By Mike Sokol
Hope you all read my guest essay in this issue so you can get a feel for just how important RV electrical hookups are for you and the industry. And since there’s been so much confusion about 120/240-volt power on groups that cover RV electricity, this is a great time to review how 20-, 30- and 50-amp pedestal hookups actually work, and how they should measure using a meter.
Then in my RVelectricity Newsletter, which is publishing on Sunday, I’ll get really deep into a discussion about wiring a residential dryer to work from a 120-volt RV circuit, rather than the 240 volts available in a home.
But let’s discuss pedestal voltages first, to prepare you for the real brain teaser tomorrow. In a typical campground pedestal you’ll find 20-, 30- and 50-amp circuit breakers feeding 20-, 30- and 50-amp outlets. Note in the picture that while the 20- and 30-amp outlets are being fed by circuit breakers with a single handle, the 50-amp outlet is being fed by a double (2-pole) circuit breaker. There’s an important reason for that, which we’ll cover shortly.
Here’s a diagram of what’s actually hooked up inside of the pedestal. You’ll see that there should be a 120/240-volt, 2-pole service coming into the pedestal, which will be properly current limited by appropriate circuit breakers. From top to bottom there’s a 50-amp/120/240-volt/2-pole circuit, a 30-amp/120-volt/single-pole circuit, and a 20-amp/120-volt/single-pole circuit.
The other thing to note is that the 50-amp circuit has 2 poles (legs) of 50 amps each, so essentially that can provide a total of 100 amps of current at 120 volts (50 + 50 = 100), as long as you don’t go over 50 amps on either leg (pole).
The outlets for each of these circuits is defined by NEMA and the NEC, with the 50-amp outlet using the exact same 50-amp/240-volt NEMA 14-50 receptacle often found in homes to power your electric oven and range top.
The 30-amp circuit is unique to the RV industry, using something called a NEMA TT-30 receptacle (TT as in travel trailer), and it’s only rated for 120 volts, not 240 volts as its close cousin the home dryer outlet, which is indeed wired with 2 poles to create a 240-volt service. As many emails as I get about this suggests that perhaps every day someone is miswiring a 30-amp TT-30 outlet with 240 volts and destroying an RV electrical system.
And last but not least (or actually, it really is least), the humble Edison outlet, which comes in both 15- and 20-amp versions. Interestingly, even a 15-amp version will safely pass a full 20 amps of current if it’s connected to a 20-amp circuit breaker and 12-gauge wiring. Note that it’s currently the only outlet on a pedestal that requires GFCI protection, which is why you’ll see a TEST and RESET button.
In terms of voltages you should measure, here are the basics. All 50-amp outlets need to measure a nominal 240 or 208 volts (plus or minus 10%) between the two hot legs. The 208-volt measurement occurs when the campground is distributing 3-phase power to the campsites, which seems scary but is actually quite safe as long as the campground electricians understand it.
This 208- or 240-volt potential between the Hot-1 and Hot-2 contacts is to create a subtractive neutral condition so the neutral wiring and contacts won’t carry more that 50 amps of current. If the campground runs a single leg to both contacts on a 50-amp outlet, the shore power contacts and neutral wires could be forced to carry up to 100 amps of current without tripping the 50-amp circuit breaker, and that’s not a good thing at all. That’s because the RV plug, surge protector and shore power cord can fry from over-current and possibly catch on fire.
You should also note that on a 50-amp outlet the Hot-1 and Hot-2 poles should each measure around 120 volts to neutral, while you can expect to see 1 or 2 volts (and up to 3 volts) measured between the neutral and ground contacts. That’s quite normal and to be expected just from typical voltage drops in the neutral wire feeding your pedestal.
The 20- and 30-amp outlets should measure exactly the same as each other, with 120 volts and their respective amperage available via the circuit breakers. Again, it’s typical to measure perhaps 2 or 3 volts between the neutral and ground on any loaded circuit, which is perfectly safe and to be expected.
In any event, be especially wary when plugging into any NEW 30-amp outlets. Some residential electricians will be confused by their resemblance to 1970’s 240-volt dryer outlets, which were (and still are) wired with 2-pole of power, resulting in 240 volts. If you plug your 30-amp shore power cord into one of these it will result in a serious meltdown of all your RV’s electrical system and any appliance unlucky enough to be plugged in at the time.
We’ll, that’s about it for now. Read my RVelectricity Newsletter on Sunday to find out how some home dryers can be converted to run from 120 volts rather than 240 volts, and what happens if you don’t wire them properly.
Keep studying electricity, and let’s play 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.