solar disconnect sizing for inverter outputs

[ocoee]

My inspector tells me that I am required to size my AC disconnect at 125% of the "calculated load" I am using a 30 amp disco to disconnect a maximum inverter output of 27 amps.

He is referencing 225.39 Here is my argument why my practice is acceptable . Please tell me I am wrong about this. Always happy to be educated.

Here is what I see….

Your starting place 225.39 "rating of disconnect" to be determined by parts 1 ,2 3 or 4 of article 220.

Article 220… 220.3 ( additional load calculation references) Application of other articles… In other articles applying the loads in specialized applications there are requirements provided in table 220.3 that are in addition to or modification of those within this article ….. "solar circuit sizing and current" 690.8

690-8 Circuit sizing and current..
Calculation of maximum circuit current. The maximum current for a specific circuit shall calculated in accordance with 690.8(A) (1) through (A)(5) (A)(3) INVERTER OUTPUT " THE MAXIMUM CURRENT SHALL BE THE MAXIMUM INVERTER CONTINOUS OUTPUT RATING."


Later in 690.9 there are specific references to "overcurrent protection" and conductor sizing A different set of calculations entirely.

I am just talking about equipment here.. Comments?

 

[Carultch]

My inspector tells me that I am required to size my AC disconnect at 125% of the "calculated load" I am using a 30 amp disco to disconnect a maximum inverter output of 27 amps.

He is referencing 225.39 Here is my argument why my practice is acceptable . Please tell me I am wrong about this. Always happy to be educated.

Here is what I see….

Your starting place 225.39 "rating of disconnect" to be determined by parts 1 ,2 3 or 4 of article 220.

Article 220… 220.3 ( additional load calculation references) Application of other articles… In other articles applying the loads in specialized applications there are requirements provided in table 220.3 that are in addition to or modification of those within this article ….. "solar circuit sizing and current" 690.8

690-8 Circuit sizing and current..
Calculation of maximum circuit current. The maximum current for a specific circuit shall calculated in accordance with 690.8(A) (1) through (A)(5) (A)(3) INVERTER OUTPUT " THE MAXIMUM CURRENT SHALL BE THE MAXIMUM INVERTER CONTINOUS OUTPUT RATING."


Later in 690.9 there are specific references to "overcurrent protection" and conductor sizing A different set of calculations entirely.

I am just talking about equipment here.. Comments?

Typically, fused disconnects are not rated for "continuous duty", and therefore need to be sized per the algorithm that sizes the overcurrent device. I.e. 1.25 * max continuous current.

By contrast, unfused disconnects typically are rated for continuous duty, which means that the 1.25 factor can be ignored for that particular piece of equipment. I.e. it is already built in to the rating of the device. So you could have an application with an 80A fused 100A disconnect, and a 60A unfused disconnect. In any case, the burden is on the manufacturer to state this in their listing and documentation, otherwise it is not rated for continuous duty and needs the 1.25 factor.

 

[jaggedben]

Tell the inspector to look at 90.3 and to take note that Chapters 5-7 modify Chapter 1-4. Also take note of 220.3 or 225.3 which refer to other articles. Thus you really only need to be looking at 690 and/or 705 for guidance here, and he should not be referencing 225.39.

Along with what Carultch said, if you have an unfused disconnect you are probably fine. A letter from the manufacturer might be needed to definitively settle the question. With that said, for a long time I didn't really understand this subject of switch ratings, and I sized a lot of 27A outputs with a 60A disco. Your inspector might not be aware of it either.

 

[Carultch]

With that said, for a long time I didn't really understand this subject of switch ratings, and I sized a lot of 27A outputs with a 60A disco.

There is a tradeoff to taking credit for a continuous duty rating. The unit will have smaller lug capacity, and this can get you in to trouble if you've had to upsize for voltage drop or intend to use AL conductors. You usually have enough for the minimum local size, but not as much margin for increasing it.

 

[jaggedben]

There is a tradeoff to taking credit for a continuous duty rating. The unit will have smaller lug capacity, and this can get you in to trouble if you've had to upsize for voltage drop or intend to use AL conductors. You usually have enough for the minimum local size, but not as much margin for increasing it.

Fair points, but I'd say if you're oversizing that much for voltage drop then the duty rating is practically a footnote; i.e. you'd better be checking your lug size anyway. The 30A discos I've got will take a #6. Maybe for bigger discos this is more of an issue, but I think for inverters that take a 35A breaker it will be okay.