Use of 120% Rule (Solar Breaker Position) 705.12.D.2.3.b

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APS Solar

Member
Location
Sonora CA
We always try to land the solar breaker on the opposite side of the bus from the Main. However there are times due to wire limitations in the main that we can't put the solar breaker in the bottom slot but very near it. My chief building inspector who always passed jobs before is now telling me that the 120% rule will not apply if we can't get it into the far slot so now he wants me to downsize the Mains breaker (to 175A) for a 5KW system with a 30A solar breaker. Does this seem ridiculous? What can be done here? He will listen if I can point to code to justify my position. Also he told me that if the panel has a center fed main breaker then this 120% rule would not apply. Again we've been doing this for years with no problems.

Advise plz.
 

romex jockey

Senior Member
Location
Vermont
Occupation
electrician
I just don't grasp the theory?
:happysad:
Why all this focus on the panel buss, not the rest of the service?

~RJ~
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I just don't grasp the theory?
:happysad:
Why all this focus on the panel buss, not the rest of the service?

~RJ~
Power from the solar panels must be put at the bottom of the buss so that the actually buss itself will help dissipate some of the extra heat. If the breaker is install at the top I guess there is a fear of overheating the main.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I just don't grasp the theory?
:happysad:
Why all this focus on the panel buss, not the rest of the service?

~RJ~

The 'rest of the service' is one directional current flow, and we know its components' behavior under max current. A panelboard full of loads and more than one source has a lot of different possible combinations of current directions and maxes and they are/were not necessarily designed and tested for all those combinations.
 

pv_n00b

Senior Member
Location
CA, USA
We always try to land the solar breaker on the opposite side of the bus from the Main. However there are times due to wire limitations in the main that we can't put the solar breaker in the bottom slot but very near it. My chief building inspector who always passed jobs before is now telling me that the 120% rule will not apply if we can't get it into the far slot so now he wants me to downsize the Mains breaker (to 175A) for a 5KW system with a 30A solar breaker. Does this seem ridiculous? What can be done here? He will listen if I can point to code to justify my position. Also he told me that if the panel has a center fed main breaker then this 120% rule would not apply. Again we've been doing this for years with no problems.

Advise plz.

Enjoy that you were allowed to do this for years even though it was not following the code. But now you are being held to the NEC and the AHJ is correct. There was a TIA issued to allow center-fed panelboards in dwellings to get the 120% applied, as jaggedben pointed out. The only thing with TIAs is that they have to be incorporated in any local electrical code that is based on the NEC. If that has not been done yet then the AHJ might not accept it but you can always try.
 

five.five-six

Senior Member
Location
california
I just don't grasp the theory?
:happysad:
Why all this focus on the panel buss, not the rest of the service?

~RJ~

this is the way I understand it:

”in theory”

top breaker can deliver 200A to the buss...OK

buss is rated for 200A


if the load exceeds 200A (say 229A), top breaker pops,


add PV 30A and 229A of load and neither breaker pops but that’s OK because up to 200A comes from the top of the buss and up to 30A comes to the bottom so at no point on the buss is there more than 200A

Now put the PV breaker anywhere other than the bottom of the buss and draw 229A from a position under the PV breaker and the section of the buss below the PV breaker is now carrying 229A on a 200A buss


In theory.
 

romex jockey

Senior Member
Location
Vermont
Occupation
electrician
The 'rest of the service' is one directional current flow, and we know its components' behavior under max current. A panelboard full of loads and more than one source has a lot of different possible combinations of current directions and maxes and they are/were not necessarily designed and tested for all those combinations.

this is the way I understand it:

”in theory”

top breaker can deliver 200A to the buss...OK

buss is rated for 200A


if the load exceeds 200A (say 229A), top breaker pops,


add PV 30A and 229A of load and neither breaker pops but that’s OK because up to 200A comes from the top of the buss and up to 30A comes to the bottom so at no point on the buss is there more than 200A

Now put the PV breaker anywhere other than the bottom of the buss and draw 229A from a position under the PV breaker and the section of the buss below the PV breaker is now carrying 229A on a 200A buss


In theory.


so i'm supposed to believe inverter ac somehow assumes a different waveform path?

i was under the impression the theory of true sine wave inversion synchronized

ergo, an amp is an amp is an amp, is it not?:?

be in in the 'panel', or 300' of UG urd....

~RJ~
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
so i'm supposed to believe inverter ac somehow assumes a different waveform path?

i was under the impression the theory of true sine wave inversion synchronized

ergo, an amp is an amp is an amp, is it not?:?

be in in the 'panel', or 300' of UG urd....

~RJ~

It has nothing to do with waveform. And yes, the waveforms are (essentially) synced.

It has do with a) Kirchoff's law and the nature of parallel connections and b) thermal dynamics.

A panelboard busbar is a conductor with parallel connections all along it. Current can only flow in one direction at a time on a conductor. If you connect two synced sources at the far ends, there is no way the currents can add together per Kirchoff. Either one source will supply all the loads in between, or the two sources' current will combine somewhere in the middle on a load, not the main busbar. However if you put the two sources near the same end, then their currents can combine to flow towards the other end, if the loads draw that much, thus exceeding the rating of the bus.

By this logic, the 120% rule could be a 200% rule when the opposite end is adhered to. But UL and the CMP say: "wait a minute, all these breaker connections heat up at full current. And we don't test these panelboards for that." Say you have a 200A bus and the main breaker feeding a 200A load right next to the main, both of those connections heat up right next to each other. They test for that. They don't test for an inverter feeding another 200A load somewhere else on the same bus at the same time, which would be twice as much heat that the busbar has to dissipate. So they've given us this 20% allowance, for which we should be thankful I guess.
 

five.five-six

Senior Member
Location
california
so i'm supposed to believe inverter ac somehow assumes a different waveform path?

i was under the impression the theory of true sine wave inversion synchronized

ergo, an amp is an amp is an amp, is it not?:?

be in in the 'panel', or 300' of UG urd....

~RJ~

For simplicity, lets just look at buss A of a single phase service

100A breaker on the top supplies 100A to the the buss

you have a 100A load on breaker 5, there will be 100A of current on the buss between the main and breaker 5. Breaker 9-23 have 120V potential to ground but no current.

now add a 30A load to position 7 and you have 130A load on the buss but the main trips.

add a 30PV breaker to pos 3 and now nothing trips but you have 130A current on the buss from Pos. 3 to pos. 5 (30a for pos 7 and 100A for Pos 5) No breaker trips but the buss-bar overheats catches fire buns the house down and kills the mom and her 3 children dad comes home and cries.


If you had put the PV breaker at the bottom of the panel, you would have 100A from the main to pos 5, 30A from pos 23 to 7 nothing trips, nothing catches fire, dad comes home to 3 crying kids and a wife angry at him for working late with his hot new secretary; wants a divorce, life is good.
 

Zee

Senior Member
Location
CA
We always try to land the solar breaker on the opposite side of the bus from the Main. However there are times due to wire limitations in the main that we can't put the solar breaker in the bottom slot but very near it. My chief building inspector who always passed jobs before is now telling me that the 120% rule will not apply if we can't get it into the far slot so now he wants me to downsize the Mains breaker (to 175A) for a 5KW system with a 30A solar breaker. Does this seem ridiculous? What can be done here? He will listen if I can point to code to justify my position. Also he told me that if the panel has a center fed main breaker then this 120% rule would not apply. Again we've been doing this for years with no problems.

Advise plz.

If you cannot place breaker opposite the feed.....120% allowance will not apply.
The AHJ really has been lenient prior.

BTW - downsizing the main breaker is my favorite strategy. I use it all the time.

Now hold on......you still cannot feed 30A of solar! Assuming a 200 rated panel with a new 175A main breaker. That adds up to 205A which exceeds the 100% rule.
(I, per se, don't see a big problem, with 30A + 175A = 205A on a 200A bus......if your solar breaker really is immediately adjacent to end position on bus......but it is in fact not to Code. If inspector says fine, you are still lucky.)
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Now hold on......you still cannot feed 30A of solar! Assuming a 200 rated panel with a new 175A main breaker. That adds up to 205A which exceeds the 100% rule.
(I, per se, don't see a big problem, with 30A + 175A = 205A on a 200A bus......if your solar breaker really is immediately adjacent to end position on bus......but it is in fact not to Code. If inspector says fine, you are still lucky.)

This is right. The AHJ is still being lenient with suggesting 175 instead of 150. But I'd also need more information to be convinced that it's not much easier to move the breaker to the opposite end, anyway.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
We always try to land the solar breaker on the opposite side of the bus from the Main. However there are times due to wire limitations in the main that we can't put the solar breaker in the bottom slot but very near it. My chief building inspector who always passed jobs before is now telling me that the 120% rule will not apply if we can't get it into the far slot so now he wants me to downsize the Mains breaker (to 175A) for a 5KW system with a 30A solar breaker. Does this seem ridiculous? What can be done here? He will listen if I can point to code to justify my position. Also he told me that if the panel has a center fed main breaker then this 120% rule would not apply. Again we've been doing this for years with no problems.

Advise plz.

Your inspector has been passing systems which have a code violation. Just because he used to do it doesn't mean he has to continue doing it. Maybe he was reprimanded for it.
 

pv_n00b

Senior Member
Location
CA, USA
Your inspector has been passing systems which have a code violation. Just because he used to do it doesn't mean he has to continue doing it. Maybe he was reprimanded for it.

AHJ passes something that the contractor thinks might be a code violation, either because the AHJ missed it, does not understand why it is incorrect, or just doesn't care about that one thing, etc. Contractor spends the next 30 years arguing with every AHJ that flags the same thing as being non-compliant saying the way they did it is code compliant because an AHJ approved it once. :lol: "I've been doing it this way for 30 years and you are the first AHJ to say I can't do this."
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
AHJ passes something that the contractor thinks might be a code violation, either because the AHJ missed it, does not understand why it is incorrect, or just doesn't care about that one thing, etc. Contractor spends the next 30 years arguing with every AHJ that flags the same thing as being non-compliant saying the way they did it is code compliant because an AHJ approved it once. :lol: "I've been doing it this way for 30 years and you are the first AHJ to say I can't do this."

And it's made even more complicated by the fact that every AHJ has its own special idiosyncrasies that may or may not have any basis in the NEC. We have a constantly evolving document that's now up to 5 pages long with AHJ's listed along with what makes them special.
 
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