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.
I just got an RV and need to use it for COVID-19 isolation since my wife is a health care provider and she doesn’t want to come in the house after her shift. The RV is hooked up in my backyard with an extension cord and everything was working fine until she ran the air conditioner and microwave at the same time – that’s when it tripped the circuit breaker in my house. I keep resetting the breaker, but it trips again. Is there something wrong with my circuit breaker? Or is it possible the RV is drawing too much power? Should I get an electrician to fix it? I have a good electrician but I don’t even know what to ask for. —Stan
Be sure to thank your wife for her bravery and service. That’s one tough and vitally important job she’s doing, and it’s great that you’re both working to keep your family safe.
You’ve run into the weakness in nearly all RV power hookups: lack of power (watts). From your description it sounds like you have an RV power cord with a TT-30 plug. If that’s so, then you’re probably using an adapter (called a dog-bone) to plug into an Edison outlet on your house. And that’s where the trouble begins.
You see, there are three versions of shore power cords your RV would have come with: a 20-, 30- or 50-amp cord. Now, I haven’t seen a 20-amp shore power cord on an RV in decades (except for an occasional pop-up or teardrop towable), since that’s equivalent to plugging everything in your house into a single electrical outlet on the wall.
The next step up is a 30-amp shore power cord (called a TT-30 or Travel-Trailer 30), which most small and medium RVs have. It has a 3-prong plug that’s connected to an outlet that looks like this. Note that while this outlet looks like it should be wired for 240 volts, it’s most definitely only 120 volts.
Larger RVs, including most 5th wheels and coaches, have a 50-amp shore power cord that plugs into an outlet that looks like this. This is called a NEMA 14-50 receptacle, and it’s wired exactly like any 50-amp 120/240-volt stove outlet in your home.
Let’s explore just how much power (watts) is available from each one of these outlets:
- A 20-amp service can supply 2,400 watts
- A 30-amp service can supply 3,600 watts
- A 50-amp 120/240 service can supply 12,000 watts (6,000 watts per 120-volt leg)
Now let’s look at a chart of how much wattage a typical appliance in your RV draws:
- Microwave = 1,000 watts
- Computer = 200 watts
- Television = 200 watts
- Air conditioner = 1,300 watt (running)
- Electric water heater = 1,500 watts
- Battery converter/charger = 500 watts
So if your RV is plugged into a 20-amp outlet at home using an adapter like this one, then the first chart shows you that there’s 2,400 watts you can use. But a microwave will draw around 1,000 watts when it’s running and the air conditioner will draw at least 1,300 watts when the compressor is running (much more for a second or two of startup current). Add in addition to that you may have a television set on drawing 200 watts, maybe your computer is pulling another 200 watts, and your RV has a converter to make 12 volts DC for the interior lighting that’s likely pulling at least 200 and maybe 500 watts. Add that up in nearly any combination and you’ll quickly get above the 2,400 watts of power you can get from your house.
If this is the case (and I’m pretty sure it is), then you need to consider having your electrician run a dedicated 30-amp power outlet on the exterior of your house near where your RV is parked. That will give you 3,600 watts of power instead of the 2,400 watts you’re getting from an Edison outlet. Your wife will still have to do a little power management since 3,600 watts of power isn’t enough to run everything in your RV at once. So I would suggest she not run the microwave while the air conditioner is on, and make sure your water heater is set to propane, not electric.
One really important thing to tell your electrician is that the 30-amp outlet for your RV is wired for 120 volts, NOT 240 volts like a similar-looking dryer outlet in your house. See my diagram comparing the two types of outlets.
If the electrician accidentally wires the 30-amp outlet with 240 volts and you plug your RV into it, you’ll probably burn up most of your RV’s electrical system in a few seconds. So show your electrician my chart and make sure you double-check the voltage before plugging your RV in for the first time.
Good luck and stay safe. And tell your wife she’s a hero to us all.
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 stay sane and 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.