I have a panel that is powered from the generator, it runs my major appliances. One of the circuits feeds my inverter/charger , which charges my batteries. A 2nd. panel receives power from the batteries, through the inverter, and powers rest of house at 110 volt. Should I keep all grounds on their own bars?.(separate from neutral) Connect ground bars from one box to other?, and run wire to ground rod outside?. I have a Trace 2.5 KW impulse correction FET power inverter, Model 2524. I use a Honda 2000 Inverter generator, or Black Max 3000 standard generator. Plug in connection runs to panel. The neutral feeds the neutral bar in the appliance panel, positive feeds the breakers, and ground feeds a ground bar. The neutral and ground are not bonded. Wire from neutral bar feeds input on charger/inverter. Positive from one of the breakers feeds the positive input, and ground from the ground feeds the inverter safety ground. Next the inverter feeds another panel. Positive from output on the inverter feeds the breakers on my other panel, a neutral feeds a neutral bar, a ground comes from the safety ground on the inverter, and feeds a ground bar. In this box the neutral bar and ground bar are bonded through a screw in the neutral bar. This panel runs my lights, TV, pump, etc..

  • Can you include the make and model of the inverter/charger?
    – Tester101
    Jan 10 '17 at 12:33
  • What make and model is your generator for that matter? Jan 10 '17 at 12:45
  • Is your generator set up with its neutral/ground bond jumper installed or removed? (Most ship with it installed per OSHA requirements for jobsite/... use) Jan 10 '17 at 23:19
  • Also, I take it this is an off-grid setup? Or are you transferring the inverter/charger AC input between generator and mains? Jan 10 '17 at 23:42

You are the electric utility -- thrice over!

Your configuration, with a generator and an inverter/charger, requires special care as you have not one, not two, but three electrical systems, all isolated from each other and completely off the mains electrical grid. This is outside the norm for a residence, and requires careful application of the NEC rules for separately derived systems starting in 250.30.

Start with the easy stuff first

The easy part of this all is making sure that all the panelboard (aka load center/electrical panel) cabinets, the inverter's safety earth terminal, and the generator frame are all solidly connected together. This makes sure you have an effective fault-current path everywhere it's needed as per 250.4(A)(5) and 250.92. These conductors can be sized as per 250.122 in your case due to 250.102(D) and this rule from 250.32(A)(2):

A supply-side bonding jumper shall not be required to be larger than the derived ungrounded conductors.

Getting down to earth (ground)

You'll need a grounding electrode, of course. This defaults to the metal-cold-water-pipe electrode (or structural steel, but you don't have that) in 250.30(A)(4), but I'd put another 250.52(A)-compliant one in to protect against plumber-induced losses of grounding, as they can leave your system completely floating which is a problem if lightning strikes nearby.

Since you have multiple separately derived systems, though, this is where things get hairball. You'll either need to:

  • Run completely separate grounding electrode conductors from the generator grounding electrode attach point and inverter AC output bonding point to each of your grounding electrode(s). This is straightforwardly permitted by 250.30(A)(6), but can lead to a proverbial spaghetti bowl of grounding electrode conductors, a minimum of four in your case, all of which must be at least as large as required by 250.66.
  • Or, run a single grounding electrode bus conductor between all the grounding electrodes and then tap it with 250.66-compliant conductors to feed the various grounding electrode attach points. This bus conductor must be enormous though, as 250.30(A)(6)(a) requires it to be a minimum of 3/0 copper or 250kcmil aluminum. That's the same size as a 200A service conductor!

In any case, you'll need to use listed grounding clamps or exothermic welds to make all grounding electrode and grounding electrode bus connections.

Bonding it all together

You'll now need to make sure that your supply bonds are in the correct places, and nothing's cross-wired between your grounds and your neutrals. This means that you will need to practice good ground bar discipline -- both of your panels must have separate ground and neutral bars, with all the neutrals going to the neutral bar and all the grounds going to the ground bar. Typical residential-sparky-in-a-hurry slobbering of everything onto one bar will not fly here!

First, the generator in your case is supplying the ground-to-neutral bond for its part of the system. This means that you must pull the service bond out of the panel for the generator-powered stuff, wherever it might be (on a modern panel, it'll be a green screw in the neutral bar). You will also need to install a ground bar kit into the panel if one is not present already and transfer all the ground wires to the new ground bar.

Second, thankfully, since you are dealing with a 24V DC system that is derived from a grounded AC system by a rectifier (battery charger), you don't need to bond the DC system to ground (as per 250.162(A) including Exception 2). I would make sure that any metallic, non-current-carrying parts that may short to the DC system, such as metal battery racks, are bonded to the generator (inverter/charger AC input) system ground, though.

Finally, since your inverter's AC output is floating, you'll need to bond it to ground somewhere as per 250.20(B) point 1. This can either be at the inverter with a jumper from the inverter output to both the inverter safety ground terminal and the grounding electrode conductors for the inverter system, or at the panelboard using the green screw or link (i.e. configuring the inverter-circuits panelboard as service equipment) as well as terminating the grounding electrode conductors onto the panelboard ground bar instead of the inverter bonding jumper.

  • I have added more on how my off grid system is wired. A ground wire comes from the generator supplied appliance panel ground bar (not bonded to neutral bar) to the inverter safety ground, then another ground wire comes off the inverter safety ground and goes to the inverter supplied panel ground bar. In this panel the ground bar is bonded to the neutral bar by a screw. Can I attach a grounding wire that goes to a grounding rod at the inverter safety ground? There is a large lug ,that is jumper to the inverter safety ground terminal at this location.
    – Joel B
    Jan 13 '17 at 3:40
  • @JoelB -- you'll want to run wires from the generator earthing terminal and the inverter panel ground bar to your grounding electrode(s). Jan 13 '17 at 4:04
  • I presently have not attached the bonding screw in the 2nd panel, If I do it will bond to the ground bar in this panel, but will also bond to the first panel ground bar through the inverter safety ground connection. This will lead to a connection of the generator neutral output (ground and neutral are bonded at generator) to the inverter output neutral in the 2nd panel at the neutral bar. Is this correct?.
    – Joel B
    Jan 13 '17 at 17:29
  • @JoelB -- that's a good point, and one I've considered briefly in the context of isolation transformers as well. It sounds worth its own question here... Jan 13 '17 at 23:08

In my case, I connect earth grounds all together between the house and the inverter. In your case, I presume that you have the generator grounded as well?

Be sure to use an outlet tester (the kind with 3 LEDs) to check that you have done it correctly.

  • This guy has 3 separately derived systems on his hands -- hooking the earth ground wires up is the easy part of his job! Jan 11 '17 at 0:02

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