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I have a 12V DC system I just built (see image below), which I intend to ground to the DC negative side (see dotted green lines) but not quite sure if it's correct / best-practice.

From what I've read the general consensus for 12V DC off-grid systems seems to be that you should run a ground wire from components such as the Inverter and MPPT Charge Controller to the DC negative bus bar, and then run a ground wire from DC negative bus bar to a grounding earth point (in my case, via the grounding bus bar in my Solar Panel junction box).

I've also read that 12V off-grid / standalone systems (like mine) don't need to be grounded, but nonetheless, I'd like to understand / learn how to do it right.

Looking for any ground wiring feedback / comments please based on my diagram below.

Circiut diagram

Photograph of equipment

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  • It doesn't really matter indoors for 12 or 24v. Outside, I actually found that grounding positive reduced corrosion build up on any exposed positive connections
    – dandavis
    Commented Oct 18, 2023 at 18:55

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Firstly, you have to make absolutely sure that your equipment is designed for negative ground. Some solar equipment is designed for positive ground systems (it's cheaper, more efficient, and more robust, but it confuses people).

Secondly, it's become more common now to earth all your earths to a ground bar, and make only one connection between the ground bar and the negative bar (or positive bar). Where I live, that's a regulatory requirement.

Thirdly, you have to check to see if you are defeating a ground-fault detection system: modern equipment may be designed with the one connection between ground and supply already included.

Where I live, it used to be common to have fully floating systems, with only the case and structure connected to earth. It's now required that residential inverters have ground fault detection, so you must include the correct ground paths, but must not include extra connections between ground and supply.

Ok, that out of the way, lacking ground fault protection, if you connect ground to supply on purpose, you're in the same position as if you connect ground to supply by accident. So, no, there is no real reason to ground one side of your supply.

But the majority opinion is that, lacking ground fault protection, it's better to earth one side of supply, and least good to have a (fully floating) system that only fails on the second ground fault.

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What I'd do here

Assuming that your inverter does not supply its own GFP (this is a reasonably safe thing to assume for most UL458 RV/boat inverters, but check your inverter's manual for details!), your best (albeit not cheapest) bet is to tie the MPPT and inverter chassis grounds to the grounding bar in the combiner/disconnect (as well as the chassis of the device I'm about to describe), then install a Morningstar GFPD-150 in between the combiner box and the MPPT before tying the negative DC busbar in your system to the system grounding bus. (I'd also have it so the system grounding bus is somewhere other than the combiner box, but that's just me.)

This way, you don't get a hazardous "floating system" situation if your solar panels experience a ground fault; the panels will still be able to float to a fault potential, but they'll be all-pole (positive + negative) disconnected from the rest of the system by the GFPD, which significantly reduces the hazard posed by a fault.

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