You can use the CT with an extension cord, bbut you have to be mindful of your amperage draw and cord length. I use mine with a 12/3 40' cord reel on a 20A circuit using the 15A-20A adapter with no trouble. I'm sure that you could calculate the voltage drop from a longer or smaller guage cord, but if you're going to be pulling nearly 20A, you'd better make sure that you use a very heavy wire. When I'm not in my shop I haul around a 50' 12/3 20A/15A cord that I made. I've yet to pop a 15A circuit breaker with it, but I think that may just be because of the soft start feature of most of the tools.