By Mike Sokol
In a recent issue of RV Travel Newsletter, you had an article that had a diagram showing the proper voltages between the contacts for a 50-amp connection and a 30-amp connection and a 20-amp connection all in one diagram. I have been searching for this diagram but cannot find it. I would appreciate it if you or someone could tell me what issue to look in, or email me the diagram. Any help would be appreciated. —BlueFlame
That’s an easy one, but only because I have lists of the hundreds of articles and graphics I’ve published over the last 10 years on RVtravel.com. That’s a lotta content to dig through manually, so here it is without you wearing out your search button.
Here’s what I think you’re asking about, which includes the general wiring hookup in the pedestal, as well as expected voltage readings for each of the outlets.
And if you’re going to be measuring voltages with a digital meter, you’ll want to set it to the 400- or 600-volt AC scale BEFORE plugging the probes into the outlet. You’ll see there’s a red probe and a black probe on the meter, but for AC (Alternating Current) it doesn’t matter which color probe goes in which contact on the outlet. However, that’s not the case for 12-volt DC measurements, which do indeed have a polarity. But more on that in a future article.
You’ll also see in my diagram above that the 50-amp outlet can measure either 240 volts or 208 volts from Hot-1 to Hot-2, but it must never measure 0 volts between the hot legs. That would indicate that the campground had miswired the outlet with a single leg of 120-volt power jumpered between Hot-1 and Hot-2 on the 50-amp outlet, which would cause the neutral current to be additive rather than subtractive. And that would allow 70, 80 or 90+ amperes of current to flow through your neutral wiring, melting your shore power plug and possibly destroying the wiring in your generator transfer switch.
The reason that it could read 240 or 208 volts is that since 2017 the National Electrical Code (NEC) has allowed 3-phase 120/208-volt power to be distributed in campgrounds. That’s perfectly safe if the electricians wiring the pedestals understand 3-phase power, but it’s possible to accidentally create some pretty serious wiring errors with 3-phase service panels that could result in massive RV electrical system damages. So that’s a topic for my advanced RV technician classes.
Finally, here’s a full-page graphic of just my voltage diagram above that you can print out and keep for reference. After 50+ years of doing electrical work I have all of these diagrams in my head (and possibly my DNA), but you’ll want to refer to this diagram until you have it committed to memory. After that, it’s smooth sailing – or smooth RVing – as the case may be….
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ 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.
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