Wiring Choices for PV to Combiner Box

Re: Wiring Choices for PV to Combiner Box

Yes, you only need to put breakers/fuses in the positive side. The negative circuits are just all combined together and sent down one large awg wire.

How much current in the negative lead... The sum of all of the positive (paralleled) solar panel currents into the positive bus bar to the common positive lead going back.

Regarding the voltage on the negative wire--technically, there is almost no voltage (assuming negative grounded system) other than the voltage drop caused by the current flow... Practically, the NEC (as I dimily remember) requires all of the wire insulation to be of the same rating when in the same conduit... For example, the green safety ground wire and the white neutral wire in Home AC wiring both have near ground potential--but they all must have the same 600 VAC insulation rating as the Black/Red hot wires. However, you can run a bare safety ground wire with no insulation just fine (not sure if bare wire is allowed in conduit--would probably be pretty difficult to pull in a long/curvey run).

Regarding adding, for example, 2 wires in parallel to make the next 3 awg larger equivalent wire... I don't think NEC allows you to parallel wires--and I would not do it anyway. It is very difficult to parallel wire runs and get balanced current between them... There is always an extra bit of resistance some where (plug, connector, connection, lug, wire nut, etc.) which reduces current flow in one of the parallel legs, and causes more current to flow in another leg (or, in a group, one leg has lower resistance and "hogs" more of the current flow). In either case, the P=I^2*R really kills you... Double the current flow, and you get 4x the heating effect.

Paralleling of wires is frequently done in electronic/computer equipment--and, many times, it turns out to be a real weakness for long term reliability (typically a burned contact or so out of 4+ shared current connections--and eventually all the remaining contacts burnout and the device fails).

You can "parallel" your negative (and positive) leads (use, for example, 6x 12 awg wires--one for each panel string), but do not combined them into "one circuit" until you get back to the main bus bar (where-ever that is). Then, you would run your one 4 awg wire back from there...

Or, depending on your actual current ratings, you can run 100' (completely made up numbers for example) of 6x 12 awg back to near the inverter (with fuse/breaker in positive leads back to panel), and combined into one 5' piece of 12 awg for the final run to the inverter--assuming the total currents from the 6 strings add up to less than 80% of the carrying capactiy of the 12 awg wire (typically around 20a*0.8=16a inside of conduit).

Typically in solar installations, you are designing for minimum voltage drop (vs $$$$ spent). So, for example, my two 4.5 amp strings on my 3.5 kWatts of solar panels are running on 12 awg wire for ~30' (times 2) for low voltage drop, not because I needed to run 20 amps through the wiring.

Does my "verbal" picture make sense?

-Bill