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tl;dr: To facilitate using any generator with the house inlet, regardless of transfer switch type or generator neutral-ground-bond status, may one leave the generator ground wire unconnected in the inlet?

longer version: The going requirements for generator usage as house power backup require no parallel neutral-ground path between the generator and the house, so current can't flow back to the generator through the ground wire, thus energizing the ground system. The two common solutions center on:

  1. Removing the neutral-ground bond from the generator. or:
  2. Using a switched-neutral transfer switch.

Another thing is that NEC doesn't like checklists. During a power outage, one must be able to connect everything regardless of procedure and have it all just work safely.

If one goes the 'remove-the-bond' route, one can use a cheaper transfer switch or even head-to-head circuit breakers with a interlock plate between, and have common neutrals. However, one must remember to put back the bond when taking the generator to a jobsite or campsite etc. Even if one has a permanently-mounted generator, if that generator should fail to start and one has to borrow a portable, the generator's bond must be removed if the house is not switched-neutral. It is a checklist item, see above. (Not to mention the owner of the generator may not like the borrower disassembling it....)

The alternative is to use a more expensive switched neutral transfer switch. However, there is apparently a possible problem with switched neutrals: If by chance the power lines connect before the neutral, then dangerous imbalances in the 120V legs can develop depending on what loads are connected at the time, with resulting damage to devices. One could remember to disconnect all loads before switching the transfer switch, but that's a checklist item again.

A thought on a third method appears when considering the standard North American utility-to-house connection:

The power from the utility comes through a transformer to a center-tapped secondary, with the center tap grounded. The power lines and neutral go to the house, where the neutral is again connected to ground in the main panel. Note that the Utility power source neutral is bonded to ground on both ends, but only delivers two powers and neutral. There is no ground wire connection in the utility feed.

In theory, if we remove the utility transformer from the house's main three-wire utility feed and substitute the typical neutral-ground-bonded 120/240 portable generator, with the case grounded to a ground rod, we have the same setup as the utility transformer: Power source neutral bonded to ground, two powers and neutral but no ground wire connection to the house, where ground is bonded to neutral again. Lack of a fourth wire for ground seems to achieve the same setup as the utility provides.

Since NEC may want the ground wire still in place in the cable if the cable gets cut, we might not connect the ground wire in the house inlet. With no ground connection in the inlet, a ground loop cannot form with the generator. Yet with the ground-neutral bond in the main panel still connected because we are not using a switched neutral transfer switch, the generator power is still referenced to the ground system in the house.

This concept of not connecting the generator ground wire in the inlet was mentioned in this answer https://diy.stackexchange.com/a/8715/140350 by @gregmac, referencing a no-longer-online Generac webpage:

If the neutral bond cannot be removed, you have two choices. The easiest solution is to lift the ground wire coming from the generator inside the transfer switch, and secure it with a wire nut, by itself. This eliminates the loop. Your other choice is to install a Switched Neutral Kit

Not connecting the ground in the inlet could theoretically make a house compatible with any generator of sufficient size one wanted or had to use, without having to modify the generator's electric setup. (My generator's neutral-ground bond is buried in the alternator, requires tools to remove or replace, and is under cover out of sight out of mind if I forget to put it back or forget to remove it depending on circumstances, which is a checklist thing again.)

Is not connecting the ground in the inlet a sufficient way to avoid complications of switched neutral and make my house compatible with any 120/240 generator? If code strikes this down, is it only because it hasn't been considered, or is it dangerous?

2 Answers 2

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There's no way to avoid needing to manage the neutral-ground situation, one way or the other.

no parallel neutral-ground path between the generator and the house, so current can't flow back to the generator through the ground wire, thus energizing the ground system.

There are several issues. One is that with two N-G bonds, normal service current has 2 available routes - via neutral, and via the generator's bond to ground to the panel's bond. It will use both. So you'll have a lot of activity on the ground wire when you should have none at all.

Further, when your system has two N-G bonds, a properly sized one and a wholly inadequate one, failures will be silent. The robust bond breaks. OK, now all fault current is taking a serpentine path out long #12 ground wire to the generator, and back the long #10 neutral wire from the generator, *and none of these wires have fuses, mind you.

So #1, the impedance of all those wires means that a bolted fault does not flow enough current to insta-trip the breaker. So now two things are on fire: the faulty equipment and the generator circuit.

And current on the N-G bond may not even be from your house. When a nearby house loses neutral from the utility, neutral current goes out their Grounding Electrode System to the dirt to any route it can find back to the transformer neutral. That includes through your N-G bond.

If by chance the power lines connect before the neutral, then dangerous imbalances in the 120V legs can develop depending on what loads are connected at the time, with resulting damage to devices.

That's a user error. They should be throwing the transfer switch rapidly and not dwelling in a halfway position. The design of the transfer switch can also favor making neutral first and breaking neutral last. Regardless, the snap action will tend to break all at once.

In theory, if we remove the utility transformer from the house's main three-wire utility feed and substitute the typical neutral-ground-bonded 120/240 portable generator, with the case grounded to a ground rod, we have the same setup as the utility transformer:

Like a lot of schemes, this only works if everything else is working perfectly. It seems harmless but it actually removes a layer of defense.

A ground rod is entirely inadequate. One of the purposes of system grounding is to assure a bolted hot-ground fault can return enough current to trip the overcurrent device. The dozens of feet of dirt between the generator's ground rod and the house's will prevent that current from flowing, causing the ground fault to fester indefinitely.

And if the generator itself has a fault, it will sit there with the chassis energized, because the dirt cannot flow enough current to clear the fault.

If the neutral bond cannot be removed, you have two choices. The easiest solution is to lift the ground wire coming from the generator inside the transfer switch, and secure it with a wire nut, by itself. This eliminates the loop.

And leaves the generator vulnerable, as above. I can see why this advice is no longer given.

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Good switching neutral transfer switches already address your concern

Many (albeit not all) switching neutral transfer setups already address the issue of voltage spiking when the neutral is broken, one way or another. For instance, the Reliance Panel/Link X series uses a sequentially switched neutral that transfers neutral completely while no power is being provided to the loads from either source, eliminating such spikes. Larger scale systems (big generator, big transfer switch/switching-gear) can use overlapped neutral poles in their ATSes to address this, as well, so it's only really a problem if you're using a 3-pole manual double throw enclosed (safety) switch, or some lower-end ATSes that have 3 poles and a split-phase capable controller, but not a dedicated neutral pole.

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  • Seems there's too many failure modes in my idea, and with good switched neutral switches available, this would be a better way to go. Oct 23, 2021 at 23:31

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