricity from a PV system is used first or preferentially over grid power. A grid tied PV system is part of a massively parallel grid consisting of sources and loads where all the loads are fed by all the sources according to Kirchoff's Laws. A load neither knows nor cares where the electrons flowing through it come from, and moving a backfed breaker around in an MDP has no effect on metering or where the energy coming through it ultimately goes. Moving a backfed breaker around does, however, affect the current density in different parts of the bus. Kirchoff again.
 
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jaggedben

Senior Member
It is an inaccurate concept that electricity from a PV system is used first or preferentially over grid power. ...
Leaving aside the vagaries of power factor...

It is accurate if you replace the word 'electricity' with 'power' or 'energy'.
 

ggunn

PE (Electrical), NABCEP certified
Leaving aside the vagaries of power factor...

It is accurate if you replace the word 'electricity' with 'power' or 'energy'.
How so? Power factor is indeed a vagary that muddies the water, but the distribution of electricity, or power or energy if you like, is better explained by Kirchoff than by a "preference" of a load to take power from one source over another. Moving backfed and load breakers around in a panel has no effect on from where a load breaker gets its power. It does have an effect on the current density in regions of the busbar, which is the reasoning behind the 120% rule's requirement that breakers feeding a busbar must be at opposite ends of the bar.
 
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mwm1752

Senior Member
It is an inaccurate concept that electricity from a PV system is used first or preferentially over grid power. .
Then explain why if you have equal pv power generated to power consumed the meter has no register of power used from the grid.
 

kwired

Electron manager
Then explain why if you have equal pv power generated to power consumed the meter has no register of power used from the grid.
Doesn't tell you where every bit of the energy came from just tells you the net of what passed through the meter. Some one site produced energy possibly passed to the grid, but an equal amount came back from the grid if the net is zero.
 
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