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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!!
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.
The plug on the tanning bed is a 6-20P plug (250V 20 Amp), but the receptacle is a 10-30R (125/250V 30 Amp), so they aren't quite the same. The tanning bed expects 2 hots and a ground wire, but the outlet provides 2 hots and a neutral (which is almost, but not quite the same)
The "correct" way to do this would be to replace the dryer plug with a properly grounded 6-20R receptacle and replace the 30 Amp breaker in the panel with a 20 Amp. The only difficulty may be in finding a code compliant way to hook up the ground terminal of the 6-20R using existing wiring.
If the existing shared neutral/ground wire is bare or green, then you can use that as-is, just make sure that it's connected to the grounding busbar in the panel. (a main panel may have shared ground/neutral busbar(s), a subpanel will have separate busbars for ground and neutral and it is important for safety to terminate grounding conductors on the correct busbar). If the existing neutral wire is not already a green insulated or bare copper wire, you cannot re-use this wire as a ground even if you mark the wire as such. If the wires go through continuous metal conduit back to the panel, you may be able to use the conduit as ground through a self-grounding receptacle in the metal box, or a ground jumper from the conduit/box to the receptacle.
Once you have a grounded 6-20R receptacle to plug into, you can use an off-the-shelf 6-20R extension cord to extend it.
For something like a tanning bed where people (likely with damp sweaty skin) will definitely be in contact with it, I wouldn't cut corners on grounding and do it the right way.
There are 6-20R to 10-30R adapters available, but they are not safe or technically legal
Check to make sure the breaker for this 10-30 is in a MAIN panel, and not a sub panel. If it's a main panel the case, then it is safe to adapt a 6-20P extension cable with a 10-30p replacement plug head. You won't have to modify the building this way, so don't worry about code. CLEARLY label the cable you make for this purpose.
If the panel is a subpanel then you will need to move the neutral conductor to the ground bus bar and yes, that violates code because the outlet is supposed to be able to supply ungrounded split-phase. So CLEARLY label the outlet, or take the opportunity to replace it with a 6-20 outlet and swap the breaker out with a 20-amp while you're at it because you must do so to protect the 20-amp outlet per code. Then use an off the shelf 6-20 extension.
