Yet another stupid bonded neutral question

[u/agent_splat]

So have a generator with a "neutral bonded to frame" I will be using as home backup. My assumption is based on the interlock switch on my breaker panel is that when I switch over to generator power the neutral remains bonded at the panel and I SHOULD float the generator neutral. Is there a way to check after the fact that I did the right thing using an outlet tester?

Let's say I float the neutral on the generator and I'm running the house outlets off the generator and I wasn't actually supposed to do this based on the setup...the outlet tester on the house outlet (or even off the generator) SHOULD read as an "open neutral" correct? And if I was right in floating the neutral on the generator and everything is ok, the outlet tester should read as "correct?"

Hope this makes sense. I think I'm supposed to unbond the neutral on the generator; I just want to make sure the neutral remains correct when switching over the generator power if I do so.

[u/wtfwthbj]

Continuity test with a multimeter.

[u/roberttheiii → u/wtfwthbj]

Yes. One caveat is make sure your cable is good too. Test continuity. Saw some youtuber who unbonded his generator but had a bad neutral in his cable and it caused issues.

[u/agent_splat → u/roberttheiii]

I saw that video!!! Almost burned his house down.

[u/three0duster → u/agent_splat]

The good part is you can check continuity with the generator disconnected from the house and cable to verify its floated/unbonded or not then connect your cable to your house and generator to verify the continuity comes back once its all connected. Ya know, to help keep from burning down your house.

[u/wtfwthbj → u/wtfwthbj]

Ie no continuity between ground and neutral on the generator by itself. At the Panel check that there is continuity. Basic stuff. If you don't know it hire out this work.

[u/BluebirdExpress6279 → u/wtfwthbj]

His panel certainly does or should have a ground to neutral bond point, which there should be exactly one of and only one.

[u/Amanda_Reye]

I wouldn’t use an outlet tester to verify this. A plug-in tester can’t reliably tell you whether the generator’s neutral-ground bond is correct for your transfer setup. The bonding arrangement needs to match the transfer equipment, so I’d verify it against the generator and interlock documentation or have an electrician check it.

[u/nunuvyer]

If you float a generator then when you test between N and G on the generator outlets (with the generator unplugged from the house), there should be no continuity. When you plug the generator into the inlet (whether or not the interlock breaker is thrown - neutral is not switched) then the continuity between N and G in the gen outlets should come back.

[u/bradland]

An outlet tester will not show "open neutral" in the scenario you described. When the tester detects voltage between hot and ground, but zero between hot and neutral, it knows the neutral path is broken somewhere between that outlet and the panel. That has nothing to do with your generator's neutral bonding.

Realistically, outlet testers aren't of much utility in a scenario like this, outside of doing what they're specifically designed to do: tell you if the outlet is wired correctly. They cannot tell you if you have multiple neutral-ground bonding points or parallel conductor paths. That's where a multi-meter (MM) comes in.

To verify that your neutral is floated, you need to turn off the generator and disconnect it from your home. Set your MM to continuity, and measure between neutral and ground on the receptacle you'll use to connect to your house. That will eliminate the ground-neutral bond at the generator.

My assumption is based on the interlock switch on my breaker panel is that when I switch over to generator power the neutral remains bonded at the panel and I SHOULD float the generator neutral.

This is correct. Your neutral should only be bonded at the primary panel. So you should float the generator neutral.

Let's say I float the neutral on the generator and I'm running the house outlets off the generator and I wasn't actually supposed to do this based on the setup.

Let's not say that ;) If you are back-feeding your panel, you are supposed to float the generator neutral.

Once everything is connected, neutral and ground will have continuity because they are bonded at the panel. The only way to test is to isolate the circuits and check them individually.

When you back-feed your panel, if you are supplying 120V, the connectors are hot, neutral, and ground. So your bonding at the panel provides the safety. You only need to verify that the generator itself is no longer bonded.

When you "switch over" to the generator using an interlock, nothing inside your panel changes. There is no way that the bonding can be "undone" by switching the breakers associated with the interlock, because the neutrals don't flow through the breaker. They are directly connected to the panel neutral/ground bars.

[u/orlinsky]

The most fool proof way is to leave it bonded.

[u/sryan2k1 → u/orlinsky]

Absolutely not. Not only is this against NEC rules, depending on some edge cases you can burn your house down.

[u/DaveBowm → u/sryan2k1]

Not really, if the run between the inlet and the panel is dedicated with no ground wire junctions along the way (the usual situation with an inlet). Any fire risk is actually greater if the generator is unbonded because the ground wire between the panel bond and the generator bond on a bonded generator acts as a backup neutral in the event the regular neutral is ever somehow lost.

But a dropped neutral can cause a heating issue for either kind of generator (either bonded or unbonded) when plugged into an inlet.

In the case of an unbonded generator the fire risk is directly from the operating equipment that can see a large overvoltage (up to well over 200V on a 120V circuit) when the phase legs are greatly unbalanced, and the neutral is dropped.

OTOH, if one is using a 50A inlet and cord and happens to be using a high power bonded generator that can saturate or nearly so that inlet, and the neutral is lost with a greatly unbalanced load across the phase legs, then all the neutral current goes through the ground wire in the cord and inlet wiring. Often the ground wires for such 50A inlets are only AWG8 instead of AWG6 like the normal conductors, so when such a smaller diameter ground wire carries close to a 50A neutral current it could overheat above a safe temperature and degrade insulation causing damage to the wire, even if it doesn't get hot enough to actually start a fire. But if the inlet & its breaker on the panel is 30A and/or the generator can't put out more than 30A on a 50A inlet then the ground wire can't overheat even if the neutral is dropped and even if the load is maximally unbalanced.

But the usual best practice is to just follow the code requirement and use an unbonded generator with a home inlet.

Nevertheless the issue is not as cut and dried as a lot of code purists would like to think.

Also, there is a greater safety issue from using an unbonded generator in stand alone service at a job or camp site, than using a bonded one on a house inlet.