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My 23 year old house has a manual transfer switch, a Gen Tran (now Reliance Controls) unit, which I’ve used off & on for 20 years. My generator is a portable 7500 watt that I purchased in 2018. I’ve used the generator/switch combo countless times over 20 years, never a problem. My generators have always been bonded to frame, but I’ve read over and over that I should switch it to a floating neutral, as this is required by NEC code. So I decided to do this.

First, I used a meter and confirmed continuity between neutral and ground at the L14 plug on the generator. Next, I removed the ground wire as directed in the generator manual, unbonding it. Then I rechecked for continuity at L14 outlet and there was none, as expected. Next, I connected my 30amp cable generator and transfer switch (Generator was off, transfer switch on utility power). Then I tested one of the 120v outlets on the generator, and there was continuity between neutral and ground. This confirmed that ground was in place at the service panel.

At this point I felt I did everything correctly and I was now in compliance with code. Four days ago the power failed, and I fired up the generator.

I started flipping the transfer switches to “Gen” and lights started to come on. About 5 seconds after all switches flipped, the generator started running very rough. If memory serves, I think lights started dimming. In a few more seconds the generator almost stalled, and then all the lights went out, and the generator recovered to normal sound. What had happened was the 240v breaker on the generator tripped. This never happened before.

A moment later by wife ran out and told me the living room was filling up with smoke! I shutoff the generator and ran inside. I didn’t find a fire, but a terrible burnt electronics smell, and hazy smoke. Not having a clue what happened, and fearing the worst, I called the fire dept., and we prepared to evac. Whatever happened seemed to be over, in that smoke seemed to be dissipating, not getting worse.

When the firemen arrived, they quickly found that the power strip behind the TV had a hole burned right through the top of it. They checked the house outlets/walls with a handheld thermal imager, and said everything was OK. Utility power came back on while they were there, and everything continued to be fine.

I am now trying to figure out what went wrong.

I spoke with a master electrician who installs generators for a living (mostly whole-house). He said it sounds like a neutral wire may be loose, which would cause the generator to be out of phase, sending a high voltage (possibly all 240) to one leg and 0 to the other. He said he would bond the generator again, and leave it that way. “I never unbond them.” I found this very surprising because my understanding is that is not correct.

I spoke with a relative who has worked his entire career as an electrical engineer. Without telling him what the electrician said first, he said pretty much the same. He said check all connections in the service panel and make sure they are tight. He also said bond the generator again.

Everything in the house has been fine since coming back on utility power. I bought an outlet tester which checks voltage, grounds, reversed wires, etc. Every outlet came up 120v and “correct.” I pulled the cover off the service panel and checked all screws. A few screws on breakers turned a little. All the white & ground wires on the bars were tight.

I’ve never seen any evidence of electrical problems in the house. Lights are all bright & solid, no dimming. Never a crackle sound when operating switches. I have a couple ductless systems, electric range, dryer and water heater, a couple refrigerators, etc.

The power strip that cooked had an LED TV, a DVD player, and a lamp with a 15w LED bulb plugged in. The lamp was probably the only thing on at the time. I haven’t found anything else in the house that’s damaged. I pulled the outlet that the strip was plugged into, it looked perfectly fine, no charring, no burnt smell, but I replaced it anyway. Inside the outlet box looks good too.

I don’t know what to do next. The generator is bonded again, but my confidence is shaky right now. I need some direction please.

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  • And APCs are generally good products - if this happened with some off-brand I might blame the surge protector, but not with an APC. Commented Jan 19 at 0:45
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    @manassehkatz-Moving2Codidact FWIW, I've found the "good" ones just burn out (albeit not this dramatically). They protect the stuff behind it, but the surge protector is toast.
    – Machavity
    Commented Jan 19 at 1:34
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    The only thing I can think of is somehow, the transfer switch is wired so that when everything is switched to generator, there is no neutral-ground bond in the house. My suggestion is to, without hooking up the generator, switch everything to generator. Then check for neutral-ground continuity between neutral and ground on the L14 inlet on the transfer switch.
    – DoxyLover
    Commented Jan 19 at 1:59
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    If it's true that the problem is a poor neutral it is probably also true that you have been running current up to 30A through the ground wire and generator chassis whenever you used the generator. You may have sacrificed a $50 power strip to expose a more serious and dangerous problem. Don't draw conclusions yet about bonding correctness.
    – jay613
    Commented Jan 19 at 4:42
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    Don't just "snug up" screws holding down wiring, especially in your panel. Go pick up a torque screwdriver (I grabbed one for ~$60 at a local big box store), get a pack of ECX bits (specifically designed for home wiring screws, no it's not slotted, nor square drive, nor Phillips - it's like the 3 of them hooked up and had some sort of weird baby) a set of one #1 and one #2 cost me ~$5, then tighten every connection to the proper torque. Do some digging online if the torque specs aren't printed on every device. That will fix a "loose neutral".
    – FreeMan
    Commented Jan 19 at 13:16

2 Answers 2

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We know one thing: either there's a fault in your generator, or there's a fault in your wiring. I think this is fairly straightforward to check, if you can wire up a couple of test cables. I also believe the fault is a poor neutral connection in your transfer switch or the wiring from it, one that tests good with a meter but which can't actually carry current.

First, test for faults in the generator. Unbond your generator again. Hook the frame to a good earth ground. Wire up a 240V generator cord so it splits to 120V outlets, one on each leg of the 240V, allowing you to be sure about which phase you're loading. Start with a small expendable load on one side, like an old school incandescent light bulb, or one that can take any voltage up to 240V like a device with a universal voltage power supply. If the bulb doesn't immediately blow, check your voltages on each side. They should be around equal, give or take. Then check the same bulb on the other 120V leg. If both those are good, try a bigger load like a space heater. Leave the small load on the other side so it's not completely unloaded. If your voltage spikes way up on one leg or drops way down on the other leg in any of these test scenarios, you have a problem with your generator that was being masked by bonding neutral to ground.

Next, test for faults in the house wiring. To do this, bond the generator again. Transfer it as usual. Check with a clamp ammeter on the wires that go to the generator. You might have to modify a cord with a fan-out so you can put your ammeter on the wires. If you're seeing any current on the ground wire at all, you have a wiring problem in the transfer system or the house that you have been masking with the bonding at the generator. Find it and fix it.

If all that shows good, well, I'm out of ideas. Most likely, I suspect your neutral is not connected well somewhere in the transfer hookup, and your ground wire has been saving you from having a floating neutral when the generator is connected.

Edit: see the other answer - it was the cable! That counts as a 'fault in the wiring' but of course it's the one I totally forgot about.

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  • Thank you very much for this detailed explanation. I am most likely going to bring in an electrician with generator experience, as this is very serious and I don't have all the equipment needed. I am getting a real education here, which I appreciate tremendously!
    – DaveinPA
    Commented Jan 19 at 13:21
  • Not sure, @DaveinPA, that I'd go with the one who admits to violating code...
    – FreeMan
    Commented Jan 19 at 13:31
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    @FreeMan Nope, I have a different one in mind!
    – DaveinPA
    Commented Jan 19 at 13:52
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First I did some testing on the generator - ran it with a 150 watt lights on each side while I had my meter plugged in, and it was a solid 120 volts on each side for 10 minutes.

Next up – the transfer switch. Its L14 receptacle has a white (neutral) that connects directly to the neutral bar, and a green (ground) which connects ground bar in the breaker panel, and. I made sure they were tight in the panel, but I wanted to check them on the inside the transfer switch. I cut off house power and opened up the transfer switch, and everything was right. Running out of ideas! Why don’t I have a good neutral connection from generator to house?? And then it hit me: the 30 amp cable.

I removed 2 retaining screws on the female end and started to separate the L14 from the rubber boot, and the entire connector popped out of the boot. The leads for the hots and ground were loose enough that they couldn’t take that little bit of strain, but here’s the winner: the screw for the white neutral wire was in the fully loose position. It was never tightened when the cable was assembled. As someone previously said, current was going down the ground wire all these years.

I was making a video when I was taking the cable apart, check it out:

Failed 30amp generator cable

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    Incredible. Your failure mode is predicted in this paper. It is recommended (36 years ago) that surge protectors include thermal fuses in addition to overcurrent ones for just this scenario. Apparently yours does not have one!
    – jay613
    Commented Jan 23 at 0:41
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    I'm still curious about your "master" electrician who says he never unbonds generators that he installs. Generators that are typically installed by professionals are shipped unbonded and do not get bonded when installed as a whole home backup generator. Of course a professional would not unbond a portable generator that is not sold or designed as a home backup generator. He's not going to disassemble a a client-bought generator.
    – jay613
    Commented Jan 23 at 1:00
  • I'm with you. How many times I've heard, "That's how I've always done it and I've never had a problem." Until a house burns down.
    – DaveinPA
    Commented Jan 23 at 1:36
  • Of course! I didn't even think of the cable. Nice save.
    – KMJ
    Commented Jan 23 at 4:25
  • I am back online now. I got my new generator cable today and ran a test with unbonded generator, and everything worked as it should. Thank you everyone for your help, I was quite distressed when I posted. This board is great!
    – DaveinPA
    Commented Jan 26 at 0:26

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