120 % rule

[Eros]

I have ran into a interesting scenario with a AHJ. i have always sized my solar back-feed breaker in a residential panel based on taking the main and /or the buss rating, multiplying by 120%, subtracting the main breaker and sizing the solar breaker accordingly - (ex. : 200a main x 120% = 240a less the main breaker - 40a back feed bkr). The AHJ has informed me that this calculation is WRONG !!! The AHJ does the calculation as follows : take the back-feed breaker and multiply by 120% then add the main breaker. If his amount is the same or less than the buss/ main rating than it is acceptable. (ex : 50a back feed x 120% = 60a + 200a main = 260a , main buss rating = 225a. this will not pass inspection)
My calculation was this : main buss - 225a x 120% = 270a - 200a main = 70a available for solar back-feed
AHJ calculation : 50a back-feed x 120% = 60a + 200a main = 260a to big for buss rating, need to reduce main to 150a and back feed to 40a
Am i missing something and what code articles can i base my argument on
Thx

 

[peterlarsen.suntec]

This might be useful Eros. It helped me to get a good understanding of interconnections.

http://www.jadelearning.com/thejunctionbox/2014-nec-705-12d2-understanding-pv-interconnections/

 

[peterlarsen.suntec]

Forgot to mention...The code references are in there.

 

[texie]

I have ran into a interesting scenario with a AHJ. i have always sized my solar back-feed breaker in a residential panel based on taking the main and /or the buss rating, multiplying by 120%, subtracting the main breaker and sizing the solar breaker accordingly - (ex. : 200a main x 120% = 240a less the main breaker - 40a back feed bkr). The AHJ has informed me that this calculation is WRONG !!! The AHJ does the calculation as follows : take the back-feed breaker and multiply by 120% then add the main breaker. If his amount is the same or less than the buss/ main rating than it is acceptable. (ex : 50a back feed x 120% = 60a + 200a main = 260a , main buss rating = 225a. this will not pass inspection)
My calculation was this : main buss - 225a x 120% = 270a - 200a main = 70a available for solar back-feed
AHJ calculation : 50a back-feed x 120% = 60a + 200a main = 260a to big for buss rating, need to reduce main to 150a and back feed to 40a
Am i missing something and what code articles can i base my argument on
Thx

You and the inspector might both be right depending on where the inverter breaker is being connected to the bus. In your method the inverter breaker would have to be located at the opposite end of the bus from the main breaker. Using his method the breaker could be connected at any point on the buss. Take a close read of 705.12(D) and you will see the subtle difference in the requirements.
Not sure if you are using 2014 edition NEC, but note that in the 2014 edition you do not use the value of the OCPD as the basis for your calcs, rather you use the inverter output nameplate value. This would also have an impact on your calculations.

 

[Carultch]

In the 2014 edition you do not use the value of the OCPD as the basis for your calcs, rather you use the inverter output nameplate value. This would also have an impact on your calculations.

Yes and no.

It is true that the calculation is changed for 2014. However, it isn't the raw value for total inverter nominal current. You still multiply by 1.25, as you do for sizing the breaker. You just don't need to let rounding errors be a show stopper.

Example:
600A main
600A bus
96A of solar inverters total
125A breaker for solar

Even though 1.2*600 - 600 = 120, you still are allowed to use a 125A breaker. Provided that it is sourced with 96A or less, since 96A*1.25 = 120A.

 

[texie]

Yes and no.

It is true that the calculation is changed for 2014. However, it isn't the raw value for total inverter nominal current. You still multiply by 1.25, as you do for sizing the breaker. You just don't need to let rounding errors be a show stopper.

Example:
600A main
600A bus
96A of solar inverters total
125A breaker for solar

Even though 1.2*600 - 600 = 120, you still are allowed to use a 125A breaker. Provided that it is sourced with 96A or less, since 96A*1.25 = 120A.

I fully understand that and that is why used the word "basis" in my comments. I was not implying that he did not need to multiply by 1.25.

 

[ggunn]

I have ran into a interesting scenario with a AHJ. i have always sized my solar back-feed breaker in a residential panel based on taking the main and /or the buss rating, multiplying by 120%, subtracting the main breaker and sizing the solar breaker accordingly - (ex. : 200a main x 120% = 240a less the main breaker - 40a back feed bkr). The AHJ has informed me that this calculation is WRONG !!! The AHJ does the calculation as follows : take the back-feed breaker and multiply by 120% then add the main breaker. If his amount is the same or less than the buss/ main rating than it is acceptable. (ex : 50a back feed x 120% = 60a + 200a main = 260a , main buss rating = 225a. this will not pass inspection)
My calculation was this : main buss - 225a x 120% = 270a - 200a main = 70a available for solar back-feed
AHJ calculation : 50a back-feed x 120% = 60a + 200a main = 260a to big for buss rating, need to reduce main to 150a and back feed to 40a
Am i missing something and what code articles can i base my argument on
Thx

Section 705.12(D)(2)(3)(b) of the 2014 code is clear. 125% of the maximum inverter current plus the rating of the main breaker cannot exceed 120% of the rating of the busbar. Any calculation involving the rating of the backfed breaker is incorrect.

120% of the busbar rating minus the rating of the main breaker gives you the headroom in the panel. 125% of your maximum inverter current must be less than that number irrespective of the rating of the breaker. The rating of the breaker is normally (but not always) the next standard size up from 125% of the maximum output of the inverter.

You are correct; you can put a 70A backfed breaker into a 225A panel with a 200A main breaker as long as the breaker is correctly sized for the inverter. (225A)(1.2) - 200A = 70A.

You could put an even higher rated breaker in there as long as 1.25 times your inverter current is less than 70A and your conductors are sized such that the breaker protects them.

 

[jaggedben]

The AHJ is WRONG !!!.

(Whether you can do anything about it, well, that's another question.)

 

[texie]

The AHJ is WRONG !!!.

(Whether you can do anything about it, well, that's another question.)

Yes, likely, as most of us would likely assume a bus fed from opposite ends. But I hesitate to say that as we don't really know the exact circumstances here. That was the point I was trying to make in my previous comment on this. For all we know this bus could be a center fed in which case the inspector would be correct.

 

[ggunn]

Yes, likely, as most of us would likely assume a bus fed from opposite ends. But I hesitate to say that as we don't really know the exact circumstances here. That was the point I was trying to make in my previous comment on this. For all we know this bus could be a center fed in which case the inspector would be correct.

The inspector would never be correct in using the rating of the backfed breaker multiplied by 1.2 in any 120% rule calculations.

 

[texie]

The inspector would never be correct in using the rating of the backfed breaker multiplied by 1.2 in any 120% rule calculations.

Yes, I agree and thought that I already pointed out that both the OP and the inspector versions were flawed. I was just trying to point out the concept to the OP that the so called 120% rule does not always apply. Guess I did not express things real well though.

 

[ggunn]

Yes, I agree and thought that I already pointed out that both the OP and the inspector versions were flawed. I was just trying to point out the concept to the OP that the so called 120% rule does not always apply. Guess I did not express things real well though.

It's no big deal, but the NEC is (uncharacteristically, some might say) clear about the calculations pertaining to the 120% rule. The OP was correct in his math; 120% of the bus rating minus the rating of the OCPD protecting the bus is indeed the electrical space left for backfed PV.