I recently had a new furnace installed, and had the electrician add a Reliance TF151 transfer switch so I can power the furnace from an portable generator during a power outage (yes, inverter generator and low power draw gas fired boiler). I'm interested to know the power requirements of the furnace to see if I can also run from a 200W battery pack. My plan was to set the transfer switch to Generator, and then plug an extension cord from a house outlet, through a Kill-a-watt energy meter, and then into the transfer switch, essentially using my house wiring to stand in for the generator. However, instantly when I plugged the transfer switch into the outlet the GFCI breaker on my house outlet tripped. This happened whether the transfer switch was set to Line, Disconnect, or Generator.

Looking at the wiring in my breaker box shows what I've sketched out in the diagram below. Everything makes sense, except wiring the white neutral from the transfer switch into the green case ground bus. This seems to also go against the TF151 install instructions that say:

Strip approximately 5/8” from the end of the white wire. Locate the neutral bar and partially unscrew a terminal screw on the bar. Insert the stripped end of the wire into the side of the bar under the screw and retighten the screw.

So my quesitons are:

  1. Does this wiring look correct, is there a good reason why the electrician chose to wire the transfer switch white to case ground green? Should I get someone in to move the transfer switch white to the white neutral bus of the load center?

  2. Does this white to green wiring explain why I'm tripping my GFCI breaker on the house outlet when I try to plug it in? In this configuration is it safe to just plug into a non-GFCI outlet to perform my wattage test? Will I pop the breaker on my generator like I was my house GFCI, or is the simple overload breaker going to be a much dumber beast than the GFCI?

  3. I've also seen another stack exchange post suggesting that Kill-a-watt meters won't work on a transfer switch configuration: Testing a furnace transfer switch, watt meter shows extremely low usage. Is there another way to measure the furnace usage, clamp meter on the red wire coming off the breaker?

wiring diagram Full panel picture Ground bus

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    First of all, this needs to be 3 separate questions. This single post is too long and convoluted to answer based on one diagram of a panelboard. For your first question, it won't be possible to answer without first seeing a photo of the real panelboard, or a description of the ground and bonding system, whether or not it's separated from the neutral bus, and any other relevant info. For the second question, you need to explain why the outlet you're using is on a GFCI circuit breaker, with photos preferably, and is it or is it not the same circuit as the furnace and/or transfer switch? Sep 29, 2022 at 14:26
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    @RobertChapin, glossing over splitting the question... added photos of the panelboard, which I omitted because, god help us it's messy and complicated. Case ground is in the lower left with all green/bare wires, plus the single white that concerns me. All other white wires connect to bus bars on the middle sides behind the breakers. Regarding the ground and bonding system, if I could reliably describe those I probably wouldn't be asking the question here. I know enough to annoy an electrician and not lick things in the panel.
    – A Holman
    Sep 29, 2022 at 15:00
  • @RobertChapin, regards to why and what GFCI outlet. First because that was what's available in my basement nearby, and second because when doing anything slightly sketchy I like to run though a GFCI for superstition. The outlet I chose is on a different circuit from furnace, and is an outlet with the built-in GFCI breaker. The meat of my second question boils down to, is the GFCI being over-twitchy and I'm fine to plug into a regular outlet, or is it tripping because there's some larger problem in the wiring scheme and plugging into a less twitchy outlet would be bad.
    – A Holman
    Sep 29, 2022 at 15:05
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    It is hard to tell what is going on from the images (hard to trace some of the wires), but if your diagram is correct, but the neutral on ground bus seems very wrong here since this is that the service disconnect enclosure where that could be tolerated. I fully agree that should be moved. I'm not sure that the GFCI problem is related. Sep 29, 2022 at 16:58
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    If you are measuring using current clamps, then putting it on the hot wire leading to the furnace is fine. That kill-a-watt test being weird had to do with how those meters work, not current clamps. Also the test procedure they were using in that question was a code violation as it allowed for two neutral return paths. (One via the panel, the other via going through the switch, through the extension cable, and then back to the panel.) Sep 29, 2022 at 17:04

1 Answer 1


TLDR: Reliance transfer switches don't play well with GFCI/AFCI

I'm interested to know the power requirements of the furnace to see if I can also run from a 200W battery pack. My plan was to set the transfer switch to Generator, and then plug an extension cord from a house outlet, through a Kill-a-watt energy meter, and then into the transfer switch, essentially using my house wiring to stand in for the generator.

That seems like a reasonable approach for testing. Here's the problem.

And it's 1 problem. You don't need 3 questions here.

These Reliance style transfer switches, including this one and the 6/8/10 circuit transfer switches, have a very hinky and 1970s mentality way of handling neutral. They simply treat neutral as if "it's all going to the same place, so it doesn't matter". But it does. With GFCI it definitely does.

The hot wire is doing exactly what you expect. it comes out of a house circuit, through Kill-a-Watt and extension cord, to transfer switch inlet, thorough the switch, back through the panel and into the furnace. Fine.

In a 21st century sensibility, the neutral wire should follow the exact same path in reverse. Thus, the two opposite-direction currents on that path are equal, magnetic fields are not emitted, and GFCIs do not trip.

However, if this was installed according to instructions, the furnace neutral simply comes from the panel neutral bar, end of topic. And so the big loop of neutral, from branch circuit to Kill-a-Watt to extension to xfer switch to furnace, is not happening at all.

The GFCI sees dissimilar current (hot current out, very little neutral current returning) and assumes the unaccounted-for current is shocking a human, and trips the circuit. Well done GFCI.

TLDR: if you want to test That 70's Transfer Switch, you'll need to use That 70's Circuit to supply it. Lose the GFCI.

The problem is, this "alternate" routing of neutral may poison the results from the Kill-a-Watt. I don't know how they work internally, if they measure neutral, they won't see the neutral side current and that would throw it off.

Issues with the installation

Now as to your installation, the cardinal rule of neutrals is that neutrals MUST go on the NEUTRAL bar. Neutral is not ground. Neutral handles regular service current and must be thermally rated for it. Ground only handles fault current, so liberties can be taken such as ad-hoc improvised ground bars like in photo. (I have one in my own panel so that's fine if mounted correctly).

In a MAIN panel, and I presume yours is the main from the labeled main breaker at top right, grounds are permitted as guests on the neutral bar since the neutral-ground equipotential bond is in this enclosure.

Some who have worked too long in the industry and forgotten their schooling think neutrals and grounds are good on any bar. No, neutrals on neutral bar only. Or possibly your guy was not a licensed electrician.

Code vio #1: neutral on an accessory ground bar. Also a 110.3 violation since it was not installed per instructions.

Code vio #2: that particular ground bar/dual lug is not rated for >1 wire per lug (unless it is, but I doubt that). That was field-installed but not by your guy: it was done by someone previously, and they did 1 wire per lug.

Root causes

The root cause for the mischief is that your neutral bars appear to be full, which isn't possible because UL doesn't approve panels that don't have enough neutral spots for every circuit to be 1-pole - and you have five 2-pole breakers taking 10 spaces but using at most 4 neutrals. So you should have loads. However, read your panel labeling - many panels allow 2 or even 3 grounds to share 1 lug on the neutral bar. UL assumes you are exploiting this fully.

So option 1, exploit this fully.

Option 2, install accessory ground bars - consult the panel labeling again and it should list certain model numbers of accessory ground bar which are designed to fit pre-drilled/tapped sites on your panel. Get the biggest ones. Ground slots are cheap. There should be several sites, mount 1 high and 1 low.

And then you can de-commission that field-applied ground lug down below, or simply empty it out, since it can take larger wires than most ground bars can. Make sure it's attached with a proper fine-thread screw, and not a sheet metal screw.

Other cleanup considerations

By the way, since your main breaker is a "backfeed style" breaker mounted in a normal breaker position, have you thought about getting a generator interlock for this entire panel, instead of mucking about with problematic Reliance gear that mishandles neutral and won't play well with GFCI/AFCI? This style is fairly easy to get a generator interlock for, since you just mount the generator breaker across from or next to the main, and a sliding plate interlocks the two. Siemens' version is under $30 and even straps down the breakers (though it does not fit this panel). I would talk to a GE dealer about such an interlock for your panel.

The electrician probably didn't mention this because these factory interlocks are cheap and there's more markup and commission on Reliance.

  • Thanks for your comprehensive answer. I'll get the neutral moved up to the neutral bus and do something about the overfilled ground lug. The panel interlock is interesting, but the furnace is the only thing that I care about keeping running. Outages are rare here, but it would miserable in an ice storm with no furnace for a couple of days. Backstory on the Reliance is I did enough research to be dangerous; when the tame electrician showed up with the HVAC crew for the furnace I handed him the switch and $100 to add to his install. For figuring current draw I'll try a clamp meter instead.
    – A Holman
    Sep 30, 2022 at 1:26

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