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I recently had some electrical work done on my home to bring it up to code - there was no GFCI protection in the kitchen or bathroom when we bought the house, so our electrician fitted GFCI breakers at the panel. He did not use the GFCI outlets because he explained that our existing outlet boxes were too small to fit the extra size of a GFCI.

When I plugged in my generator to my transfer switch though, and flipped the main breakers in the transfer switch from LINE TO GEN, the GFCI breakers in my main service panel immediately tripped.

I have read that this may have something to do with the fact that most portable generators come with the ground + neutral bonded to the frame. Apparently, this makes the generator a SDS (separately derived system) compliant with OSHA for use on job sites. I can confirm that my generator is like this. It says so right on the power panel.

However, when connecting to the house, I have heard that the ground + neutral are actually already bonded together in the panel, so you should "float" (disconnect) the neutral wire at the generator. However, I have also heard this increases shock risk on the generator side.

I am not positive that this is why the GFCI breaker in my main panel is tripping, but it's a hunch. It could just be that the breaker doesn't like the power signal coming from the generator.

Regardless, I'd still like to have power to my kitchen from the gen.

Any suggestions as to why this could be happening?

  • Additional info: The generator has four 120V GFCI receptacles. It also has an L14-30R 30A twist lock connector that I use to power the house - however, this receptacle does not appear to be GFCI protected. – rk15000 Jan 18 '18 at 15:08
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    You haven't given us enough information about how the circuit is configured. Particularly we'll need a lot more about the generator transfer switch. Neutral at the generator needs to be part of rhe circuit. You have more flexibility whqt to do with ground. – Harper - Reinstate Monica Jan 18 '18 at 15:54
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That's to be expected. The neutral from the circuit is going through the GFCI breaker, but the hot is coming from the generator. The GFCI sees this as an imbalance, since there's no current flowing through the hot side of the breaker.


Here's what your setup likely looks like (if it was drawn really quickly in MSPaint).

Bad MSPaint Generator Transfer Switch diagram

If you follow the current through from the generator, you'll see that it returns through the GFCI breaker.

Bad MSPaint Generator Transfer Switch diagram showing current flow

However, since the current is not flowing through the "hot" side of the breaker. The breaker sees an imbalance, and trips.


The cutaway in this image shows the internal circuitry of the GFCI breaker, and depicts the current flowing only on the neutral wire.

Bad MSPaint Generator Transfer Switch diagram showing current flow and CT in breaker


WARNING:

If the generator or transfer switch does not provide GFCI protection, the kitchen will not have GFCI protection when on generator power.

  • Thanks for the detailed drawing. So based on what you know, what is the least risky option? To let the breaker trip or remove the neutral bond on the generator? I can't imagine the transfer switch provides GFCI protection (it seems kind of old) - the only outlets on the genny that appear to be GFCI protected are the 120V, not the 240V. So i would imagine that yes, my kitchen is powered but not protected via GFCI anymore when on generator power. – rk15000 Jan 18 '18 at 17:58
  • @rk15000 Removing the neutral bond on the generator will not change anything. – Tester101 Jan 18 '18 at 19:10
  • Tester, the way it's physically wired is a huge hank of hot wires from xfer switch to breaker panel. Current goes from (GFCI) breaker "hot" to the transfer switch, through one leg of an SPDT, through the common, back to the panel, then out on the branch circuit wire that used to land on the breaker's "hot" wire. The gen plugs into the xfer switch. In gen mode, the SPDT is thrown to gen side, and neutral for all circuits travels via the one neutral wire from xfer switch to panel. It's very, very ghetto. – Harper - Reinstate Monica Dec 21 '19 at 18:46
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The decision to use a GFCI breaker was a correct one.

The problem is this style of transfer switch is about as "Bangkok wiring" as you can get - the worst series of code violations I have ever seen in a consumer product*. In modern panels with modern needs, they're a very bad fit, and this is a case in point.

These days, hot and neutral need to be handled together on a per-circuit basis. This thing doesn't do that, and that's the problem.

A better transfer switch (cheaper, too)

Obtain another service panel. Get enough spaces for every circuit you might conceivably ever want on generator or PowerWall + 4 more spaces. Spaces are good. 40 spaces is not excessive. Prefer the brand and type that matches the GFCI breakers you already have, because then you won't have to replace them (ouch)!

Then, add a generator interlock to it. This is a moving piece of metal that prevents two double breakers from being on at the same time - a regular double breaker, interlocked with either the main breaker or another double breaker.

Siemens makes the cheapest generator interlocks, Square D "QO type" is the runner-up, both for main-lug (no main breaker) panels. Code requires the interlocked breakers be bolted down (Siemens' kit does this). Some jurisdictions require it work with the panel cover missing (which eliminates some types).

Shop carefully for your panel and interlock, balancing cost of interlock vs cost of not being able to reuse breakers. Look at the breakers, not the panel: if you have alien HOM breakers in a GE panel (bad), look at HOM panels not GE.

Installing

Link it up with the original panel through several rigid metal conduit pipes. If they're within 24" you only need 1-2 large ones.

Install the generator interlock. If this panel has a main breaker, reserve that for utility. Make the interlocked breakers large enough to supply the subpanel, but any size you please - they are just switches and their breaker ability won't be used. 60A breakers are the cheapest.

Come off your main panel with thru-lugs, subfeed lugs, or an actual breaker. Use wire big enough to be safe at the max possible amperage. For instance if you lug off a 200A main panel to a 200A sub, your max current is 200A so the wire must be 3/0 copper or 4/0 aluminum. It's fine to have a 200A, 3/0 wire feeding a 150A subpanel breaker, as long as the wire physically fits; you can't whittle it down.

Move branch circuits (hot(s) AND neutral) over to the new panel, either by moving their supply Romex to the new panel, or extending hot+neutral into the new panel via the conduit pipes.

  • There is no limit to how many circuits you can move to this panel (hence my reco for a 40-space).
  • No spaghetti. The interior of this panel will look exactly like a normal service panel.
  • It does the right thing with the neutrals, so happy GFCIs.

  • This setup is future-PowerWall-friendly.

  • It is also friendly to someday deprecating your original main panel and making this panel your new main.

I'm not really addressing removing the neutral-ground bond on your generator. This type of installation renders that a non-issue operationally but it's still a Code issue.

Operation

To switch, shut off all your breakers. You generally want to switch no-load or low-load. Then shut off the present source, then switch on the desired source. Then turn on breakers of the things you want to power.

Why does this play nice with GFCI? Because you're making the generator selection before the GFCI breaker. In both modes, the GFCI breaker pulls hot off the bus and neutral off the pigtail, and supplies protected hot+neutral to the whole circuit.




Footnotes


* You couldn't hand-build a transfer switch like this, you'd get a sea of red-flags. I suspect it's in a gray area between what the UL White Book allows in an appliance and what NEC allows in field wiring.

The general concept of these transfer switches is there is a row of SPDT switches. One side taps the generator hot, the other side goes to the panel and taps a circuit's hot off the breaker, and the "common" goes back to the panel and joins the hot wire that used to go to the breaker. There is one single neutral wire that handles all generator current. That's ... ghetto, but workable for a 1960s panel. Here's what it looks like inside.

enter image description here

Look, ma, no neutrals!

enter image description here

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