electricalextension
I have a 220v tanning bed.
The bed has a 4ft cord 
; need to extend another 10 feet to reach 220v dryer outlet 
.
The bed came with a homemade extension, but plug doesn't fit our outlet.
Homemade extension wire is 10 gauge; 15 feet long.
Is it safe to just replace the plug on the homemade extension with one that fits my outlet?
Thanks in advance!!
You can make a cord using listed components. A 10awg 3 with ground SO cord (4 wires) and the correct cord cap you can make the length you need and it would be legal. a link to the wire and the plug (normally called a cord cap). I would use crimp connectors on the end that attaches to the dryer because the SO is fine braided wire like this. Here is a link to a cord cap not sure if it is the same as you have but it will give you an idea if you were not sure what I described cord cap link.
Thanks for the pictures! It looks like the problem is that your extension cord is polarized, but your Christmas lights aren't. The best solution is to replace the electrical receptacle at the end of the first set of lights with a polarized one. They sell replacement receptacles for Christmas lights.
You could use a ground plug adapter to do this, but you would need to trim the polarized neutral plug to fit in a non-polarized hole which is a bad idea. It will be fine for this season, but once you take your lights down for the season you'll forget that you have a modified adapter that could be reused for something else and hurt somebody.
Best Answer
The only solution, is to install a code compliant grounding conductor, circuit breaker, and receptacle.
The circuit breaker is easy. Since you're going from 30 amperes down to 20, you can simply swap the 20 ampere breaker in. However, since you're changing it to a 20 ampere branch circuit, AFCI and/or GFCI protection may be required.
The receptacle is also easy. Just swap out the old receptacle for the new one.
Installing a code compliant grounding conductor may require pulling a new cable. Depending on the home, you might be able to find another way. Though without being on site, it's difficult to suggest a solution.
Once you have the proper circuit protection, wiring, and receptacle. Then you can buy an extension cord, and safely extend the reach of the cord.
If you end up having to pull new cable anyway, you might consider simply installing a new receptacle closer to the tanning bed.