60 Amp AC combiner panel/feeder to 200A loadcenter

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tortuga

tortuga

Code Historian
Location
Oregon
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Electrical Design
Greetings all I was out to look at a enphase AC solar system about to be installed on new home, the electrician wiring the house (not affiliated) ran a 6/3 from the service to the enphase combiner panel (3-ES i believe)
And landed a 60A breaker at the bottom of all the installed breakers but not the bottom of the loadcenter.
This got me thinking the 60 amp feeder from AC combiner panel is a "power source output circuit" and the breaker needs to comply with 705.12(B)(3) right?
So for a 200A panel the max size breaker is 40 amps and the breaker needs to be moved the the opposite end of the bus from the main right?
The actual output from the microinverters is less than 5kw, well below 40 amps.
 
The breaker size is not regulated, just 125% of the sum of the microinverter output currents. But yes, every panel between the microinverters and the utility needs to comply with 705.12(B)(3).

Cheers, Wayne
 
Well just an update on this, I did two jobs with two different inspectors one passed one failed (different AHJ's), the one that failed the inspector is arguing that the 60 amp breaker violates 705.12(B)(3)(3). I just re-read the section and it does say output current rating (at 125%) and the rating of the overcurrent device protecting the busbar. So ill have to convince him that the 20 amp PV breaker in the SMA combiner is the OCPD thats protecting the busbar not the feeder breaker.
 
tortuga said:
So ill have to convince him that the 20 amp PV breaker in the SMA combiner is the OCPD thats protecting the busbar not the feeder breaker.
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Huh? The feeder breaker is the one protecting the busbar (from the utility source). The sum of that rating and 125% of the total microinverter continuous output ratings has to be less than 120% of the busbar rating. The 60A rating on the PV breaker doesn't matter.


Cheers, Wayne
 
wwhitney said:
Huh? The feeder breaker is the one protecting the busbar (from the utility source). The sum of that rating and 125% of the total microinverter continuous output ratings has to be less than 120% of the busbar rating. The 60A rating on the PV breaker doesn't matter.


Cheers, Wayne
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Woops I meant he is quoting 705.12(B)(3)(2):
Code: 
Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar.
Saying both the 125 percent of the power source(s) output circuit current and the OCPD's protecting the busbar
 
tortuga said:
Saying both the 125 percent of the power source(s) output circuit current and the OCPD's protecting the busbar
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Right, so, I'm not following what you're saying the inspector is saying the problem is.

In the OP, you said the total microinverter output (should be based on maximum output current) is less than 5 kW, so say it is 20A. Then the computation would be:

200A (breaker protecting the busbar) plus 125% * 20A (power source output circuit current) = 225A < 240A = 120% * 200A (busbar rating).

The 60A PV breaker rating doesn't come into it.

One possible way to misread that is that "Power source output circuit" is a defined term at the beginning of Section 705. So "power source output circuit current" is "(power source output circuit) current," the actual (maximum) current put out the by the power sources. But you might misread that as ". . . (circuit current) . . ." and think it means the rating of the circuit. 705.28(A) clarifies this.

Cheers, Wayne
 
Or are you saying that the phrase "the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar" is being read to mean 125% * (power source output circuit current) + 125% * ( rating of the overcurrent device protecting the busbar)?

It is true that the isolated phrase "125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar" could be read as 125% (power source output circuit current + rating of the overcurrent device protecting the busbar). But in the full phrase that's obviously not the case for two reasons:

(1) "the sum of" is a phrase that expects at least two objects, "the sum of A and B". So the the word "and" is separating those two summands, and the modifier 125% percent therefore applies only to the first summand, not both summands.

(2) That interpretation would render the rule useless, as for the simple case of an OCPD protecting a busbar at its rating, 125% of the OCPD will never be less than 120% of the busbar rating, let alone after adding in 125% of the power source output circuit current.

Cheers, Wayne
 
Be it correct or incorrect I think many inspectors will be looking for a 40 amp breaker.
 
phipsi1237 said:
Everything I read outside actual NEC text, says 40 amp max added on a 200 amp panel unless you derate it.
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Probably a lot of what you read is out of date for the current version of the NEC. The rules were expanded in the 2014 NEC which means most places adopted the changes around 2017 and a few still haven't adopted them.
 
phipsi1237 said:
Everything I read outside actual NEC text, says 40 amp max added on a 200 amp panel unless you derate it.
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The most inverter current (as referenced by the published maximum current for the inverter(s)) one can add to a 200A busbar with a 200A main breaker is 32A.
 
ggunn said:
The most inverter current (as referenced by the published maximum current for the inverter(s)) one can add to a 200A busbar with a 200A main breaker is 32A.
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For one of the 3 rules for protecting busbars that are capable of supplying multiple branch circuits or feeders.

For one of the others the maximum is 0A (so not a particularly useful rule in this case). And for the last one, the maximum is 160A, which would require the busbar to have no other breakers.

Cheers, Wayne
 
wwhitney said:
For one of the 3 rules for protecting busbars that are capable of supplying multiple branch circuits or feeders.

For one of the others the maximum is 0A (so not a particularly useful rule in this case). And for the last one, the maximum is 160A, which would require the busbar to have no other breakers.

Cheers, Wayne
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Yes, of course, but that would have to be in a panel that is, obviously, not an MDP for a service. We use that option for AC inverter combiner panels but it still leaves the problem of how to interconnect it.160A would not be less than 20% of any residential service that I have heard of, so that panel could not be a residential subpanel. It would have to be supply side interconnected.
 
ggunn said:
Yes, of course, but that would have to be in a panel that is, obviously, not an MDP for a service. We use that option for AC inverter combiner panels but it still leaves the problem of how to interconnect it.160A would not be less than 20% of any residential service that I have heard of, so that panel could not be a residential subpanel. It would have to be supply side interconnected.
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The third rule, the 'sum of all breakers' rule, occasionally works for a residential MDP.
 
wwhitney said:
So it's a potentially useful, if rare, exception to the "rule" stated in posts 10, 11, and 13. Which was my point.

Cheers, Wayne
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Um, well, OK but it isn't a "rule", it's a rule, one of six in the latest rev.

I assure you, I am very familiar with all the subsections of 705.12(B)(3), and I have used all of them except for the new bits about centerfed panels.
 
ggunn said:
Um, well, OK but it isn't a "rule", it's a rule, one of six in the latest rev.
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I would say the rule is that you must comply with one of the six options. So while one option gets used the most, it's the not the full rule, and shouldn't be presented as the only option. Doing so leads to issues such as mentioned in post #11.

Cheers, Wayne
 
wwhitney said:
I would say the rule is that you must comply with one of the six options. So while one option gets used the most, it's the not the full rule, and shouldn't be presented as the only option. Doing so leads to issues such as mentioned in post #11.

Cheers, Wayne
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Well, I guess you win the "argument". :D
 
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