ECN Forum Forums General Topics of the Trade General Discussion Area Portable generator grounding question

My portable generator pulls double duty as a home backup and is also occasionally used on the job, so I always just leave the neutral bonded to the frame.
Doesn’t OSHA require a generator used on a jobsite to have the neutral bonded to the frame?

Maybe manufacturers could put a switch on their portable generators to change the configuration, instead of having to manually remove and install a jumper wire. Then again, that might just make things more confusing.

IMO, no matter what perceived hazard exits because of this situation, from a safety standpoint, things are still light-years ahead of where they were 10-years ago.
It’s got to be much safer than backfeeding into a dryer receptacle, like so many people used to do before all these home transfer switches and generator-ready load centers became readily available.

I suppose we should look at why we ground services

250.4(A)



Those are not scenarios that are present with a portable generator. You are not driving a rod so you are not going to ground the system anyway. Bonding the neutral really serves no purpose.

If you are connecting to the house with switched neutral transfer equipment (taking the MBJ out of the circuit) you would need to bond the neutral in the generator but if the transfer equipment is only switching the ungrounded conductors you should leave the generator unbonded.

The neutral must be bonded once, and exactly once, in any given electrical system. If the neutral is not bonded, equipment that relies upon the neutral being close to ground (like old ranges and dryers) can be become dangerously energized during fault conditions. Additionally, ground the neutral effectively ties the hot wires to ground; if it was left to float, instead of -120 and +120 phases, you may end up with -5V and +245V, which is considerably more dangerous.

The neutral cannot be bonded in multiple places because the neutral current will travel through the ground system, which creates all sorts of other safety issues. And yes, this is considered a "ground fault" just as much as accidentally shorting a neutral somewhere to ground is.

ALL small portable generators with receptacles on the side have an internal ground-neutral bond because they're intended to run standalone. When you grab one of these off-the-shelf and plug it into a house, you've now created a double-neutral bond, with neutral current running through the generator cable's ground wire, and potentially energizing the generator chassis. You need to do one of two things to remedy this:

1) Disconnect the ground/neutral bond in the generator. I don't like this, because it makes it dangerous to use as a standalone generator. If you do this, disable the outlets!

2) Install a transfer switch that transfers the neutral. (Note: have to move the ground/neutral bond point to the line side of the transfer switch; you can't leave it in the panel.)

The ground rod on the generator is simply for extra safety, and ensuring the generator chassis and ground wires are all at the same potential as the earth.

Nobody has given me a compelling reason to ever bond the neutral in a portable generator that never gets connected to premises wiring. If anything you are creating a hazard that didn't exist. Like an ungrounded delta, the first fault is free.

Greg, Wouldn’t a fault to ground in a piece of equipment being powered up not have a return path back to the source?
I know some portables, like mine, don’t have the GFCI protected duplex outlets on them. I use a plug-in GFCI adapter, but I’m sure there are still a few people out there who don’t.

If there is no path back to the source, where is the danger? That is the theory of isolation transformers on lab benches.
In that situation there is no "hot" or "neutral", just 2 conductors with some volts between them. There is no reference to ground. You can be sitting on a metal bench in a puddle and there is no fault path unless you get phase to phase.

We ground services because of the things that can happen to overhead lines (lightning strikes, contact with high voltage secondaries, surges etc). You don't have that exposure with a portable generator.

Actually, there probably enough leakage current via capacitance coupling to produce enough current to shock or kill a person. Without the bond, there will never be enough current to trip any overcurrent protective device.

Plus without a reference to earth, the output voltages can float anywhere above or below earth potential. This could result in the failing of insulation in extreme circumstances.

I would be curious just exactly how much capacitive coupling there really is but since small Honda generators are shipped without a bonded neutral and we have not seen dead bodies dropping all over I doubt it is a problem.
Again, if nothing is grounded, where is the fault path?
I imagine the EGC would measure 60v to each phase and zero to a water pipe. A megohm resistor to either phase would make the 60v go away.

Wouldn’t the guy standing in the puddle with one hand hand on the grounded building steel and leaning up against a piece of ungrounded equipment with its frame energized become a conductor and provide the path to ground?

There would be a single path to ground but no return path to complete the circuit. No current would flow.