By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – What’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
Dear Mike (aka J.A.M.),
Mike, I take delivery on a new Class A at the end of the month with 50-AMP service. My wife and I plan to travel the country for the next 3+ years. I see and read about various dogbones used to connect from power sources with less than 50-AMP service. With my limited knowledge of electricity I am wary of connecting this expensive coach without knowing I won’t harm the electrical system. I have already purchased a Surge Guard protector. Have you written about the do’s and don’ts of using dogbones? If not, it would be helpful. Saw you in Hershey last year and plan to see you this year. Thanks – Bill Evans
Dear Bill,
That’s an easy one. The thing to remember is to pick the proper size surge protector for your RV’s shore power plug, then use an appropriate adapter on the pedestal outlet that matches the surge protector plug. See the diagram on the right, which you can click to enlarge.
So if you have a 50-amp/240-volt RV, then buy a 50-amp/240-volt surge protector. If you need to plug into a 30-amp outlet, then add a 30-amp to 50-amp adapter. If you need to plug into a 15- or 20-amp outlet, then add a 15-amp to 50-amp adapter. (As a side note, 15- and 20-amp plugs can both carry 20-amperes of current. Only the sideways contact on the plug keys it to fit only 20-amp outlets.)
Same thing goes for anyone with a 30-amp shore power cordset. Get a 30-amp/120-volt surge protector, and add a 15-amp to 30-amp adapter if you need to plug it into a 15- or 20-amp outlet at a campground or your house. Of course, you’re still limited in the amount of wattage you can draw from each outlet, so a 20-amp outlet can only supply 2,400 watts (120 volts x 20 amperes = 2,400 watts), while a 30-amp outlet can only supply 3,600 watts (120 volts x 30 amperes = 3,600 watts), and both of these outlets are a big step down from a 50-amp/240-volt outlet, which will supply 12,000 watts (120 volts x 50 amps x 2 legs = 12,000 watts).
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Learn about Mike’s upcoming seminars in Hagertown, MD on June 8.
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.
##RVT892
You did not address if you are a 30 amp and have only 50 available. Use a 50 to 30 dogbone and the 30 amp protector. No more power than if you are plugged into a 30.
The danger of that kind of hookup is that you now have a shore power cord rated for 30-amps of current being fed by a 50-amp circuit breaker on the pedestal. Not all RVs with 30-amp shore power have an incoming 30-amp circuit breaker in their load panel, so the panel can pull way more than 30 amps. In fact, I’m answering a question about that very situation on another forum as I write this. So you’ve now created a condition where you can pull 40 to 50 amperes of current through smaller wiring and connectors only rated for 30 amps. I know that RVers get away with a lot of non-code things all the time, but if I were to do this in a professional capacity I could be liable for the resulting fire. So if you’re going to do this, you need to monitor your incoming shore power current carefully, and proceed at your own risk. Yes, it really is that dangerous.
So, to summarize: if I want to charge my batteries at home off the 15 A, I should:
1. Leave the 50 A surge suppressor attached.
2. Add a 50/30 stepdown
3. Add a 30/15 stepdown.
Correct, Mike?
Better if you use a single 15-to-50-amp dog-bone adapter for that purpose. One less connector. And you can still use your 50-amp surge suppressor on the RV itself.