I did a little test to consider the EV manufacturer advice not to charge via extention cords (cables).
I connected my iMiEV charging from its 'portable' 10A EVSE unit at the end of 100m + 20m + 10m + 10m of extention cables.
The cables were just dumped in a loosely bundle on the ground.
Total 140m of cable. (not that this is my normal charging method !)
These are all 1mmsq '10A' types.
Input voltage is 235V 9.41A 2205W pf 0.99 (arlec power meter)
Output voltage is 203V 9.41A 1864W pf 0.99 (iMiEV will charge down to about 90VAC from the specs)
(the power meter needs more significant digits so P=VxIxpf accurately!)
So some 300W is being lost in the resistance of the extention cables. (16%) Not the end of the world, but a loss none the less. And of course a longer charging time.
Heating of the cable if coiled or rolled up is the biggest issue. But then usually the need for an extention cord means that the cable will be out straight.
After charging for an hour, there was a just decernable temperature rise in the 100m loosely coiled cable. Other than that, all is well.
At only about 2W/m, the cable would need to be rolled up for there to be a temerature issue.
Coming back to a say 20m extention cable, loss would be around 42W in some 2000W (2%).
Water on the extention cable plugs is another consideration. Take care in the rain.