Grounding Lug on MPPTs

[WorldwideDave]

I have an off grid setup, panels up on unistrut, battery box made of wood that also has inverter, 3 MPPTs, 2 bus bars, 1 kill switch, 1 t-class fuse, and 2 shunts mounted to it. The MPPTs, panels, and the unistrut are not 'grounded' that I know of.

THe only problem I'd like to solve is where is the ground for the MPPT supposed to connect to? Noticed larger MPPTs have grounding lugs. I have a 250/70 and FM80 for example (totaling 150A max charge in perfect conditions). I see various recommendations on what to ground it to, but it is not a vehicle so no chassis ground. Just a wood box. Should I be grounding to the bus bar, or put a connector on the unistrut and connect to that, or get a new 4 post bus bar just for those grounds (2 MPPTs, 1 inverter, and a wire to the unistrut)? Open to suggestions. Does not have to be perfect. Just seeking options for the MPPTs.

I will do panels to unistrut as well. Unistrut attached to a cement wall with rebar in it - hoping that will carry any potential shock.

 

[robbob2112]

On a permanent install-

Your house has a ground rod
That is connected to the main electrical panel, in that panel there is a N-G bond
That is in turn connected to all the outlets and everything in the house.

This makes up your house grounding system.

Your inverters, mppt, and any other device with a case ground connects to this ground system. You just need one wire to where that stuff is and you can daisy chain from there. So, all the cases connect together and if there is a metal battery rack it gets grounded. You also take an EGC from the mppt to your panels and attach that to the frames. The metal mounting and unistrut also gets attached to the EGC. There are special unistrut and frame connectors that cut into the metal verse the normal clamps that don't cut through the coating.

 

[WorldwideDave]

On a permanent install-

Your house has a ground rod
That is connected to the main electrical panel, in that panel there is a N-G bond
That is in turn connected to all the outlets and everything in the house.

This makes up your house grounding system.

Your inverters, mppt, and any other device with a case ground connects to this ground system. You just need one wire to where that stuff is and you can daisy chain from there. So, all the cases connect together and if there is a metal battery rack it gets grounded. You also take an EGC from the mppt to your panels and attach that to the frames. The metal mounting and unistrut also gets attached to the EGC. There are special unistrut and frame connectors that cut into the metal verse the normal clamps that don't cut through the coating.

For off grid, meaning that the panels, strut, MPPT, inverter, plastic battery box sitting on wood shelf that are 35 feet from the house panel and ground? I have a copper pool pipe going through the cement. Can that be my ground and connect it all to that?

 

[robbob2112]

Everything i said still applies if you have a grid connection.

If you are collecting solar and storing in battery then using it later, or just plain inverting it and feeding the power to a critical load panel... everything still ties to the ground system at the main panel a d to that main panel ground rod.

Running off-grid with a grid connection means you are zero-export with a grid interactive inverter. This means it needs anti-islanding so it doesn't burp to the grid while the grid is down.

Huge note - if you have an on-grid inverter and set to zero export it WILL still export little burps of power to the grid that are detectable by the poco.
You will need an interconnect agreement to cover these burps

And, yes, the poco can detect the burps of power that are a fraction of a second and are only a few hundred watts... even with the old style meters unless they are super old ones that are just a rotating disc... even the newer ones of those can catch it and will report there was an attempt to run it backwards. You never know until you get the huge fine.

If you have an off grid inverter that can pull power from the grid to charge your batteries you are an island and you don't need an interconnect agreement unless your AHJ requires one because you are installing panels.

So, for the first you can't avoid the interconnect agreement and for the second you would use a L1, L2, N, and G from the main panel and that wire would be hooked to the grid side and the load side would have your critical panel on it.

For the question on using the pipe by the pool.... no, the EGC is required to be in the same conduit as the current carrying PV wires.

Make sense?

Bonus answer - the best thing is an auto-transfer switch. This switches the hots... the ground stays connected always... what happens to the neutral depends on if the switched item is a generator with a N-G bond in it. In that case the switched N is not connected.... so the N-G bond is done in the generator and the critical load panel is disconnected from the main panel.

I am not sure which wires are switched with a solar system connected.

 

[WorldwideDave]

Maybe I’m not saying anything correctly. There is a pool. Everything for the pool goes into a little junction box. Two wires leave the junction box and go to my inverter. The inverter connects to the battery. Never at any point is the pool pump or pool heater connected to the house system. Neither are the solar panels. Neither are the batteries. I must not be understanding something.

The only time there has been something you could call a great connection is when I ran a 50 foot extension cord from an outlet on the side of the residence to a lithium battery charger that is 15 A, put two clamps on top of the battery to wake up The BMS, and once the sun came out, I disconnected the clamps and put away the charger and the extension cord.

 

[robbob2112]

Gotcha

In that case you can use the separate ground if you are certain it is a good ground. The N-G bond must exist in the inverter, this can be software that controls a relay... I prefer to disable that and manually tie it together with a physical ground screw.

That ground ties to the inverter ground screw, the mppt ground screw, panel frames, and any other equipment cases. You want every piece of metal that connects to or touches the pool tied to that ground so it is all the same potential.

NOTE: check the rules for electricity around the pool with your AHJ.

If you ever connect the house ground to the pool ground it will make a loop and possibly cause issues.

If you need to run an extension cord to charge a battery that is fine as it will only connect to the battery with dc.