reply to SwedishRider
Re: Voltage Drop Tolerances Pull the cable off the inlet breaker back at the panel and short the hot and neutral together. Plug in the cord and measure the resistance between hot and neutral with your meter back at the end if the cordset. Use ohm's law (V=IR) to calculate voltage drop with the resistance you measured (in ohms) and your worst case current (30 A).
Edit: 240v circuit, short the two hots, ignore the neutral.