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Grounding at detached structure, solar backfeed

azazel1024

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Jan 17, 2023
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MD
I'd appreciate any input on this one. I've been doing wiring for a lot of years, but usually stuff not terribly far off the beaten path/formulaic stuff you'll run across doing the basics like adding circuits, adding sub-panels and wiring a new main panel and addition. That isn't meant as snark, that stuff seems all very standard NEC stuff.

What I am hoping to do this fall or next spring is build a detached workshop/shed and power it with solar and battery storage. All good. What I'd like to do that is off the beaten path (at least in my mind and trying to do some research), is I'd like the ability to backfeed from the detached workshop back to my house in the event of a power outage. I am currently running a 3.2 or 6.2kw generator to a standard generator hookup and one of my two panels has an interlock on it. I've rebalanced the panels and all of my important loads are on my one panel, and the other panel is mostly the high use stuff like my range and A/C compressors as well as a few not very important 120v loads. Critical things like fridges, freezer, most lights and outlets, well pump, boiler electrical, etc. are all on the other panel.

I have to double check if both panels are bonded to neutral at the panels, or in the meter box. I think both are bonded at the panel (possibly also in the meter, I don't have access to that without cutting off the utility's lock).

How the heck should the panel at the detached workshop be setup? I plan to run back either 10ga or 8ga wire in NM conduit (for extra protection for the buried wire). Total power, I am looking at maybe 20 amps max. it'll be around 130-150ft from the workshop to the main panel, so likely 8ga.

Should the workshop panel have the neutral and ground bonded at the workshop panel? My plan was to have a switch to select between generator input or workshop input for the interlock breaker to the house. Depending on the switch, this should allow the neutral and ground from the workshop (and from the generator, though it is a portable generator, so not hooked up all the time) to always be connected back to the main panel is and its neutral to ground bond. I am aware that sub-panels are not to have neutral and ground bonded to prevent ground loops.

But how the heck do you handle that when the subpanel in question is getting its own power separate from mains? A switch that breaks neutral? A switch that breaks neutral AND ground? Not a concern at all and this is the way?

The more I've thought it over, I think this is fine and I don't want a separate ground rod at the detached workshop. I also don't want to bond neutral and ground at the subpanel (and I'd want to avoid an inverter that does this). Grounding and neutral to ground is accomplished back at the main panel. And separate power being delivered is not an issue in this scheme with only neutral and ground connected back and then bonded at the main panel.

Or is this a quick way to let out the magic smoke from an inverter and/or induce dangerous voltage on the house neutral/ground bus bars that can leak into the house circuits? I can't wrap my head around HOW though (for the latter).
 
How the heck should the panel at the detached workshop be setup?
As a standard Sub panel in a detached building.
Should the workshop panel have the neutral and ground bonded at the workshop panel?
Absolutely not.
But how the heck do you handle that when the subpanel in question is getting its own power separate from mains?
It makes no difference, which way the power is flowing. As long as the neutral is not broken (switched) at the transfer switch.
The more I've thought it over, I think this is fine and I don't want a separate ground rod at the detached workshop. I also don't want to bond neutral and ground at the subpanel (and I'd want to avoid an inverter that does this). Grounding and neutral to ground is accomplished back at the main panel. And separate power being delivered is not an issue in this scheme with only neutral and ground connected back and then bonded at the main panel.
Your thought process is spot on.
 
As a standard Sub panel in a detached building.

Absolutely not.

It makes no difference, which way the power is flowing. As long as the neutral is not broken (switched) at the transfer switch.

Your thought process is spot on.
Sorry for the long-time coming reply, thank you!

That was what my thinking was. I was thinking it shouldn't matter that the house is on mains and the detached building is on solar with neutral and ground connected back to the house main panel from the workshop sub-panel.
 
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