When is a ground not really a ground?
Dear Readers,
I recently realized, after trying to explain to an electrician the facts of how electricity gets from the electric pole into your RV outlets, that there were no basic diagrams to be found. So I’ve started making drawings of the entire campground power distribution layout, which I’ll publish here. While this looks complicated at first glance, it’s really pretty straightforward.
Today we will cover how it’s all hooked up according to the National Electrical Code, which also shows how the neutral and ground wires are bonded (connected) together in the power company incoming service panel, but nowhere else. And Part 2 next month will cover all the various load and fault current paths, including how to integrate a generator with a floating neutral into your RV’s electrical system.
Where to begin?
Most of you have never seen a schematic diagram of what’s on the electric pole coming into the campground or your own house, so here’s where the power begins for you.
Starting at the left side of the diagram you’ll see there’s a transformer either on the pole or on a pad being fed with around 7,000 volts of electricity. Yes, those are what we call primary wires since they carry electricity from the substation to the campground’s power transformer. The reason why the voltage is up as high as 7,000 to 11,000 volts is to reduce the voltage drop over the miles of wire connecting the campground or your residence to the power company.
And indeed it starts out as much higher voltages on the big high-tension lines you see going across the fields, with 110,000-, 250,000- and 500,000-volt lines being pretty common. Again, higher voltage equates into less current for a given amount of wattage, and less current equals less voltage drop. Take my word for it now, but someday I’ll show you all how Ohm’s Law works.
TransformersÂ
The purpose of the power company transformer is to step-down the incoming 7,000 or 11,000 volts to 240-volts. And as you can see, that 240-volt winding is also split down the middle to create a pair of 120-volt outputs (which is why it’s called split-phase). You’ll also find that the bottom of most electric poles in the USA have a ground rod/pad installed, which functions as a place for lightning strikes to find “ground.” OK so far?
This power company transformer then feeds 240/120-volt power to the campground’s incoming service panel. You have the same thing in your own house, which might be a 100- or 200-amp service. For future discussion you should know that a 100-amp service can supply 100 amperes of current at 240 volts, or 200 amperes of current at 120 volts. And a 240-volt, 200-amp service can supply 400 amperes of current at 120 volts. Many campgrounds have a 400-amp (or larger) service panel because they need to feed power to multiple pedestals.
Neutral-ground bonding
You should also see that the incoming service panel includes a Neutral-Ground bonding point, which is typically a green screw in the neutral bus bar which bonds (connects) it to the metal box and service panel grounding rod that’s installed at the campground. This is how modern electrical code works, but it’s possible to find really old installations that do some strange grounding things. More on that MUCH later.
Now let’s take a look at what’s inside of a campground pedestal. You’ll see that it has incoming Hot 1, Hot 2, Neutral and Ground wires. The two Hot Legs (as they’re often called) go to several circuit breakers powering the various outlets in the pedestal panel. So there’s a double-pole 50-amp breaker feeding the 50-amp/240-volt outlet (50 amps x 2 = 100 amps at 120-volts), a single-pole 30-amp breaker feeding the 30-amp outlet, and a single-pole 20-amp breaker feeding the 20-amp outlet. Yes, the 20-amp outlet must be GFCI protected according to code, but for simplification I’m going to ignore that on this diagram.
What, no ground rods or neutral bonds?
One thing that you’ll see missing on the campground pedestal is a separate ground rod or neutral-ground bonding screw. There will be a ground wire bonding screw from the green/ground bus to the metal box, but that’s NOT connected to the incoming neutral wire.
Again, the G-N (Ground to Neutral) bonding point should only be done in the incoming service panel, and NOT the sub-panel or campground pedestal. This ground wire is technically called an EGC or Equipment Grounding Conductor, and is only allowed to carry FAULT current (when something goes wrong), and not LOAD current (which is what you draw to power your RV appliances).
Again, this G-N bonding point (as well as an earth grounding rod) is only included on the incoming service panel, NOT the campground pedestal, which is wired just like a secondary circuit breaker panel in your house.
The RV connection
Finally, the power is fed from the outlet in the pedestal into the power distribution center of your RV, where it has its own incoming 20-, 30- or 50-amp circuit breaker, and outgoing 15- and 20-amp breakers to feed your air conditioner, 12-volt converter, microwave, etc.
I’ve simplified this diagram without the output breakers to show that there’s definitely NO Neutral-Ground bonding screw in this box, but there IS an EGC (Equipment Grounding Conductor) connected to an RV chassis bonding point. This connection is what eliminates any hot-skin/stray-voltage on your RV chassis (and skin) in the event of a fault current caused by normal internal appliance leakage, or even something like a metal screw poked through a wire accidentally. (Hey, stuff happens.)
Time to power down…
Wow, I think that’s enough for now. As you can see, there’s a very specific order as to how all of this hooks up, and there can only be ONE Ground-Neutral bonding point at the campground’s incoming service panel. And there are no ground rods at your RV or the pedestal.
So what’s all this mysterious naming of Grounds, Ground-planes, Earth Grounds, Ground Rods, Equipment Grounding Conductors, and Neutral-Ground Bonds? Yes, they’re all different things and have different functions. But if you’re going to wire ANYTHING connected to 120 volts, then you need to know the difference. So stay tuned tor more info on RV and campground grounding soon.
Let’s play safe out there (especially around electricity)… Mike
Hi Mike,
We are at a well maintained campground which recently upgraded all the campground electrical including pedestals. My neighbor had an issue with his hw and was trying to figure out, I was helping him with checking for power to hw tank and with my ncv realized he had a hot skin situation. We went through the breakers and found the circuit which I told him to leave off until fixed. I gave him this newsletter info to subscribe to. He got an electrician in next day and everything checked out on the rv and he traced the problem to the pedestal, ground issue. The electrician checked the ground at the main distribution panel and ended up putting a ground in at the pedestal which resolved the issue. Seems this has happened to other pedestals here as well.
The rv owner had no idea of the dangers of hot skin, neither did Park officials. So 2 things, thank you for sharing your expertise, I read all your emails and have gained valuable knowledge and you have gained another subscriber. Second is adding the ground to the pedestal seems to have fixed the situation but is it the proper fix.
Thanks
Allan
al_cardinal@live.com
One interesting thing you didn’t show in your Po Co diagram are the fuses. I know they have them, at least my power pole does because I’ve heard them blow and it sounds like a very loud gunshot. I assume they protect the transformer but I’m not sure. My Po Co just installed some kind of a safety device they said was to protect the fuses, but they didn’t say how it worked. I’m in a rural area with lots of trees which have a bad habit of dropping limbs which find their way onto power lines during wind and snow storms. Mike, do you know how the fusing works at the power pole/ transformer?
A trouble man resetting a fuse told me about this. First I’d heard of it. A sensor box down the street has 3 phase going to it. When it senses a short from a tree limb or a squirrel or anything else, it switches to another leg faster than the fuse can blow. If a temporary short, all is well. If short lasts the fuse blows and has to be reset.
Mike, thanks for another good article. A couple questions. As far as I know if you run a sub panel to an external building (like my barn) you need to have a ground rod but you leave the neutral/ground un bonded. Why wouldn’t a pedestal which is basically a sub panel at an external building also require a ground rod? Is there any problems or dangers if every pedestal at a camp ground had ground rods? Would the extra ground rods hide problems like the other comment here with the hot skin?
Another potential topic for discussion would be cord winders. They have brushes. In a prior motorhome, I experienced neutral failures in 2 of its cord winders resulting in damage to a TV, a satellite receiver and the ATS. The insulation on the neutral wire from the winder got charred. After the cord winder was removed, there were no more electrical problems. The wonders were replaced under warranty but the shop blamed the failures on power durges although an external surge protector was always used and the winder was never rotated after plugging into the pedestal.
Ground and hot skin, if I understand what I’ve read hot skin requires two things: a current leak to chassis/ground/skin and an improper or missing ground.
I’m trying to reconcile the above with a condition I ran into a campground. Underneath a TT roll up against the axle, I’m covered in sweat, ground is damp and I feel an electric tingle. Start to try to find the problem when a neighbor walks over and says every RV in the area has a low level of hot skin. Shoved one probe of the voltmeter in the dirt and touched the frame with the other probe. Did this on five or six RVs all showed 10 to 12 volts.
Any thoughts?
It could be a reflected hot-skin voltage. Read this: https://open.substack.com/pub/rvelectricity/p/whats-a-reflected-hot-skin-voltage?r=1txmtl&utm_campaign=post&utm_medium=web&showWelcomeOnShare=false
During a storm, a limb fell and pulled my neutral line off the feeder from the pole. I didn’t know this at the time. Neutral conductors are commonly used to secure/support the aerial line from the pole to the house. I noticed some anomalies with the power but only with the A/C running.
The power company responded about an hour later. The were amazed that anything worked in my house, let alone the A/C. Turns out the ground rods at the house were supporting the load that should have been on the neutral. So in sandy wet soil, grounds can actually carry a substantial load.