When tying into subpanel, wHich breaker counts for 120% rule?

Zee

Senior Member
Location
CA
Hey guys,
I am tying into a subpanel with a (N) PV breaker.

Am i right in saying that - when applying the 120% rule to the MAIN service panel:
the 2008 NEC counts the breaker feeding the subpanel (or series of subpanels) as the backfed breaker,
whereas
the 2011 NEC simply counts my original (smaller) PV breaker?
 

Smart $

Esteemed Member
Location
Ohio
Hey guys,
I am tying into a subpanel with a (N) PV breaker.

Am i right in saying that - when applying the 120% rule to the MAIN service panel:
the 2008 NEC counts the breaker feeding the subpanel (or series of subpanels) as the backfed breaker,
whereas
the 2011 NEC simply counts my original (smaller) PV breaker?
IMO, 2008, 2011 makes no difference. The text was moved around but it still essentially has the same requirements. Its the sum of PV inverter output breakers' ratings plus the panel's utility supply breaker's rating.
 

GoldDigger

Moderator
Staff member
IMO, 2008, 2011 makes no difference. The text was moved around but it still essentially has the same requirements. Its the sum of PV inverter output breakers' ratings plus the panel's utility supply breaker's rating.
Whereas, IMO, for non-battery grid tied inverters, it is the sum of the feed breakers and the subpanel main that is used for the subpanel bus sizing, and the size of the actual individual breaker at the main panel which leads to the subpanel or the size of the main breaker in the subpanel (whichever is smaller) that is used in the calculation at the main panel.
For hybrid grid-tie with energy storage, it is a 125% multiple of the sum of the maximum rated outputs of the inverters (regardless of breaker size) which is used at each panel along the way.

Definitely a subject on which (even) reasonable people disagree, so I am not surprised that there are differing opinions here. :)
 

Smart $

Esteemed Member
Location
Ohio
Whereas, IMO, for non-battery grid tied inverters, it is the sum of the feed breakers and the subpanel main that is used for the subpanel bus sizing, ...
I'm okay with that... :D

...and the size of the actual individual breaker at the main panel which leads to the subpanel or the size of the main breaker in the subpanel (whichever is smaller) that is used in the calculation at the main panel.
...but where did you get this :?
 

GoldDigger

Moderator
Staff member
...but where did you get this :?
Primarily from my interpretation of 705.12(D)(1)

(2) 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.
It seems sensible (always a dangerous thing) that the applicable overcurrent device number to be used for each circuit would be the smaller/smallest of multiple overcurrent devices that protect only that circuit.

I can say that in the experience of several PV installers, inspectors seem to be applying that test.
It does get interesting when you have a 100A subpanel that supplies loads as well as serving as the PV connection. If the PV backfeed in the subpanel is two 20A breakers, no inspector that I have heard of has used the 100A feeder breaker for that subpanel in the main panel or the 100A main breaker in the subpanel when calculating the backeed amps for the main panel.
But the language certainly does not state that plainly.
 

Zee

Senior Member
Location
CA
Yes, sensible is a dangerous thing......... :)

Thanks guys. I found the answer which is exactly as Smart$ said: we can use just the smaller PV breaker amp rating (instead of the larger, subpanel feeder breaker) it is actually specifically allowed - even in 2008.
That is great news.

690.64 (B) (2)
Bus or Conductor Rating.
[.....] 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.
 

GoldDigger

Moderator
Staff member
Yes, sensible is a dangerous thing......... :)

Thanks guys. I found the answer which is exactly as Smart$ said: we can use just the smaller PV breaker amp rating (instead of the larger, subpanel feeder breaker) it is actually specifically allowed - even in 2008.
That is great news.

690.64 (B) (2)
Bus or Conductor Rating.
[.....] 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.
That is good and bad. If you follow that strictly, it means that if you have four 8A GTIs, each connected with 20A breakers into a subpanel, you will have to treat the feeder to that subpanel from the main as 80 amps even if the subpanel feeder has a 40A breaker at each end. Fortunately, again, inspectors do not seem to be doing that.
 

Smart $

Esteemed Member
Location
Ohio
That is good and bad. If you follow that strictly, it means that if you have four 8A GTIs, each connected with 20A breakers into a subpanel, you will have to treat the feeder to that subpanel from the main as 80 amps even if the subpanel feeder has a 40A breaker at each end. Fortunately, again, inspectors do not seem to be doing that.
Yeah... can be good or bad. Should be per smallest. In this case should be 40A towards main panel sum, but as the Code is worded it must be figured using 80A.
 

GoldDigger

Moderator
Staff member
Yeah... can be good or bad. Should be per smallest. In this case should be 40A towards main panel sum, but as the Code is worded it must be figured using 80A.
As it stands, the only way to recapture the difference between the output of one inverter and the next size breaker which meets the requirements (1.25 x 1.25) would be to combine two or more inverters using only switch disconnects and no OCPD, then select a closer matching OCPD as the "first" after the combiner.
But I don't think that would be acceptable given the installation instructions for most if not all inverters.
 

Smart $

Esteemed Member
Location
Ohio
As it stands, the only way to recapture the difference between the output of one inverter and the next size breaker which meets the requirements (1.25 x 1.25) would be to combine two or more inverters using only switch disconnects and no OCPD, then select a closer matching OCPD as the "first" after the combiner.
But I don't think that would be acceptable given the installation instructions for most if not all inverters.
I agree...
 

Smart $

Esteemed Member
Location
Ohio
As it stands, the only way to recapture the difference between the output of one inverter and the next size breaker which meets the requirements (1.25 x 1.25) would be to combine two or more inverters using only switch disconnects and no OCPD, then select a closer matching OCPD as the "first" after the combiner.
But I don't think that would be acceptable given the installation instructions for most if not all inverters.
BTW, the ocpds ratings for the AC side of inverters are not 125% ? 125% the rated output.... just 125%.

Also, that's not the only way. You can use industrial control-cabinet type breakers (which are available in non-standard ratings) and terminals (both DIN rail mount, for example) to make up a combiner box... that is, as long as the manufacturer's instructions don't rule it out.
 
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