Q&A’s from other forums
I spend a lot of time on dozens of other RV forums answering questions about electricity. Here is one really interesting topic that keeps cropping up. Just how many amps can a 50-amp shore power outlet supply?
From the Holiday Rambler Group:
Q: Dear Mike,
Saw this page on the web describing a 50 AMP service. I have a problem with some of the wording saying that each leg is 50 amps at 120 volts. That has made some RVers think they can run 50 AMPS on each leg. I think the wording should be clearer that there are only 50 AMPS total available not 100 AMPs. What are your thoughts? —Dick K. / 2015 Holiday Rambler Ambassador 38DB
A: There’s just no easy way to put this, so I’m going to rip off the bandage fast. You are wrong and they are right. That is, a properly wired, 50-amp, 120/240-volt campground pedestal does indeed have 100 amps of current available at 120 volts as long as you don’t exceed 50 amps on either leg. That’s because a properly wired 50-amp service includes two separate 120-volt legs (Hot 1 and Hot 2) of up to 50 amps each. So you can draw 30+30 amps, or 40+40 amps, and up to 50+50 amps and still be within limits. See my diagram on the left that shows how this works.
Note that if you add 50 amps plus 50 amps you get 100 amps total, which is 12,000 watts of power (120 volts x 100 amps = 12,000 watts), and power is the only thing that really counts. Now don’t confuse this wiring diagram with the 240-volt power like you would use in your house to run an electric stove. In that case you definitely only have 50 amps of total current at 240 volts.
But because nearly every appliance in an RV is wired for 120 volts (with only a few exceptions), then you have up to two times the amperage available on a single leg. And even if you do have an RV with 240-volt appliances (yes, the larger Cheap Heat systems use 240-volts), then you should be able to read the amperage supplied to a hard-wired 240-volt heater on both of your amperage meters. So you don’t have anything to worry about until you get over 50 amps of current on either leg.
Electrical Code Alert!
One important thing to know is that according to the NFPA 70 National Electrical Code, a 50-amp circuit breaker and associated wiring is only rated for 50 amperes of non-continuous power. In reality, you have to derate it to 80% for continuous duty appliances such as electric heaters. So a 50-amp RV outlet is really closer to 80 amps (100 amperes x 0.80 = 80 amperes) if you’re in constant power draw mode. And space heaters can definitely demand constant power. Just one more thing to know about electricity.
And yes, your house wiring has the same derated amperage percentage. You just don’t normally have to think about it because a 200-amp panel in your house actually has 400 amps of current available at 120 volts or 200 amps at 240 volts, or any combination thereof. So even a 50-amp, 120/240-volt pedestal can’t keep up with the power available to your house, no matter how much you want it to.
As a side note, also be aware that the 20-amp wall outlet in your house is only rated for 16 amps of continuous current draw, which is why domestic space heaters typically top out at 1,800 watts, because 1,800 watts divided by 120 volts equals 15 amperes, and that’s below the 16-ampere continuous duty rating for appliances such as space heaters and hair dryers.
Email me at mike (at) noshockzone.org with your questions.
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.