Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. Today I discuss neutral/ground bonding plugs, inverter generators and EMS/advanced surge protectors.
While trying to figure out how to use my surge protector with my generators, I found your article about fixing the open ground problem with an inverter generator. I have two Cummins Onan P4500i generators for my 50-amp camper, which I run in parallel to have enough power to operate both AC units.
Then I had the same issue where the surge protector would not allow power to travel due to the “open ground” message. After reading your article I understand the reason now and the fix you describe. I have two questions I’d like to have answered to make sure I don’t cause any damage:
- My Cummins Onan P4500i has two 20-amp Edison outlets. The article and information related to the Southwire neutral ground bonding plug say to plug this into the 15-amp Edison outlet. But since mine are 20-amp outlets, is it safe to use this plug with a 20-amp outlet?
- I run my two Cummins P4500i in parallel to get the needed power for two AC units. Will I need a Southwire neutral ground bonding plug for each generator when using the parallel function?
Thank you for your time. —Travis
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Those are two great questions that are pretty simple to answer. For those of you lurking in the shadows, most (if not all) inverter generators have something called a floating or unbonded neutral. That’s just technospeak way of saying the neutral conductor is not connected (bonded) to the chassis ground of the generator.
While this doesn’t matter for a lot of things, if you’re using an EMS/advanced surge protector that can shut off the power in the event of an open ground, it’s a problem. That’s because the control circuitry in the EMS/advanced surge protector misinterprets this floating neutral as an open ground. Here’s a picture of what this looks like to a 3-light outlet tester. Note that it shows “open ground” when it’s actually a “floating neutral.”
My simple generator bonding plug fix….
My neutral/ground bonding plug invention simply connects (bonds) the ground and neutral conductors together at the power source (the inverter generator), which is exactly how all power pedestals and residential outlets are supposed to be connected to the service panel. This is not a trick to circumvent a missing ground conductor in old residential wiring. In fact, all installed generators in RVs are internally bonded just like this.
What about 15-amp plugs in 20-amp outlets?
The trick is that you can insert a 15-amp plug into a 15- or 20-amp outlet, but you can only insert a 20-amp plug into a 20-amp outlet. See how the sideways blade on the 20-amp plug is keyed so it won’t fit into a 15-amp outlet. Electrically, the internal contact areas are the same for both 15-amp and 20-amp versions of these “Edison” outlets.
However, 20-amp outlets tend to be built more heavy-dutily [editor: Yes, that’s a Mike term, not a typo], which is why we like them for higher-power connections. In your case, a 15-amp neutral/ground bonding plug will work perfectly in a 20-amp outlet.
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What about bonding parallel generators?
There only needs to be ONE neutral/ground bonding plug for inverter generators connected in parallel. If you have a companion/standard pair of generators, it should be plugged into the generator with the twist-lock 30-amp outlet.
I’m not exactly sure how your pair of Cummins Onan generators are linked together for parallel operation. But you only need one neutral/ground bonding plug inserted into a spare 20-amp outlet. That will bond the generator pair properly. Above is a picture of how it works with two Honda generators in parallel, one of which is a companion model.
However, I suspect you may have something that looks more like this parallel kit. If that’s the case, you should be able to insert the 15-amp bonding plug into either of your generator’s spare 20-amp outlets. But you may need to confirm that your parallel connecting cables are polarized, at least by marking one pair with white tape. Normally this isn’t an issue if the parallel wiring is swapped, but it can accidentally reverse the polarity of the neutral/ground bonding plug.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Send your questions to me at my RVelectricity forum here.
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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