120% rule from sub to service entrance w/ no breaker provisions

[deadshort]

Hi everyone,

So i got into a "discussion" with our engineer (i know that was my first mistake) about the 120% rule.

Scenario: We are installing a 100A PV sub panel being fed by a 70A breaker from a 225A panel board (inside garage) which in turn is being fed from a 200A service Entrance main (outside garage). The main is different in that it has no branch breaker provisions and merely feeders going to the 225A panel board.

From what I see, the 120% rule is good to go. 225A x 1.2 = 270A -200A =70A. We are putting in less than 60A (@ISC).

Our engineer is adamant about downsizing the 200A main breaker to 175 to "protect" the "bussing" going out to the utility feed. Huh? Am I missing something? I was under the impression that the 120% rule only applied to what is in between the solar breakers and the main, in this case the feeder wires and bus bar in the 225A panel board and sub. I attached some pictures for clarification.

Thanks!

 

[ggunn]

Hi everyone,

So i got into a "discussion" with our engineer (i know that was my first mistake) about the 120% rule.

Scenario: We are installing a 100A PV sub panel being fed by a 70A breaker from a 225A panel board (inside garage) which in turn is being fed from a 200A service Entrance main (outside garage). The main is different in that it has no branch breaker provisions and merely feeders going to the 225A panel board.

From what I see, the 120% rule is good to go. 225A x 1.2 = 270A -200A =70A. We are putting in less than 60A (@ISC).

Our engineer is adamant about downsizing the 200A main breaker to 175 to "protect" the "bussing" going out to the utility feed. Huh? Am I missing something? I was under the impression that the 120% rule only applied to what is in between the solar breakers and the main, in this case the feeder wires and bus bar in the 225A panel board and sub. I attached some pictures for clarification.

Thanks!

The 120% rule is a factor in both panels.

In the sub: (1.2)(100A) - 70A = 50A electrical space in the sub.
In the main: (1.2)(225A) - 200A = 70A electrical space in the main.

Isc has nothing to do with it; that's a DC number. If your PV breaker or 125% of your rated inverter output (depending on your code cycle) is less than 50A, you are good in both panels as long as the PV breaker in the sub is at the opposite end from the feed and the breaker in the MDP feeding the sub is at the opposite end of the MDP bus from the feed.

If you are wanting to connect 60A in the sub there isn't a problem in the main (no need to downsize the breaker) but you have a problem in the sub unless you can qualify the bus under 705.12(D)(2)(3(c), i.e. the sum of all breaker ratings connected to the bus, source and load, disregarding the OCPD that feeds the bus, is less than 100% of the rating of the bus.

 

[ggunn]

If you are wanting to connect 60A in the sub there isn't a problem in the main (no need to downsize the breaker) but you have a problem in the sub unless you can qualify the bus under 705.12(D)(2)(3(c), i.e. the sum of all breaker ratings connected to the bus, source and load, disregarding the OCPD that feeds the bus, is less than 100% of the rating of the bus.

That last part is true if you are under the 2014 NEC. If not, it gets a little stickier. For many/most AHJ's you can declare the sub an aggregation panel and put no loads in it without having to deal with the 120% rule.

 

[deadshort]

The 120% rule is a factor in both panels.

In the sub: (1.2)(100A) - 70A = 50A electrical space in the sub.
In the main: (1.2)(225A) - 200A = 70A electrical space in the main.

Isc has nothing to do with it; that's a DC number. If your PV breaker or 125% of your rated inverter output (depending on your code cycle) is less than 50A, you are good in both panels as long as the PV breaker in the sub is at the opposite end from the feed and the breaker in the MDP feeding the sub is at the opposite end of the MDP bus from the feed.

If you are wanting to connect 60A in the sub there isn't a problem in the main (no need to downsize the breaker) but you have a problem in the sub unless you can qualify the bus under 705.12(D)(2)(3(c), i.e. the sum of all breaker ratings connected to the bus, source and load, disregarding the OCPD that feeds the bus, is less than 100% of the rating of the bus.

Right, I misspoke regarding the ISC comment, I meant the 125% rule. Still less than 60A total. My main question is WHY would it be necessary to downsize the main in the service entrance if everything downstream complies with the 120% rule?
The engineer's argument is to protect the bussing going out to the utility as it is "likely" only rated for 200A?
How could this bussing ever be overloaded? If the PV is exporting power that would mean not all loads are drawing power so the utility wouldn't "push" extra amps in. If the PV is being used by all the loads and the utility is filling in the extra power needed then the PV circuit wouldn't be exporting power across the bus bar at the service entrance. Does that make any sense? Again, I am talking about the "bus bar" that is directly attached to the utility feeders going out of the meter/main.

 

[GoldDigger]

Yes, the conductors going out to the utility are not an issue as long as the PV is no more than the service rating.
But the bus between the main and the feeder breaker(s) technically is subject to the 120% rule.

 

[jaggedben]

The 'engineer' is simply wrong. :rant:

Those busbars aren't load side of the service disconnecting means, so they aren't subject to the rules in 705.12(D).
Practically speaking, there is no way that more than 200A can flow through any conductor when bottlenecked by a 200A breaker.

Note that next year when California goes on the equivalent of the 2014 NEC, the 120% rule will apply only to a panelboard with breakers at opposite ends. It will no longer be relevant to any other 'conductors' (and there will be new rules for tapping feeders at more than 120%).

If it makes you feel better, your engineer is not the only person I've heard of who has made this dumb mistake.

 

[ggunn]

Right, I misspoke regarding the ISC comment, I meant the 125% rule. Still less than 60A total. My main question is WHY would it be necessary to downsize the main in the service entrance if everything downstream complies with the 120% rule?
The engineer's argument is to protect the bussing going out to the utility as it is "likely" only rated for 200A?
How could this bussing ever be overloaded? If the PV is exporting power that would mean not all loads are drawing power so the utility wouldn't "push" extra amps in. If the PV is being used by all the loads and the utility is filling in the extra power needed then the PV circuit wouldn't be exporting power across the bus bar at the service entrance. Does that make any sense? Again, I am talking about the "bus bar" that is directly attached to the utility feeders going out of the meter/main.

Your engineer is mistaken. Tell him to read 705.12(D)(2)(3) and adhere to it. It's all there in black and white.

By the way, a common error in interpreting this stuff when there is PV in a subpanel fed by a breaker in the MDP is to think that the rating of the breaker feeding the sub somehow figures into the 120% rule in the MDP. It doesn't. Either 125% of the inverter rated output current or the rating of the OCPD fed by the inverter (again, depending on code cycle) figures into all calculations all the way back to the service.

 

[david]

(again, depending on code cycle) figures into all calculations all the way back to the service.

Today I borrowed a 2014 NEC handbook to check something since we are on the 2008 cycle here.

In 2008 there was language about panels in series “In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.”

If I understand the OP question, and I’m looking for the answer myself worded a little differently

Where in 705.12(D) 2014 does it address the (service disconnect) panel past the sub panel I’m going to read this a couple of times but I’m getting the impression that the calculations stop at the panel the inverter is tied to.

If the series information is in 2014 I would like to read it

 

[ggunn]

Today I borrowed a 2014 NEC handbook to check something since we are on the 2008 cycle here.

In 2008 there was language about panels in series “In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.”

If I understand the OP question, and I’m looking for the answer myself worded a little differently

Where in 705.12(D) 2014 does it address the (service disconnect) panel past the sub panel I’m going to read this a couple of times but I’m getting the impression that the calculations stop at the panel the inverter is tied to.

If the series information is in 2014 I would like to read it

The only thing that has changed in 2014 is that you can use 125% of the rated inverter output in your calculations instead of the OCPD rating.

 

[deadshort]

Okay, so I spoke with my engineer again. He explained to me that the purpose of downsizing the main breaker is to protect the feeder going TO the 225A panel board (not the feeders going to the utility, he said it wrong) in the event someone were to tap into this and add more loads thus exceeding the ampere rating of the conductor. I'm still a little confused as to why this responsibility falls on us but it does make a little more sense now... sorta.

This is a theoretical situation that would probably never happen but I think it is mostly a C.Y.A. provision since we were unable to confirm the size of the feeder so he assumed it was only good for 200A.

Either that or because he already ordered the 175A breaker:lol:

Anyways, looks like the code is much more clear in the 2014 version.

 

[david]

The only thing that has changed in 2014 is that you can use 125% of the rated inverter output in your calculations instead of the OCPD rating.

Trust me I’m willing to take your word for it, but if I state that I'm going to have to be able to connect the dots and show someone else where 705.12 (D) 2014 Point of connection addresses anywhere beyond (toward) the service if a load side connection is being tied to a sub panel in a garage.

With the series language absent. Where does 705.12 (D) direct me to apply calculations upstream towards other panels in series. Or am I just missing the language

 

[ggunn]

Trust me I’m willing to take your word for it, but if I state that I'm going to have to be able to connect the dots and show someone else where 705.12 (D) 2014 Point of connection addresses anywhere beyond (toward) the service if a load side connection is being tied to a sub panel in a garage.

With the series language absent. Where does 705.12 (D) direct me to apply calculations upstream towards other panels in series. Or am I just missing the language

I could not find where the code says it explicitly, but it is implicit that if an inverter feeds a subpanel it also feeds the main panel which feeds the sub.

 

[GoldDigger]

Trust me I’m willing to take your word for it, but if I state that I'm going to have to be able to connect the dots and show someone else where 705.12 (D) 2014 Point of connection addresses anywhere beyond (toward) the service if a load side connection is being tied to a sub panel in a garage.

With the series language absent. Where does 705.12 (D) direct me to apply calculations upstream towards other panels in series. Or am I just missing the language

The breaker in the main which feeds the garage sub is also a backfed breaker. The only question is what value of backfed current you assign.
Pre 2014 it is the sum of the breakers closest to the inverter(s). In 2014 it can be the sum of inverter rated output(s) times 1.25.
Pre-2014 it also arguably applies to the feeder wire going to the garage (your engineers viewpoint.)

 

[ggunn]

Okay, so I spoke with my engineer again. He explained to me that the purpose of downsizing the main breaker is to protect the feeder going TO the 225A panel board (not the feeders going to the utility, he said it wrong) in the event someone were to tap into this and add more loads thus exceeding the ampere rating of the conductor. I'm still a little confused as to why this responsibility falls on us but it does make a little more sense now... sorta.

This is a theoretical situation that would probably never happen but I think it is mostly a C.Y.A. provision since we were unable to confirm the size of the feeder so he assumed it was only good for 200A.

Either that or because he already ordered the 175A breaker:lol:

Anyways, looks like the code is much more clear in the 2014 version.

Of course, a design engineer is free to overengineer a system any way he wants for whatever reason he wants. The code only directs the minimum requirements he must adhere to. If downsizing the main breaker makes him more comfortable, so be it, but the NEC does not require it.

 

[david]

The breaker in the main which feeds the garage sub is also a backfed breaker. The only question is what value of backfed current you assign.
Pre 2014 it is the sum of the breakers closest to the inverter(s). In 2014 it can be the sum of inverter rated output(s) times 1.25.
Pre-2014 it also arguably applies to the feeder wire going to the garage (your engineers viewpoint.)

Without doubt I see that the code directs that calculation in 705.12 (D)2008 and 2014 to be applied to the sub –panel say in a garage that an inverter would be tied to.

And I clearly see depending on consumption at the garage the current will back feed the service panel and in 2008 there was language about panels in series in article 690 that talked about applying calculation to panels in series.

I need to find language in 2014 that directs me to apply those calculations to panels in series with the sub panel or I have to conclude that those calculations do not apply to any panel past the panel the inverters are tied to.

If the language is no longer there I cannot apply it based on what was there in the past. I need to find the language in the 2014 code

 

[GoldDigger]

Without doubt I see that the code directs that calculation in 705.12 (D)2008 and 2014 to be applied to the sub –panel say in a garage that an inverter would be tied to.

And I clearly see depending on consumption at the garage the current will back feed the service panel and in 2008 there was language about panels in series in article 690 that talked about applying calculation to panels in series.

I need to find language in 2014 that directs me to apply those calculations to panels in series with the sub panel or I have to conclude that those calculations do not apply to any panel past the panel the inverters are tied to.

If the language is no longer there I cannot apply it based on what was there in the past. I need to find the language in the 2014 code

IMHO there does not need to be language about panels in series as long as there is language about any panel (or conductor) containing backfeed.

 

[ggunn]

I need to find language in 2014 that directs me to apply those calculations to panels in series with the sub panel or I have to conclude that those calculations do not apply to any panel past the panel the inverters are tied to.

No, you do not have to conclude that. I certainly don't. Do you think you can backfeed 1000A into a dedicated 1000A subpanel and connect it through a 100A breaker into a 200A MDP?

Build a system that way and get into it with an inspector, and report back here how it comes out. I'll be very interested in seeing the result.

Sometimes you actually have to use common sense. Anything you backfeed into a subpanel is also backfed into the MDP.

 

[jaggedben]

Okay, so I spoke with my engineer again. He explained to me that the purpose of downsizing the main breaker is to protect the feeder going TO the 225A panel board (not the feeders going to the utility, he said it wrong) in the event someone were to tap into this and add more loads thus exceeding the ampere rating of the conductor. I'm still a little confused as to why this responsibility falls on us but it does make a little more sense now... sorta.

This is a theoretical situation that would probably never happen but I think it is mostly a C.Y.A. provision since we were unable to confirm the size of the feeder so he assumed it was only good for 200A.

Of the two or three dozen AHJs I work in, only one cares about this interpretation, and they are willing to let us put placards on the equipment that say 'Do not tap feeder.'

As a designer, knowing what the 2014 code says, I think it's really silly to incur expenses for this reason.

Either that or because he already ordered the 175A breaker:lol:

Save it for the next job where you really need it.:thumbsup:

 

[jaggedben]

The language about connections in series was removed in 2014 because, with the change to using 125% of the inverter output, the clarification about which breaker ratings to look at was no longer needed.

Personally I think that it wouldn't hurt to have some language at the beginning of 705.12, applying to all types of connections. Something along the lines of 'Where source output connections are made in series, each connection shall comply with the rules of this Article that are applicable to the location of the connection.' But for right now one just has to use common sense, per ggunn.

 

[david]

The language about connections in series was removed in 2014 because, with the change to using 125% of the inverter output, the clarification about which breaker ratings to look at was no longer needed.

Thank you, I heard discussion that the 2014 was changed to give the installer a choice between the first breaker with comes in standard sizes or the 125% of the inverter output. (continuous Duty)