120% rule 705.12(D)(1)

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I want to be sure I understand this point. Here's what I believe to be true.

With a 200A service and 200A panel/bus, it would not be possible to add any PV EXCEPT with this provision. The maximum would be 40A (200*1.2 - 200) located at the far end of the bus from the service.

With a 200A service and a 225A panel/bus, it would be possible to add a 25A OCPD for PV anywhere in the panel (the 100% "rule": 225 - 200), or 70A at the far end of the bus (225*1.2 - 200).

Please clarify any mistakes I've made.

I'm also in the midst of a knock-down-drag-out about the "bottom of the bus" point. His point is that it doesn't make sense, there's no way that position on the bus makes any difference, so long as the bus is rated sufficient.

Here's the only logic I've come up with: a panel can be (and routinely is) "overloaded", that is, due to load calculations etc. the total of load OCPD's in a panel exceed the service/main breaker. Generally that shouldn't matter, in a worst-case scenario the main breaker protects the bus from overload/overheating (due to the fact that the rating of the bus is at least as large as the main). HOWEVER: adding another supply (PV) means that any loads AFTER the PV breaker (on the other side of it from the main service) can draw from both. In the extreme case, consider putting ALL the loads after the PV breaker (so PV is at the top of the bus): they can draw up to the total of PV + service. Which, unless one is sticking with the 100% rule (no PV breaker greater than the bus rating - service), will overload the bus (in this extreme example).

That's good as far as it goes. Moving the PV to the bottom of the bus helps resolve that, but is there any particular justification to the 20% excess (that is, the 120% rule) specifically? Is it just a rule of thumb style number? I don't quite see why it's not 150%, or 200%? Is it just a safety margin thing, and is there any reason for THAT safety margin?

Thanks so much for any help getting this nailed down. I'm new to this level of analysis, so any help with concepts OR jargon/languaging :) will be appreciated. I need to be able to explain this stuff coherently. For example, in talking with another inspector, I mistakenly called the PV OCPD a "load", and he bristled, and thought it was wiser to call it a disconnect or supply OCPD so as not to confuse the issue. Helpful clarification...
 

Smart $

Esteemed Member
Location
Ohio
As best I can determine, your assessment is correct.

Why 120%, rather than some other percentage over 100%? IDK.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I'm also in the midst of a knock-down-drag-out about the "bottom of the bus" point. His point is that it doesn't make sense, there's no way that position on the bus makes any difference, so long as the bus is rated sufficient.

His point is irrelevant if he wants to conform to code. Tell him to read 705.12(D)(7).

But there is some logic to it. There is no requirement that limits the total of the ratings of the load breakers in a panel to the rating of the main breaker. For example, if the panel busbar is rated the same as the main breaker, and if the backfed breaker is mounted right next to the main, and if the sum of the load breakers is greater than the rating of the main, and if the sum of the loads on the load breakers exceeds the rating of the main but is less than the rating of the main plus the current coming onto the bus from the backfed breaker, then the current in bar between the backfed breaker and the loads will exceed its rating and the main breaker will not open.
 
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GoldDigger

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... Which, unless one is sticking with the 100% rule (no PV breaker greater than the bus rating - service), will overload the bus (in this extreme example).

Thanks so much for any help getting this nailed down. I'm new to this level of analysis, so any help with concepts OR jargon/languaging :) will be appreciated.
Here is some advice on languaging:
Usually when we refer to the 100% rule, particularly for [2014], we mean that the sum of all potentially feeding sources (main, PV, Cogen, etc.) must be less than or equal to the bus rating, not that the PV breaker(s) plus the service size be less than equal to the bus rating. The main breaker is allowed to be smaller than the nominal service size.
 
Here is some advice on languaging:
Usually when we refer to the 100% rule, particularly for [2014], we mean that the sum of all potentially feeding sources (main, PV, Cogen, etc.) must be less than or equal to the bus rating, not that the PV breaker(s) plus the service size be less than equal to the bus rating. The main breaker is allowed to be smaller than the nominal service size.

yes, thanks, I knew that but it's not what I said. Good.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I want to be sure I understand this point. Here's what I believe to be true.

With a 200A service and 200A panel/bus, it would not be possible to add any PV EXCEPT with this provision. The maximum would be 40A (200*1.2 - 200) located at the far end of the bus from the service.

With a 200A service and a 225A panel/bus, it would be possible to add a 25A OCPD for PV anywhere in the panel (the 100% "rule": 225 - 200), or 70A at the far end of the bus (225*1.2 - 200).

Please clarify any mistakes I've made.

I concur that this is 100% correct.

I'm also in the midst of a knock-down-drag-out about the "bottom of the bus" point. His point is that it doesn't make sense, there's no way that position on the bus makes any difference, so long as the bus is rated sufficient.

If the bus rating is equal to or greater than the sum of all sources of power, then he is correct. If the rating is not equal, then he is not correct and the 120% rule comes into play.

... Moving the PV to the bottom of the bus helps resolve that, but is there any particular justification to the 20% excess (that is, the 120% rule) specifically? Is it just a rule of thumb style number? I don't quite see why it's not 150%, or 200%? Is it just a safety margin thing, and is there any reason for THAT safety margin?

No particular reason for 20%. Just that John Wiles and the code making panel thought that 120% was a relatively safe and conservative number, given the fact that panels are not tested by UL for these sorts of potential overload conditions. Probably also a feeling that 20% would be 'enough' for most solar customers to cover their usage, which is maybe mostly true (but I've still felt the pain from my share of service upgrades and cancelled sales).

I've had a member of a (different) code making panel comment to me that if a customer has never had a problem with nuisance tripping of their main breaker, then it should be safe to have up to 200% of the bus rating because we know that the load has never exceeded the busbar rating. (Of course, if someone then adds some major loads thinking they have all this solar power they can use, that bet could be off...)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I've had a member of a (different) code making panel comment to me that if a customer has never had a problem with nuisance tripping of their main breaker, then it should be safe to have up to 200% of the bus rating because we know that the load has never exceeded the busbar rating. (Of course, if someone then adds some major loads thinking they have all this solar power they can use, that bet could be off...)

I think that the OP is talking about something different. The 120% he is asking about is the supply ratings to the bus, not loads on the bus. If the active loading on the bus (not the ratings of the load breakers) never exceeds the rating of the main breaker there is no reason why the sum of the load breaker ratings couldn't be 200% or even more of the rating of the main. If that is the case and everything gets turned on at once, the main breaker trips and the bus is protected. If there is a backfed breaker on the opposite end of the bus that is rated at 20% of the main and that happens, the bus is still safe because there is no load which could draw power from both supply breakers from the same direction on the bus. The 20% limit, is, of course, somewhat arbitrary.

But you knew that, right? :D
 
If the active loading on the bus (not the ratings of the load breakers) never exceeds the rating of the main breaker there is no reason why the sum of the load breaker ratings couldn't be 200% or even more of the rating of the main. If that is the case and everything gets turned on at once, the main breaker trips and the bus is protected.

But you knew that, right? :D

When you said "sum of load breaker ratings" did you perhaps mean "sum of supply breaker ratings"?

Which raises another question... Well, just an observation. That is, in principle one could imagine installing 200% of the main rating (the sum could be 300% of the main) in PV, since it could then supply a fully-loaded panel AND backfeed the full main rating for grid "storage". Though that does go a wee bit beyond the 20%... And I think anything beyond 100% of main rating risks overloading the bus if it's not bigger.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
When you said "sum of load breaker ratings" did you perhaps mean "sum of supply breaker ratings"?

I think he meant what he said. Read it again.

Which raises another question... Well, just an observation. That is, in principle one could imagine installing 200% of the main rating (the sum could be 300% of the main) in PV, since it could then supply a fully-loaded panel AND backfeed the full main rating for grid "storage". Though that does go a wee bit beyond the 20%... And I think anything beyond 100% of main rating risks overloading the bus if it's not bigger.

You really can't ever assume that loads will be consistently there, so under no circumstance could you ever have backfed supply that exceeds the utility source breaker. And a 'fully loaded' panel should not pull 100% of the main breaker... that would cause nuisance tripping. 'Calculated load' is supposed to be higher than real max load.

My guess is that for most panels the 120% rule could be the 190% rule and not be dangerous or generate nuisance tripping. (An actual 200% rule heightens the risk of nuisance tripping at max backfeed). But the code needs to cover all situations, not most situations. Older obsolete panels might not be okay. And the higher you make that number, the more critical enforcement of the 'opposite end' rule becomes. And you've got to account for people who might follow behind who are either not knowledgeable or not scrupulous.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
When you said "sum of load breaker ratings" did you perhaps mean "sum of supply breaker ratings"?

No, I meant "sum of load breaker ratings". The code does not restrict the number and ratings of load breakers in a panel to sum to less than 100% (or 120%) of the main breaker or busbar ratings, in part because of the assumption that all the load breakers will not be fully loaded simultaneously. If they are and they are, then the main will trip, anyway, so there is no case where the bus can be overloaded.

Which raises another question... Well, just an observation. That is, in principle one could imagine installing 200% of the main rating (the sum could be 300% of the main) in PV, since it could then supply a fully-loaded panel AND backfeed the full main rating for grid "storage". Though that does go a wee bit beyond the 20%... And I think anything beyond 100% of main rating risks overloading the bus if it's not bigger.

Irrespective of whether or not you or I think a specific system could ever overload the bus, the code DOES restrict the sum of the ratings of breakers feeding a bus to 120% of the bus rating, and it directs that if the rating of the main breaker plus that of a backfed breaker total to between 100% and 120% of the rating of the bus, they must be placed on opposite ends of the bus. Most AHJ's extend this rule to require that only one backfed breaker be allowed in a panel with a main breaker and load breakers.

I have heard that the 2014 NEC backs off this rule a little to consider only the maximum current deliverable by a utility interactive inverter rather than the rating of its backfed breaker to calculate conformance to the 120% rule, but I haven't got my 2014 copy yet.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
When you said "sum of load breaker ratings" did you perhaps mean "sum of supply breaker ratings"?

No, I meant "sum of load breaker ratings". The code does not restrict the sum of the ratings of load breakers in a panel to less than 100% (or 120%) of the main breaker or busbar ratings, in part because of the assumption that all the load breakers will not be fully loaded simultaneously. If they do and they are, then the main will trip, anyway, so there is no case where the bus can be overloaded.

Which raises another question... Well, just an observation. That is, in principle one could imagine installing 200% of the main rating (the sum could be 300% of the main) in PV, since it could then supply a fully-loaded panel AND backfeed the full main rating for grid "storage". Though that does go a wee bit beyond the 20%... And I think anything beyond 100% of main rating risks overloading the bus if it's not bigger.

Irrespective of whether or not you or I think a specific system could ever overload the bus, the code DOES restrict the sum of the ratings of breakers feeding a bus to 120% of the bus rating, and it directs that if the rating of the main breaker plus that of a backfed breaker total to between 100% and 120% of the rating of the bus, they must be placed on opposite ends of the bus. Many AHJ's extend this rule to require that only one backfed breaker be allowed in a panel with a main breaker and load breakers.

I have heard that the 2014 NEC backs off this rule a little to consider only the maximum current deliverable by a utility interactive inverter rather than the rating of its backfed breaker to calculate conformance to the 120% rule, but I haven't got my 2014 copy yet.
 
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GoldDigger

Moderator
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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I have heard that the 2014 NEC backs off this rule a little to consider only the maximum current deliverable by a utility interactive inverter rather than the rating of its backfed breaker to calculate conformance to the 120% rule, but I haven't got my 2014 copy yet.

The online public free access to the 2014 is a royal pain presumably designed to be unusable, but as several members have pointed out, if you use the Public Input (formerly Proposals) interface you can actually navigate, search and even cut and paste freely.
 

Smart $

Esteemed Member
Location
Ohio
The online public free access to the 2014 is a royal pain presumably designed to be unusable, but as several members have pointed out, if you use the Public Input (formerly Proposals) interface you can actually navigate, search and even cut and paste freely.
I believe one has to get into the 'edit' stage to cut or copy.
 
I think he meant what he said. Read it again.

Gah! Yes, sorry.


You really can't ever assume that loads will be consistently there, so under no circumstance could you ever have backfed supply that exceeds the utility source breaker. And a 'fully loaded' panel should not pull 100% of the main breaker... that would cause nuisance tripping. 'Calculated load' is supposed to be higher than real max load.

My guess is that for most panels the 120% rule could be the 190% rule and not be dangerous or generate nuisance tripping. (An actual 200% rule heightens the risk of nuisance tripping at max backfeed). But the code needs to cover all situations, not most situations. Older obsolete panels might not be okay. And the higher you make that number, the more critical enforcement of the 'opposite end' rule becomes. And you've got to account for people who might follow behind who are either not knowledgeable or not scrupulous.

Right. Makes sense. Thanks.
 

Zee

Senior Member
Location
CA
Yes to your original Q's.

One caveat: 40 amps PV max IF you do not reduce main breaker rating.
(I am currently reducing main breaker from 200A to 175A. That gives me 25 plus amps = 65 A.)

Logic of "opposite feed" requirement?
Current thru main breaker will cancel out current flow from PV if located at opposite end. At no point on bus will it exceed rating.
However, if main and PV are "co-located on bus" then the currents they SUPPLY are additive ===>danger in overloading bus.

(Apologies if repeat of another's explanation.... i couldn't read all of above posts...:weeping: )
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yes to your original Q's.

One caveat: 40 amps PV max IF you do not reduce main breaker rating.
(I am currently reducing main breaker from 200A to 175A. That gives me 25 plus amps = 65 A.)

Logic of "opposite feed" requirement?
Current thru main breaker will cancel out current flow from PV if located at opposite end. At no point on bus will it exceed rating.
However, if main and PV are "co-located on bus" then the currents they SUPPLY are additive ===>danger in overloading bus.

(Apologies if repeat of another's explanation.... i couldn't read all of above posts...:weeping: )
You did, but phrased a bit differently. No biggie.
 
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