Grounding a DC lighting system in a building served by AC

[BretHeilig]

Hi all,

I am building a very small off-grid PV array (68W) on my roof, to demonstrate that solar power works (never ceases to amaze me how many people simply do not believe this) - it will power a few LED lights. It will have a charge controller and a battery so that the lights can be used after sundown. Every current-carrying wire in the system will be sized such that it can handle perhaps double or triple the short-circuit current available in any fault situation.

Because the system involves metal parts on a roof, these must be bonded to the building electrode. Where I'm worried is, I have a vague/general understanding that the battery negative should also be bonded to ground. But is it okay to bond a battery negative to an AC grounding system? Can't a voltage develop between the AC neutral and the battery? On the other hand, could I just let the battery float? Would there be any practical disadvantage to that?

Thank you for any advice!

Bret

 

[broadgage]

If the battery is left floating, then 2 pole breakers will be required, or fuses in both poles.
If the battery negative is grounded, then fuses or breakers are only required in the positive.
Single pole breakers are cheaper and introduce less voltage drop than double pole.

The short circuit current of a PV module is only slightly more than the normal load current, therefore overcurrent protection is pointless and would never open.

The battery can supply hundreds, perhaps thousands of amps if shorted, overcurrent protection is therefore vital for anything connected to a battery.

With only 12 volts, voltage drop can be a problem.
A rule of thumb for small installations at 12 volts, avoiding calulations, is to use wire one size larger than required by code.
For a small simple system I would use a couple of 15 amp circuits, in at least #12.

Automotive type fuses and connectors are often used, but this is a violation as such are not listed for use in building wiring.
Strict code compliance is a challenge for such installations.

 

[BretHeilig]

Thanks Broadgage.

I'm still not sure if bonding the battery negative to an AC Grounding System is a bad idea on its own or not, but that's a good point about battery shorts, and a good idea for an alternative (just fuse both poles of the circuit). I'm not too worried about voltage drop; I'll just upsize the conductors, and the loads will only be a couple of feet from the battery anyway.

As an aside, 99.9% of the time it's an esoteric point (especially in a 1-string PV system), but the fuse is there not just to protect the conductor from the module, but also to protect the module from a fault that might originate somewhere else.

 

[GeorgeB]

Where I'm worried is, I have a vague/general understanding that the battery negative should also be bonded to ground. But is it okay to bond a battery negative to an AC grounding system?

This is a little off topic, but in industrial systems, we commonly (usually, almost always) connect 24VDC common, frequently identified as 0V, to chassis where L2 of our 120 control power is connected. I've never seen any problem.