Sizing a GEC for 250.166(B)

[Lappedrider]

I am curious how others are interpreting this. Specifically what conductors should be considered part of the "system" as referenced. I am reading this as the GEC would need to be equal in size to the largest conductor used from the modules all the way to the point of interconnection, including both A/C amd D/C sides of the system.

Thanks for any thoughts.

 

[Marvin_Hamon]

My take on it is that the system in our case is the DC array and DC side of the inverter. The DC GEC goes from the inverter ground to the grounding electrode system.

250.166 says to use A or B except as permitted in C through E. I have found that one of C through E will be applicable in most installations so the size of the DC GEC does not have to match the largest DC conductor. If I had to use B then the largest conductor would be taken from the DC side and not the AC side.

 

[Lappedrider]

Thanks for the reply. Our office almost exclusively uses exception C. We are divided however on the interpretation of "system" for rule B. Half agreeing with you that it is the DC side of the array the other half referencing 690.2 and the definition of Solar Photovoltaic System as the system in question.

 

[Marvin_Hamon]

Interesting interpretation, but since the AC and DC sides are isolated I don't see it myself. 250.166(B) uses the term "system" twice, first as "DC system" then "system" and it would be hard to argue that they both do not refer to the same thing, i.e. the DC system.

If you look at Mike's Solar PV Systems book you will find that he only thinks C and D apply in off grid DC systems. So I guess only A and B apply according to him. As you can see there are multiple interpretations. Of course as a business you all have to come to accept one way to do it and do it in all your projects.

 

[Lappedrider]

Agreed, there is certainly more than one way to interpret the code. The second mentioning of system however refers to the "largest conductor supplied by the system". If the article is referencing the DC side, would not the conductors being supplied by that system be the AC side?

 

[jaggedben]

The second mentioning of system however refers to the "largest conductor supplied by the system". If the article is referencing the DC side, would not the conductors being supplied by that system be the AC side?

Hard for me to see how that could be. For one thing, theoretically a grid-tied system can 'supply' any AC conductor on the grid, so if AC is included it becomes nonsensical to decide which conductor is the largest supplied. (IOW, how literal do you want to be?)

My impression is that the intention of 250.166(B) is merely to ensure that the DC GEC can carry the same maximum amount of DC current that the system is designed to operate at (and that it be mechanically sound, hence minimum #8). I think 'supplied by' is simply being used here in the sense of 'that carries the (dc) current of'. I think it was written with DC utilization systems in mind, not (all) systems that meet the definition in 690.2.

 

[Lappedrider]

I agree. We were just trying to come to a consensus as an office and trying to look at it from all angles. I thank you both for your input. We do a good deal of inspectional work and hear all kinds of code interpretations.