Need help with NEC690.7 calculation

CVN-71

Hi, I'm going thru the "Worksheet for PV system Plan Check" and am stuck on a few things. For the NEC690.7 calculation for maximum system voltage for my array. I have 6 panels mounted in three strings of 2 panels in series. The VOC of each panel is 22.7V and -15F is min expected temp. Do I multiply 22.7x2, then times 1.13 (from NEC table)?
This would be 512V, am I wrong on this one?

Also, under the Wiring and overcurrent protection, how do I figure the:

1. maximum PV source current
2. Minimum PV source conductor ampacity
3. Minimum PV output circuit conductor amacity
4. Minimum inverter output circuit conductor ampacity

Any help or direction to formulas would be appreciated.

hillbilly

Re: Need help with NEC690.7 calculation

You're temperature corrected voltage should be 51.3V (22.7*2*1.13=51.302) As for the others:
the maximum PV source current would be the ISC*# of parallel modules * 1.25

The minimum conductor ampacity of the both PV circuits would be the max circuit current from above * 1.25 (yes you are multiplying the ISC by 1.25 twice) The conductor needs to be sized (minimum) to handle this much current, but you would also want to go further with temperature derating, and conductor fill correction factors AND account for Voltage drop which typically means upsizing a couple of gauges beyond this. Note that the PV input source current is just 1 module in parallel, and the output in your case would be 3 modules in parallel.

The inverter output circuit conductor ampacity would be the inverters rated continuous output *1.25

Some basic terms: the PV source input is basically the wire run from PV panels to the combiner boxes. These are then paralleled in the combiner box and from there to the charge controller or inverter they would be the PV output circuit (in your case 3 times the current of the PV input circuit)

The minimum conductor ampacity figure is used for sizing circuit breakers/fusing as well.

Does this answer your questions? If you don't have it already, a copy of the NEC code handbook is very helpful with lots of diagrams and examples.

CVN-71

Re: Need help with NEC690.7 calculation

Thanks! That explains and helps a lot. The checklist terms were different than those used in the PV books, so that confused me a bit.