Grounding Daytime Use only 24V system

The more I do research the more confused and frustrated I get. According to the NEC 2011 code, what is the proper way to ground and bond equipment for daytime use systems only? Below are the equipment sepcifications. There are no batteries, inverters, charge controllers. Its is a straight 24vdc system (really it runs at 30vdc).

(2) 235watt panels in parallel
(1) top-of-pole racking
(1) steel pole, Buried 4 feet, 6 feet above ground
Panels are connected to over current protection breaker
Panels power a small 24v BLDC pump with the use of other electronics.

Just need some clarification. Thanks,

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: Grounding Daytime Use only 24V system

    Assuming you are in lighting country... 6 AWG cable from the panel frames and metal structure to a 8-10 foot ground rod driven at the foot of the array (or attached to home ground rod/water pipe if very near array). If this is next to a well, attach a ground wire to metal well casing.

    If this is an outdoor system that does not bring power into an occupied home/office, you can stop here.

    If you get lots of lightning strikes in the area and work with/round the system, I would suggest grounding one of the PV leads (negative ground is traditional, positive ground is used in telecom systems so in ground pipes do not get corroded by stray electrical currents if there is a short to earth--see cathodic protection systems).

    If you are really worried about lightning, then I would make sure that all above ground wiring is in metal conduit and install Midnite's surge suppressors at the load's electrical panel, and probably at the array's combiner box. Where ever the surge suppressors are installed, they (and the metal electrical box, metal pump frames, etc.) should have a short 6 awg wire going to a local ground rod/well casing/etc.

    Ground wire should be kept short, rounded corners (soft ~18" radius bends, not sharp corners), not brought into the building (say roof mount solar array, do not bring ground wire down middle of building, always place lightning ground on outside wall).

    That is the short and sweet version. I am not a lighting expert--but the above are the minimum recommendations that I understood. Some locations may let you use smaller awg wire for grounding and shorter ground rods (or just attach to metallic cold water pipes, etc.). Lightning travels in the outside surface of the wire (skin effect) so too small of wire diameter is not as good. There is lightning grounding cable that is braided that is better for the job--but probably a lot more expensive. (we used flat braided cable in computer systems for even better grounding).

    In the end, lightning is a "radio frequency" type event (something like 7.5 kHz) and does not follow the "DC" wiring practices where we assume current just simply follows the wiring. "Impedance" (AC "Resistance) is very important too.

    By the way, you may not need circuit breakers if the wiring is sized correctly for the solar array (two panels in parallel do not need breakers to protect themselves either). If you have small gauge wire that leaves the may power bus, you can put fuses/breakers there).

    By the way, are you after safety, reliability, or what with the grounding requirement? Is this system out in the middle of a field, or part of a larger installation, etc.?

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