It depends. On a transformer-based buzz-box, the welding current will drop slightly for a given setting when using an extension lead. The effect will be greatest at the highest current setting, since that will produce the greatest voltage drop in the supply cabling. On an inverter-based welder, the welding current will be changed little, if at all, by using an extension cable, but the maximum welding current would tend to reduce with cable length, though not by much.