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

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david

Senior Member
Location
Pennsylvania
In Pennsylvania we are under the 2008 but we are 20 minutes from the Ohio line and we are getting a lot of questions from the guys in Ohio who work in both PA and Ohio about the 2014 code.

I have looked very closely at the language in 2008 setting article 690 aside for a minute.

I am told to do a calculation add the sum of the amp rating of the overcurrent devices suppling the panel board. Then depending on the result im told where the breaker could be placed in the panel.

Then for series rated panels it simply tells me to do the same thing with language making it clear that the value to use for the up stream panels is rating of the first overcurrent device directly connected to the output of a utility-interactive inverter

So all this section is telling me in relation to the up stream panel breakers is move the breaker to the opposite end on the buss away from the main feeder Or main over current device

So there is no language in article 705 2008 that directs me to extend the 120% rule up stream towards the service panel so look at article 690 in 2008 and the language there tells me to extend the 120% rule upstream

The language in 2008 about series panels and buses and conductors may have been there to direct you to the correct overcurrent device in doing up stream calculations. But in 2014 you place this information under a section that its sole content is about the panel that the inveter output is connected to, you are only left with common sense to apply it upstream.

Most of us would like to have something in writing when we are told the code requires it.
Anyhow you guys helped the OP on this and gave me something to think about when trying to explain to guys under the 2014 the section requires you to apply this calculation upstream

Article 690 2008
Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor. 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.

Article 705 2008
(7) Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. 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. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So all this section is telling me in relation to the up stream panel breakers is move the breaker to the opposite end on the buss away from the main feeder Or main over current device

So there is no language in article 705 2008 that directs me to extend the 120% rule up stream towards the service panel so look at article 690 in 2008 and the language there tells me to extend the 120% rule upstream

But you agree that it makes no sense at all not to, right? Otherwise my ludicrous example of installing a huge subpanel fed from a small MDP and loading the sub to the gills with PV could be compliant.
 

david

Senior Member
Location
Pennsylvania
Do you think you can backfeed 1000A into a dedicated 1000A subpanel and connect it through a 100A breaker into a 200A MDP?

But you agree that it makes no sense at all not to, right? Otherwise my ludicrous example of installing a huge subpanel fed from a small MDP and loading the sub to the gills with PV could be compliant.

You want me to make a common sense assessment on this without any NEC code guidance?
 

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
You want me to make a common sense assessment on this without any NEC code guidance?

In your diagram the 1000A of solar would trip both the 100A breakers and possibly the 200A breaker depending on timing and coordination. That clearly defies a common sense assessment, no?
 

david

Senior Member
Location
Pennsylvania
In your diagram the 1000A of solar would trip both the 100A breakers and possibly the 200A breaker depending on timing and coordination. That clearly defies a common sense assessment, no?

a thousand amps pushed through a 100 amp breaker would over load the breaker..

I was thinking the common sense assessment he was asking for was in applying the 120% rule up stream of a sub-panel
 

david

Senior Member
Location
Pennsylvania
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.

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.

Is it your understanding from the OP comments the 225 amp buss garage panel is a MLO panel
 

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Is it your understanding from the OP comments the 225 amp buss garage panel is a MLO panel
What OCPD feeds the sub from the main panel? I don't see that in your drawing.

It's possible that the main is a feed through to the MLO sub with no OCPD between them; in that case the main and the sub are essentially one panel.

But here's the thing: if you consider why the 120% rule exists and think about the vulnerability for hot spots on the busbars, it makes no sense whatsoever not to apply the rule to every series panel all the way back to the service.
 
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david

Senior Member
Location
Pennsylvania
What OCPD feeds the sub from the main panel? I don't see that in your drawing.
It's possible that the main is a feed through to the MLO sub with no OCPD between them; in that case the main and the sub are essentially one panel..

The OP indicated the service disconnect was a feed through (no means for branch breakers)

But here's the thing: if you consider why the 120% rule exists and think about the vulnerability for hot spots on the busbars, it makes no sense whatsoever not to apply the rule to every series panel all the way back to the service.

from a common sense prospective would not placing the back feed breaker opposite end of the main
or main feeder take care of hot spots on the buss.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The OP indicated the service disconnect was a feed through (no means for branch breakers)



from a common sense prospective would not placing the back feed breaker opposite end of the main
or main feeder take care of hot spots on the buss.
And since when has the NEC been about common sense.
More seriously, with opposite end placement you might expect a 200% (or more accurately a 100%/100% rule.
But the panels were never tested under those conditions (think of putting 200% worth of branch + feeder breakers in, for example), so the 120% rule was a baby step in that direction that all CMP members felt comfortable with.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The OP indicated the service disconnect was a feed through (no means for branch breakers)
That's not what I was asking. What, if any, OCPD is there between the MDP and the sub?
from a common sense prospective would not placing the back feed breaker opposite end of the main
or main feeder take care of hot spots on the buss.

The same could be said for the 120% rule as a whole; the 120% figure is completely arbitrary. But if you accept the 120% rule and wish to apply it consistently, then you have no choice but to apply it to every panel all the way back to the service.

Question: are you dealing with this stuff in the real world or are you just arguing about it for the sake of arguing? I deal with it every day in my job, and I know how the AHJ's whom I must satisfy with my designs interpret this section of the NEC. The NEC is a complex evolving document with no single author; finding inconsistencies and ambiguities in it is like shooting fish in a barrel. Designing a system utilizing such an inconsistency or ambiguity just because your interpretation of the NEC says you can is not necessarily a good idea.
 

david

Senior Member
Location
Pennsylvania
That's not what I was asking. What, if any, OCPD is there between the MDP and the sub?
As long as the Service Disconnect ( MDP) feed through breaker is sized to protect the the 225 amp MLO panel (sub-Panel) a head of the feeder supplying it, it is the OC protection for the sub
 

david

Senior Member
Location
Pennsylvania
Question: are you dealing with this stuff in the real world or are you just arguing about it for the sake of arguing? I deal with it every day in my job, and I know how the AHJ's whom I must satisfy with my designs interpret this section of the NEC. The NEC is a complex evolving document with no single author; finding inconsistencies and ambiguities in it is like shooting fish in a barrel. Designing a system utilizing such an inconsistency or ambiguity just because your interpretation of the NEC says you can is not necessarily a good idea.

I didn't know i was arguing i thought this was a discussion. yes i deal with this stuff in the real world.
we could stop discussing it if you would like.

That's not what I was asking. What, if any, OCPD is there between the MDP and the sub?


The same could be said for the 120% rule as a whole; the 120% figure is completely arbitrary. But if you accept the 120% rule and wish to apply it consistently, then you have no choice but to apply it to every panel all the way back to the service.
 

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I didn't know i was arguing i thought this was a discussion. yes i deal with this stuff in the real world.
we could stop discussing it if you would like.

I have no problem discussing it, but I am struggling to see what you are getting at. I cannot figure out what you are showing with your drawing; could you maybe draw it more conventionally showing breakers and their values where they are? I don't get the arrows; current can only flow in one direction at any one time.

Scenario: If you have an MDP with a 225A bus fed by a 200A breaker you have (1.2)(255A) - 200A = 70A electrical headroom in that panel. If a 100A breaker in the MDP feeds a 125A MLO subpanel you have (1.2)(125A) - 100A = 50A headroom in the sub.

You can land (0.8)(50A) = 40A of PV maximum current in the sub on a 50A breaker. The PV counts 50A toward the 120% rule in both the main and the sub, the PV breaker must be at the opposite end of the busbar from the feed in the sub, and the 100A breaker feeding the sub must be at the opposite end of the busbar in the MDP from its feed.

If the MDP were only rated at 200A with 200A OCP feeding it, then you would be OK in the subpanel but fail the 120% rule in the MDP: (1.2)(200A) - 200A = 40A.

Irrespective of vagaries in the NEC, that's how it works with the AHJ's around here.
 
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david

Senior Member
Location
Pennsylvania
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). Thanks!

could you maybe draw it more conventionally showing breakers and their values where they are?

I think I did show the breaker locations and values as the op described, at least i tried to.


If the MDP were only rated at 200A with 200A OCP feeding it, then you would be OK in the subpanel but fail the 120% rule in the MDP: (1.2)(200A) - 200A = 40A.

I was thinking that was the OP engineers concern. It seems the OP engineer may want the 120% rule applied to the 200 AMP SER conductors between the 200 amp feed through and the 225 amp main lug panel using the 56 amp inverter out put X 125% = 70 amp value

you seem to be saying you must apply the 120 % upstream and yet say the engineer is wrong

Our engineer is adamant about downsizing the 200A main breaker to 175 to "protect" the "bussing" going out to the utility feed.

200 X 1.2 = 240
240 - 200 = 40 amps

200- 175 = 25 amps (170 amp may have been a closer value to get to the desired protection)
40 + 25 = 65 amps
 

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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yes.
There is absolutely no need to "protect" the utility feed any more than it is, but downsizing the 200A main to 175 will remove any possible argument about the stretch of 200A SEU between main and next panel down.
And for some AHJs that is a valid concern.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I think I did show the breaker locations and values as the op described, at least i tried to.




I was thinking that was the OP engineers concern. It seems the OP engineer may want the 120% rule applied to the 200 AMP SER conductors between the 200 amp feed through and the 225 amp main lug panel using the 56 amp inverter out put X 125% = 70 amp value

you seem to be saying you must apply the 120 % upstream and yet say the engineer is wrong



200 X 1.2 = 240
240 - 200 = 40 amps

200- 175 = 25 amps (170 amp may have been a closer value to get to the desired protection)
40 + 25 = 65 amps
I said that the engineer was doing something that was not dictated by the NEC. He is not doing anything that is prohibited by the NEC. There are no loads on the SER conductors between the MDP and the service but there are loads in the MDP (I assume) between the breaker feeding the sub and the OCPD of the MDP. That's the difference and it's why the 120% rule applies.

I believe I remember seeing something way back about the engineer being concerned about a future tap of the SER conductors, and that being the reason he wants to downgrade the MDP OCPD. That's OK, of course, but not necessary under the current state of the service and system. Current can not flow in both directions simultaneously; if the loads are pulling current and the PV system is producing power, the PV can only subtract from the current through the SER conductors as long as there are no loads tapped into them.
 
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