Wiring questions

I'm planning on installing solar panels myself at an off-grid cabin. I have done basic house wiring, and have a couple of neighbors up there who are very capable with electrical matters, and one who has installed his own PV system.

Do I need to keep the positive and negative lengths of wire from the panels to the combiner box the same length? I may have just enough MC-4 cable to position the combiner box in a nice location under the eaves(roof mounted panels) to run the pos. of one string of panels for 27 ft and the neg 23 ft.

I plan on using 4 ga. wire to go the almost 50 feet from the combiner box to the controller. (32 amps max at 48 volts, so it should keep the voltage loss to less than 3%). Does the ground wire from the combiner box to the controller need to be 4 ga. as well?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Wiring questions

    No, there is no reason (that I can think of) why you would need to keep + and - runs the same length.

    For various reasons, it is a good idea to keep +/- wires running bundled (or in conduit) next to each other. One basic reason is to reduce the antenna effect. If you have a big loop of wires (like a circle or other arbitrary shape) and strong radio waves (like a near by lightning strike) can couple into the wiring better than if they where kept next to each other (or even twisted every few inches--which is not usually practical for house sized wiring).

    Also, if there is radio noise from the charge controller--keeping wire pairs together (and crossing AC wires at right angles) can help keep AM/FM radio interference down.

    If you have excess wire to store (panels you can move around the property, long extension cords for short distances, etc.)--wrap them in figure "8" (eights) instead of coiling them--it will help reduce RF coupling too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • albertaalberta Registered Users Posts: 10
    Re: Wiring questions

    Thanks for the reply. Can I use a smaller ground wire from the combiner box to the controller than the 4 ga. I'm planning on using for the positive and negative runs?
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Wiring questions

    The smallest recommended bare ground wire, I think, is 6 awg. Any smaller, and if run through earth, may corrode/fail too easily.

    Also, the ground wire should be heavy enough to "handle" the short circuit current.

    For example, from this NEC derived chart--6 AWG is good for at least 55 amps (more if not following NEC).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • albertaalberta Registered Users Posts: 10
    Re: Wiring questions

    We do not have iany inspections at our location, but safety is obviously important. I could use either welding cable or THHN wire for the run of 4 gauge from the combiner box to the controller, but since part of the run will go through the cabin and down through an interior wall, I presume THHN would be prerable (and likely code-compliant), whereas I wouldn't think welding cable would comply?
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Wiring questions

    Welding cable is a generic term--so while there may be some very good stuff out there--It is difficult to know if it will have problems (age hardening of insulation, long term sunlight / ozone / water / oil resistance, etc.). If you have conduit, obviously it is difficult to pull. Also welding cable is very fine copper wire and not easy to get a good connection in electrical panels not designed to hold such fine wire).

    Most likely, if you have it, it is hard to go and pay $$$ for THHN.

    On the other hand, My old 70 year old home has that rubber insulated house wiring with fabric braid--that rubber just about shatters like glass now from age. And there is my in-laws place that was wired with aluminum wiring in the mid-1970's. I can't see welding cable being any worse. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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