Please help me figure out this electrical box...

mike95490 wrote: »

The 3 links in my .sig below, have many photos

BB. wrote: »

Yes, and you need to add a green ground wire from the bottom ground bus to the ground rod/common system ground point (single point ground). Typically a 6 awg wire from ground bus to ground master ground rod.

A second 6 awg from battery negative ground common/bus to master ground rod (same rod as AC ground). This "negative ground common/bus" point is now both your DC Return common and DC safety ground green wire common connection point (basically just like you made your AC panel the common Return+Green wire ground point for all your AC power). Make sure you do not "touch"/"bond" AC and DC grounds together any place else in your system (this means your AC box and the next door charge controller sheet metal do not touch, and you should not connect Inverter Chassis to AC Panel with metal conduit)--This prevents multi-point ground connections.

If you install a additional ground rods, connect your 6 awg (minimum) cable from your "Master Ground Rod" to any additional rods (do not connect AC ground to one rod and DC ground to a second rod).

-Bill

BB. wrote: »

This part... This can be done by running a heavy ground cable (size of your DC inverter wiring) from Chassis ground to ground rod. And from DC battery - bus common to ground rod.

Or, you run heavy wire from DC Inverter Chassis to Battery Bus common ground; And a smaller (6 awg minimum) from DC battery - bus common to ground rod.

It does not matter too much where you make your single point of ground bonding--It can all be at the Master Ground Rod--Or you can create "local" common grounds (like the AC Neutral+Green Wire ground in your AC panel, and a second DC common ground at your battery negative Bus connection. And from each of those "common local grounds" run a single green wire (6 awg minimum) from the common "local ground" to the master ground rod.

The issue with mixing AC vs DC grounds is that the DC side may have 10x the current vs the AC side (example of 120 VAC vs 12 VDC inverter--10x more current on the DC side for the same power--Power=Voltage*Current)--So you have to be very careful that DC Fault Current does not somehow flow through AC grounding cable(s).

I.e., a 14 gauge wire can will have a 15 amp breaker per NEC. But the cable on the DC side of the inverter would need to be rated ~150 amps (actually, with losses and "worst case engineering" >~168 amps for a 1,200 watt AC 12 volt input inverter). You do not want a 175 Amp DC circuit to find a short circuit through your 14 AWG green wire AC grounding.

That is why you either need to keep your AC common ground "separate" from your DC common grounds. Or you need to cable (at least some) of your AC/Common grounding with much heavier cable. If you mix/match grounds, you could be forced to replace all your 14 AWG ground wires with 175 amp rated (2/O to 4/O) ground wire (again assuming a 1,200 watt 12 VDC input inverter)...

If you keep your DC side wiring separate from your AC side wiring--Then the AC side is "sized" for your 120 VAC AC currents and your DC side is sized for your ~10x larger currents.

Those two "local AC and DC grounds" are then connected to the Master Ground Rod with 6 AWG cable. Since no DC faults can flow through a separate AC power system, there is no DC Fault Current flowing through the 6 AWG cable at the ground rod (6 AWG is plenty heavy enough for most AC systems up to full grid connected AC mains).

If, for example, you connect your AC inverter Chassis Ground (which can short to the DC main input power) to the Master Ground Rod, then you need a similar sized cable from the master ground rod to the DC coming negative bus (to manage short circuit current). This may, worst case, mean you need a few feet more of the more expensive larger gauge cable.

Your first inverter was a 12 volt @ 1,500 watts--A pretty good sized inverter. I think now you are looking at a MorningStar 300 watt (600 watt for 10 minutes) which, if I recall correctly is a 6 AWG cable per their manual. So a 6 AWG DC Safety ground cable and the standard 6 AWG ground cable are the same with this smaller inverter.

300 watts * 1/0.85 inverter eff * 1/10.5 inverter battery cutoff * 1.25 NEC derating = 42 Amp branch circuit. 6 AWG is fine.

Now, if you plan on running the inverter closer to 600 watts for ~10 minutes, you might want to look at 4 AWG just to keep voltage drop lower and cabling cooler.

Sorry for the long explanation--Grounding is not simple subject. And proper grounding can be done using different methods--But each has its own "gotchas" you need to avoid.

My short answer/suggestion (after all of the above :roll:)--Create an AC main ground in your main AC panel-Connect with 6 AWG to master ground rod.

Make your DC negative battery bus your "common/local" DC ground. Run all DC safety grounds (and negative return cables) to this common/local DC ground. Then connect a 6 AWG cable from common/local DC ground to the Master Ground rod.

Any other "ground wires" (antenna grounds (that enter the home), metal water and propane plumbing, etc.) all have their single point ground connected to the master ground rod too (again, usually 6 AWG, although I have seen the phone companies use smaller gauge cable--That rule for "other grounds" would be the green wire to the ground rod should be at least as large as the cabling system you are grounding with 6 AWG being the maximum required).

When you ground your PV array (panel frames) and metal frame work). Run the 6 awg ground wire directly to the master ground rod (if pretty much a straight run down the roof and on the outside of the building to your master ground rod.

If the array is more than ~10 feet away (sideways) from the master ground rod, drive another ground rod local to the array. And (ideally) run another 6 awg ground cable from the remote ground rod to your main ground rod (ground rods are for lightning protection, 6 awg connection between local and remote ground is for electrical grounding in the case of short circuits, people nearby to avoid shocks).

Have I lost you yet--Or gone completely crazy myself and none of this makes any sense?

-Bill

BB. wrote: »

Looks perfect.

-Bill

BB. wrote: »

It is turning on for 1/2 or 1 cycle (of 60 cycles per second) looking for a load--So I could understand hearing a small tick every time it does it.

But I don't own one, so I cannot deny or confirm... I am sure somebody else can.

How about some pictures and a small write up on your system and how it is working for you (after a few weeks) and any suggestions you may have for the next person.

You are very welcome.

-Bill