Hi Mike,
I need to add a 25-foot extension cord to my shore power cord to reach the outlet in my garage while my RV is parked in the driveway. I don’t need 50 amps since this is just for charging the batteries and running the air conditioner when the kids want to hang out in the RV. I was going to get a 20-amp to 50-amp adapter and an extension cord, but there’s a lot of price difference between the orange cords I can get at Home Depot and the heavy black cords from the RV stores. Why is there such a price difference, and just how do I pick the right extension cord. I love your articles, and keep up the good work. —Steve
Hi Steve,
Glad you like my articles, and I’m happy that you’re asking these questions before you get into trouble. No, not all extension cords are created equal, and here’s why. It all comes down to amperage needed, how much voltage drop you can put up with, and the temperature rise allowed.Â
So let’s start with a simple 30-amp circuit. As you may know from one of my previous articles, that can provide up to 3,600 watts total (120 volts x 30 amps). Here’s a chart of the wire size needed to pass 30 amperes of current safely. 
Let’s say that you do get the cheap orange 16- or 14-gauge extension cord from Home Depot and manage to hook it up to your RV with a series of dog-bone adapters. Looking at the chart you’ll see that a 14-gauge cord is only rated for 15 amperes of current and you’re asking it to pass 30 amperes. Not only will you increase the voltage drop down to less than 110 volts (as low as 100 volts with a long cord), you’ll also get a lot of heating of the extension cord itself, which can be dangerous and a fire hazard. Here’s a video I made of what happens when you overload a 15-amp extension cord with 30 amperes of current.
That’s why you really need at least a 10-gauge extension cord for any 30-amp circuit. As you can see from the chart above, it’s rated for 30 amperes of current so it will have much less voltage drop (easily half as much as a 14 gauge cable with the same current). Plus there will be much less heating.  
I recommend that you get a proper pedestal outlet installed in your garage and a 10-gauge extension cord for a 30-amp circuit or a 6-gauge extension cord for a 50-amp shore power circuit. You’ll pay more up front, but all your RV appliances will operate properly and you won’t worry about getting hot around the collar when it’s time to crank up the air conditioner. Let’s play safe out there….
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.
##RVT796
The right gauge extension cord for an RV depends on its power circuit. 10 AWG extension cords are typically ideal for heavy-duty use, so they are compatible with RVs, which are below 30-amp circuit. If you have a big 50-amp circuit RV, you may require a specialized and more heavy-duty extension cord.
Hello my name is Tasha Jones . we r rving and usinf thw air condition what drop cord do we use
Mike, you suggest 10 gauge cord for 30 amp through 25 feet of extension cord. What gauge cord would you recommend for 20 amp with a 50 foot extension cord. We only need it to extend slideouts and run the refer prior to a trip.
Mark
Good point. The picture of the coiled mess is from an article I wrote about HOW NOT to store an extension cord. And you’re correct that an extension cord tangled and jammed into a small space can cause overheating. However, it’s not due to Eddy Currents or any other magnetic or inductive effects. It’s simply a lack of airflow that causes the heat building.
The same idea of magnetic coupling causing overheating is also an old-wives tale in the pro-sound industry. Since we generally transport big extension cords on reels, the normal practice is to unreel the entire length as you suggest for RV users. That’s also good advice to prevent overheating, but again, for the wrong reason. I can show this is true via the math, but not even my sound engineers believe me. Perhaps I need to set up an experiment and do a video of the temperature rise of extension cords in various configurations. Plus I have test gear that can measure the actual inductance of the wire. That would be an interesting experiment, I think.
Mike;
I noticed at the side of your article about ‘extension cords’ that there is a photo of a jumble of orange cord in a pile.
This is a usual situation until the pile is sorted out and in use. However……..
Some dimbulb out there might construe this as an acceptable way to put a cord into service. It is Not!
The cords should not be used in a pile or a coil, but rather should be laid out in a line or zig-zagged when in use.
THIS to prevent overheating/fire due to magnetic induction causing back-flow. (A coil is a coil, right?)
In reading your book, “No-Shock” and other articles, I do not recall seeing this advisary anywhere.
Perhaps the ‘Hoi-Poloi’ should be given a heads up on this matter.