20m is 65'. So OP is talking about at least 200'.
There's a calculator here
https://www.tlc-dire... Min Cable Size
which suggests you're OK up to about 50m with standard extensions, which are 1.5mm squared. A 2K only draws about 8A so you're OK in theory There's a bit of voltage drop as you see.
If you can find a heavy-duty cable it would be better.
Keep an eye on the cable and make sure it is fully unwound otherwise it will act as a heating element and get a bit hot.
Just make sure they're not light-duty 6A extensions.
Edited by Mark Dunn, 08 February 2016 - 02:31 PM.