RV Electricity – Does ground size in extension cord matter?

Dear Mike,
When I went shopping for a 50-amp extension cord for our 5th wheel, I was surprised (shocked?) to find that they seem to all contain three #6 gauge conductors and one #8 gauge conductor.

I’m assuming that the #8 gauge line is the ground. But whether it is ground or neutral, I’m surprised that one of the leads can be seemingly underrated (the common rating for #8 gauge I’ve seen is 40 amps, although I’ve seen higher and lower for special uses). How can this be safe?

Thanks, and thank you for all your contributions to the RV community. —Al

Dear Al,
Actually, code only calls for a #8 gauge grounding conductor for this type of circuit. That’s because the ground wire should never have to carry any of the neutral return current. The reason that the neutral needs to be the same size as the hot wires is that it’s possible for all the loads to occur on one side of the 120/240-volt supply. In that case the neutral could carry up to 50 amps of current for a sustained amount of time.

However, in the event of a short circuit to chassis, the equipment grounding conductor (EGC) only needs to carry a few hundred amperes of peak current to trip the circuit breaker quickly, not carry a sustained load. And a #8 gauge conductor can easily do that.

So a ground wire really only has two jobs to do:

#1) Keep the overall voltage of any conductive surfaces in your RV close to actual earth potential (grounding).

#2) Provide a fault current path back to the neutral-ground bonding point at the incoming service panel in the event of a line-to-chassis short circuit (bonding), which will trip the circuit breaker quickly.

In both cases, a #8 conductor can easily handle the current of grounding and bonding, so that’s why it’s allowed to be undersized compared to the neutral and hot wires.

Let’s play safe out there….

 

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

##RVT830

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7 Thoughts to “RV Electricity – Does ground size in extension cord matter?”

  1. Dick HIme

    Great explanation for the difference in ground conductor gauge! I’ve actually wondered about that in the past. Makes perfect sense in the way you clarified it. Thanks.

  2. Tom

    Great idea, but how does one turn off the converter? I am not aware of any switch on my converter, schumaker mity mite.

  3. Jim Larsen

    Mike,
    My RV, like most others, has a bunch of 12 v. circuits and a couple of 120 v. circuits. All of the plugs operate only when the RV is on shore power or generator power. I was recently boondocking at Quartzsite and an RV owner there had an easy fix to have all of his 120 v. plugs operate on battery power (so he could watch TV without his generator running). He connected an extension cord from his inverter to the shore power inlet of the RV and turned off his converter. That made all of his plugs run off from his batteries. Is this safe (assuming you have a large enough inverter and enough battery power) ?

    1. Tommy Molnar

      This is EXACTLY how I have had my trailers set up for over 20 years. I’ve got solar power on my roof and a 2500 watt Cobra inverter running off my golf cart batteries. This system has been working flawlessly since day one. We just have to be sure we turn off the CONverter before turning on the INverter.

      I wonder if I met Jim Larsen in Quartzsite last year . . . Ha.

      1. Roy Christensen

        I did that with my last motorhome, too. Besides turning the converter off, I also make sure the refrigerator is set to propane so it doesn’t run on 120v and run down the batteries.

        1. Tommy Molnar

          Me too, Roy. I just forgot to mention that.

    2. Mike Sokol

      I don’t see any downside to this, but I’ll roll it around a bit in my head to be sure. Basically you’re using your on-board inverter to act like a generator powering the 120-volt AC outlets in your RV. As long as you have a big enough inverter to power what you need to turn on it should be OK. And if you draw too much current the inverter will protect itself and turn off. Interesting trick.

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