AC power and grounding from the electric pole to your RV
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 layout, which I’ll publish here. While this looks complicated at first glance, it’s really pretty straightforward. Part 1 today 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 (click any image to enlarge) 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 volts is to reduce the voltage drop over the miles of wire connecting you to the power company. And indeed it starts out much higher than that on the big high-tension lines you see going across the fields, with 110,000-, 250,000- and 500,000-volt feeders 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 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. You’ll also find that the bottom of every electric pole in the USA has 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 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 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 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 is only 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 to the next RVelectricity issue #13 and learn how it all works. See you then.
In the meantime, let’s play safe out there (especially around electricity)…
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.
##RVT869
I don’t have lights on the right side of my RV. Inside. What can I do to get my lights on
Thank you
Jackie Aman
Set up my 30 amp single post breaker in my sub panel and for some reason its reading 240v at the breaker and at the RV 30 amp receptacle. How is this even possible?
At my campsite I have had odd electrical issues. Several appliances have had board burn out. At the pedestal the skin read 123 volts with black lead in neutral outlet. I opened the pedestal and found corrosion, pitting, mold and a broken connector. What surprised me most was the neutral bonded to ground. I explained this to the owner who then put a new 30 amp outlet in without addressing the broken connector or anything else. I decided to let it go.
Today though I was repairing burnt wires on my hot water heater element and later the limit switch. To verify the element was operating I would place my meter leads into a outlet inside the RV and watch for a voltage drop when turning on the remote power switch. I placed only the black meter lead into the neutral and noticed 3 volts. I placed my finger on the red lead and read 41 volts. Other circuits did the same. Also with a analog. I disconnected from the pedestal and the 30 amp outlet did the same. Another site read 90 volts. Why?
At a campground we stayed at recently our neighbor was having a problem with the power. His higher end class A rig had an automatic switch to turn on the generator if the shore power went out. It shuts off the shore power at the same time. His system would not allow the shore power into the rig because it was showing about 3 volts between the ground and neutral. What would cause this issue?
Most people assume that the ground and neutral will always be at the same electrical potential (zero volts) all the time. But that’s only true if there’s no electrical load on that particular branch circuit. If there’s an unbalanced load in that part of the campground (very normal and not dangerous at all), it’s quite common for the neutral wire to have 1 or 2 volts of drop compared to the ground wire, and certainly even 3 volts is within code limits. It sounds like your friend’s automatic switch was set a little too low. Do you know the manufacturer and mode of the automatic switch you’re referring to? If so, I’ll call their engineering department and find out what’s going on.
In the article you say there should be no ground rod at the campsite pedistal. Most if not all of the campgrounds I’ve been at have a ground rod at the site pedistal. Is this against code or is it done to keep from running the fourth ground conductor all through the system?
The key to understand this is to realize that code does not REQUIRE a grounding rod at each campsite, but it’s certainly optional. However, the pedestal grounding rod cannot be a substitute for the 4th conductor (the ground wire, or more specifically the EGC / Equipment Grounding Conductor) that must connected to the ground bus in the pedestal back to the campground’s service panel Ground-Neutral Bonding Point. Additional ground rods do improve lightning protection a bit, and aren’t a bad idea. However, they will do little to nothing to prevent a hot-skin/stray voltage condition if the actual ground wire in your RV (the aforementioned EGC) doesn’t have a solid (less than 1 ohm) connection back to the service panel.
Mike
Nice explanation. it is straight forward but can be confusing. I am solid with the basic RV connections and power. But, two questions:
1) in your diagram showing the wiring of the pedestal. If I take my VOM (set on AC) and put the leads across the hot blade on the 30 and and to the hot blades of the 50 amp, I would see 240 vac on one of the 50 amp blades and zero volts across the other – correct? If the pedestal is wired correctly. Technically, I should see the same with the 120 VAC recip on the pedestal. Not a big thing, but just curious.
2) Regarding grounding – is there any advantage to have those grounding straps that I remember on some vehicles that drag on the ground and would touch the ground when the vehicle is parked? The would be attached to the frame or bumper to make sure they are connected to the frame.
Mostly interested in your opinion to question no. 2. I am pretty sure about Q1.
Thanks for a great blog. I enjoy it.
Steve
1) in your diagram showing the wiring of the pedestal. If I take my VOM (set on AC) and put the leads across the hot blade on the 30 and and to the hot blades of the 50 amp, I would see 240 vac on one of the 50 amp blades and zero volts across the other – correct? If the pedestal is wired correctly. Technically, I should see the same with the 120 VAC recip on the pedestal. Not a big thing, but just curious.
From Mike: Yes, you should read 0 volts when you’re on the common leg, and 240 volts when you’re on opposite legs.
2) Regarding grounding – is there any advantage to have those grounding straps that I remember on some vehicles that drag on the ground and would touch the ground when the vehicle is parked? The would be attached to the frame or bumper to make sure they are connected to the frame.
From Mike: No, grounding straps have nothing to do with actually “grounding” your RV while plugged into shore power. They were originally designed to help drain off static charges on your vehicle due to tire movement. I think that was an issue in the 50’s and 60’s because tire rubber formulation had very high electrical resistance (low conductivity). But with IIRC there was a change in tire chemistry that increases the rubber conductivity (lowered the electrical resistance) sufficiently so that any static voltage buildup on the vehicle would be drained to ground automatically. But I’ll ask Roger Marble to be sure. As a kid I remember seeing these grounding straps on school buses and gasoline delivery trucks, but haven’t seen them in years.
Late to the party here, but great diagrams Mike, as usual.
Thanks. I think that drawing out schematic diagrams makes it much easier to understand how these systems work, especially when you have to troubleshoot them.
Great article; however, your drawings need a spell-check. I think you want credit to “RVelectricity 2018.”
Well, I never said I could spell, so thanks for the heads up. Of course I not only got the spelling wrong on the first graphic, I simply copied and pasted it into the next two graphics.
A subject of general interest for publication and to include at Quartzite would be how to install an inverter in a RV that only has a converter. 15 years ago larger 5 ths came with both; no longer so.
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At Quartzite would I have to attend all 3 days to see all your seminars? Do you have the schedule yet.
Chuck and I don’t have a plan for which seminars I’ll be doing on any particular day, which is one of the reasons I did a survey. We should know more in a few weeks and will do an announcement then.
I got to *attempt* to explain to a park owner (it’s a mobile home park with one site converted to an RV site) that the 30 amp plug is not 240 V, it’s 120. The owner had been a licensed electrician in a previous life, and was sure he was looking at an old 240 volt dryer plug. Fortunately, apparently the first person who attempted to plug in, tested voltage (or had a good surge protector in his setup) and discovered the problem and fixed it before any rigs got destroyed.
In my case he replaced the 30 amp plug with a 50 amp plug and circuit breaker, properly. The pedestal, being installed for a mobile home originally, had a 200 amp service.
As I’ve noted elsewhere, I get at least several emails like this every month, and sometimes a few every week. Even though the TT-30 receptacle is plainly marked to 125 volts maximum, many old-school residential electricians simply can’t believe it’s “only” 120 volts. That’s one of the reasons I think that EVERY RV owner with a 30-amp connection needs an Intelligent/EMS surge protector. It only takes a few seconds of 240-volts on a TT-30 outlet to destroy most of the electronics in your RV. An intelligent surge protector that can disconnect you from incorrect voltage is way cheaper.
Well done!
Thanks… I carry this around in my head, so it’s about time I put it all down on paper.
Hi, Mike.
This is a timely article. I was just in a campground that had a ground wire/rod on the pedestal. Is this simply unnecessary, or harmful? The pedestal was mounted on a “telephone” pole. If harmful, how is this different from the style of pedestal that is metal all the way down to the ground? If it makes any difference, this campground is on a flood plain, and floods frequently. (I don’t think it makes a difference, but I’m not the expert!)
On a separate note, a possible other rally topic would be an SVP gathering if/when you could get enough attendees to make it worthwhile. Perhaps Quartzsite? Looking forward to meeting you there.
-Al
An extra ground rod and wire at a pedestal isn’t a problem, and it’s not a code violation. However, it’s not a substitute for a proper low-impedance (less than 1 ohm) ground wire connection to the EGC neutral/ground bonding point back in the incoming electrical service panel.
Yeah, I get a least a few emails a month about electricians and home owners who install a 30-amp/120-volt outlet for their RV. The TT-30 (Travel Trailer-30 receptacle) does indeed resemble an early 30-amp/240-volt dryer outlet found in many homes, So wiring one with 240-volts will do a lot of expensive damage in seconds if you plug your RV into one.
Well done. A request I get a lot as an electrician is to install a 240 rv outlet. First question I ask is 30 am or 50 amp. Most of the time it’s 30, so I have to explain it’s a 120 volt, not a 240 volt. A lot of people see the big plug and automatically think it’s 240, or they think the big prongs are 2 hots and the 3rd is a ground, not realizing it’s 1 hot, 1 ground, and 1 neutral. I even had 1 guy trying to rig up 2 extension cords to feed an rv outlet. He figured plugging 1 end in the top of the outlet would give 120 and the other end in the bottom would give him another 120. That was a tough chore explaining it doesn’t work that way.