Question re DC on NMD90 cabling

[Ambrotos]

I'm planning an off grid ~4kW solar array, and am putting together a parts list. I plan on using the appropriate outdoor-rated 8awg cable in a conduit for the run between the array itself and where the cabling will enter the house. However, the array will be at the opposite side of the house from where the inverter/batteries will be installed. There is already an unused run of NMD90 6/3 cabling running almost exactly where I need it to. It would be very convenient if I could just use this existing cable instead of having to pull new cable across the entire house.

6awg is good for upwards of 50amps, and my array will likely be in a 8s1p configuration, operating at around 13amps. So voltage and amperage are within spec for the cable, and it's not being used outdoors. Is there any other reason I shouldn't use this existing run of NMD90 for the last ~50 feet of my PV circuit?

Thanks,

-A

 

[Supervstech]

Im sure there could be some reason for issues with it, but i cant think of any.

I would use it.

 

[Supervstech]

I would want to PHYSICALLY inspect the entire run to be sure there isnt a junction box or any splicing anywhere on the cable.

 

[Ambrotos]

Sage advice. Physical inspection shouldn't be too much trouble -- it runs the length of the house through the basement above a drop ceiling to the breaker panel. I should be able to pop up enough tiles to see all of it. The previous owners used it on a separate 50a breaker for a hot tub, so it's probably a dedicated circuit, but can't hurt to make sure.

-A

 

[Ambrotos]

follow-up question... I noticed while I was doing my inspection that the printing along the length of the cable indicates it's rated for 300V. The panels I'm looking at have a Voc of ~40v, which means @ 8s I'd be running at 320v and that's without some margin for cold temperature. Is this maybe the reason not to do this we were looking for? What makes NMD90 only rated for 300v? The thickness of the insulation? Lack of metal shielding? Interference problems specific to AC? Is that just as far as they test residential cable?

 

[Supervstech]

follow-up question... I noticed while I was doing my inspection that the printing along the length of the cable indicates it's rated for 300V. The panels I'm looking at have a Voc of ~40v, which means @ 8s I'd be running at 320v and that's without some margin for cold temperature. Is this maybe the reason not to do this we were looking for? What makes NMD90 only rated for 300v? The thickness of the insulation? Lack of metal shielding? Interference problems specific to AC? Is that just as far as they test residential cable?

Residential cable has low quality insulation.
Usually pvc, and the conductors bundled together lower the voltage capabilities.

Residential voltage RMS is 240, but peak to peak is over 300V so, the ratings stop at 300.

Commercial wiring like thhn-2 has a 600V rating.

 

[Ambrotos]

Makes sense. Thanks for the background. I suspected it wasn't a conductor-related issue.

My inverter will support MPPT on up to 400V in, so my original plan was to use as high a PV voltage as I could in order to minimize line loss over the ~200ft run. Now I'm thinking maybe that's not necessary. I found a calculator and did some rough estimates and going from 8S1P to 4S2P only increases my losses from ~2% to ~4.5%. That would bring my Voc down to 160V, which I'd feel more comfortable putting across that cable.