Wire Selection

Wire Gauge

The Kyocera instruction manual suggests wire gauge from 14 to 10. I believe they limit it to AWG 10 because you just can't get a larger size to wrap around the #10 terminal screws in the connection box. In fact, its a real chore to fit AWG 10 wire on those screws.

The total length of DC wiring in my 5.4 kW system is 797 ft. The string loops range from 40 ft. to 51 ft. in round-trip length. During much of each sunny day the current in this wire will be about 6 amperes. The I2R copper loss on a cold day would be at least 73 watts for #14, 29 watts for #10, and only 13 watts for #6. I chose #6 in spite of the Kyocera recommendation.

My inverters are located very close to the PV panels that they serve. I suspect the DC copper losses for roof-mounted arrays might become pretty significant.

Wire Type

Important wire parameters for PV applications are:
UL listed type
90 degree C wet location insulation
Sunlight resistant unless in conduit

And, in my specific case, flexibility because of moveable PV panels. My research revealed the following possibilities:

THWN-2 (Thermoplastic Heat resistant Wet location Nylon jacket)
due to additional expense and labor to install required liquid-tight flex conduit and fittings, and my belief that thermoplastic insulation will deteriorate on a DC circuit in what may become a wet location.

RHW-2 (Rubber-like Heat resistant Wet location)
Sometimes this type is dual-rated as USE-2 (Underground Service Entrance). It would be usable in lighter gauges, but the coarse stranding makes the #6 size far to stiff to be usable in the confined spaces of the PV junction boxes and inverter combiner board.

TC (Tray Cable)
Though widely available and often used for PV wiring, I believe using this 2-conductor cable to be the hardest possible way to interconnect series-wired panels. Furthermore, running paired conductors through the small openings of the PV panel's sealing bushings limits the wire size.

DLO (Diesel LOcomotive Cable)
This fine quality ultra flexible wire is Class 1 stranded (61 strands of tin-coated #24 wire) with ethylene propylene rubber (EPR) insulation and a chlorosulfonated polyethylene (CSPE) jacket, sometimes identified by the tradename Hypalon. Good stuff, Hypalon - often used around here to tent water reservoirs. Most manufacturers of DLO have it UL listed to RHH/RHW and USE specifications.

But its 2,000-volt insulation made it a bit tight through the sealing bushings. I found a 1,000-volt version but the chief county inspector told me he would not approve it because the UL ratings were not printed on the jacket. This seemed to be a big deal for inspectors.

3BEVA Zero Halogen Cable
Originally developed by AT&T for use in the construction of cellular telephone block houses, this highly-specified wire is used in a wide variety of industrial applications. Like DLO it is Class 1 stranded 61/24 but with a "rope lay". The ethylene vinyl acetate insulation/jacket makes it a bit less flexible than DLO, but easier to strip. Best of all it has UL, RHH, RHW, USE, VW-1, CSA, RW-90, FT-4, SUNLIGHT RESISTANT, 105 deg C printed all over the jacket.

PostScript:  This is the wire I used. As it turned out, when my system was inspected (by both county and SDG&E inspectors) no one even looked at the wire.


Of course #6 wire must be fitted with terminals for connection to the #10 screws in the PV junction box. It is important to know that Class 1 stranded wire requires different terminals than ordinary stranded wire. Here is the UL certified combination of crimper and terminals I used - not your average Home Depot variety.