By Mike Sokol
I spend a lot of time on dozens of other RV forums answering questions about electricity. Here is one really interesting topic that’s getting a lot of attention: Just how do you ground a sub-panel?
From the Montana Owners Club:
Here’s what I’m trying to do: I have a new 10’x12′ wooden shed for our home-base location. Power at this location comes from a typical RV campground pedestal. I have outputs for 50-amp (used by my Montana), 30-amp (unused) and a 20-amp (currently used by crappy, old shed that is going away in a few weeks.)I want to take the 30-amp power to my new shed, then break it off into two 15-amp circuits. This will allow a refrigerator and a freezer in there to have their own circuits as required in their manuals. (Also, non GFCI, which is oddly required.)
So today I purchased a 30-amp, 2 circuit sub-panel and two 15-amp breakers for it. I’m in the process of ordering a 50′ 30-amp UL rated cord. (This is TT-30 to L5-30R.) I’m also going to order an L5-30 Inlet Box. My plan is to mount the inlet on the outside of our shed, connecting it through to the wall to the sub-panel. (Yes, I know this isn’t ideal, but the requirements of our camping club here state that I cannot have underground wiring and sheds may not be hooked up to permanent power. This RV-power cable would qualify more as temporary power on an extension cord and will keep me out of trouble.)
I think all of that is sound, but here’s my question. Do I need to ground the sub-panel in the shed? If I do, that would probably require driving grounding stakes, right? This is where it becomes clear that I don’t really know much about electricity. The only things that are going to run regularly are the two appliances and maybe a charger for the LED light bar we’ll have there, but that will be rare. Lightning is very, very rare in our area (outside of Seattle). The distance from the power to the shed is roughly 35 feet, though I’ll be running it through a 50 foot 10/3 cord. Not sure which of these factoids are relevant, just throwing out thoughts.
I purchased a grounding bar for the sub-panel, but it’s not going to do me any good without stakes, will it? (It’s quite possible that sub-panel isn’t the right term here. I’m connecting via an extension-type cord to an RV pedestal; I’m not bringing in direct leads from a primary panel. Obviously this is where my lack of expertise becomes an issue.) I watched one video where a guy has a true sub-panel and he referenced tying a bar back into the neutral, but I have no idea if that’s valid or viable. So please, electricians, put me on the right path here. (And also, please excuse any incorrect terminology I just used!)
Dear Jeremy (and the dozens of RVelectricity and RVtravel readers who have asked about “grounding” a pedestal at their home base),
Here are the basics. First of all, Jeremy doesn’t need a grounding rod for this type of power extension, which is basically just a sub-panel being fed from the pedestal he already has installed at his house. Remember, a grounding rod should probably be thought of as a lightning rod, since its real job is to shunt the current from any lightning strikes into the earth. You only need ONE lightning (grounding) rod at any location, and the service panel feeding your house should already have one. Now, code doesn’t forbid adding extra grounding rods at a location, and in fact the latest code calls for something called equi-potential grounding around swimming pools and such, which does add additional grounding rods. But since this is essentially “tiny shed” power, there’s no requirement or real necessity for adding a grounding rod at your proposed sub-panel.
Now as far as the “ground” strap inside of the sub-panel you’re going to build: In actuality, this is really a “bonding” strap which bonds (connects) the incoming neutral and ground (Equipment Grounding Conductor) lines together. However, there can only be ONE of these Neutral/Ground bonds in any given service entrance, which is the circuit breaker panel feeding your house from the power company’s transformer on the pad or pole.
So any sub-panel in your house – the sub-panel that is your existing 50/30/20-amp pedestal or the new sub-sub-panel you’re building – CANNOT have a secondary neutral-ground bond. There needs to be ONE and only ONE neutral-ground bond on your local power distribution system. Adding a “ground strap” or “ground screw” in your sub-panel will intermingle the normal neutral load currents with the EGC ground fault currents, and the EGC is only there to trip the circuit breakers and protect you in the event of a hot-to-chassis short circuit. The manufacturers supply you with a green grounding (bonding) screw or strap just in case that panel ends up as an incoming service panel. That’s the only time it’s neutral/ground bonded.
So always remember there needs to be and can be ONE and ONLY ONE ground-neutral bond in any electrical service, and that bond belongs at the incoming service panel of the campground or house electrical system. And you can add additional grounding rods if you’re in a high-lightning area like Florida, which can help reduce equipment damage if there’s a lightning strike in the area. But you don’t really need to add additional grounding rods for an RV pedestal or any other electrical sub-panel feeding a shed in your backyard.
See how simple this is once you understand the difference between “grounding” and “bonding”?
Until next time, let’s be safe out there…
Mike Sokol is an electrical and professional sound expert with 40+ 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.