Use this calculator to estimate cable voltage drop to size conductors.  The calculation assumes uncoated copper conductors operating at a temperature 75C (140F) and is based on the largest ac/dc resistance or impedance from NEC 2002 Tables 8 and 9.  The allowable ampacity of each conductor size in the dropdown menu below is based on NEC 2002 Table 310.16 for insulated conductors rated 0 through 2000 volts with not more than three current carrying conductors in raceway, cable or earth with an ambient of 30C (86F).

Note that the actual ampacity and voltage drop for your application may differ from these results but in most cases will be better than those shown here.

Units herein are American Wire Gauge (AWG) and English (feet).

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Voltage drop for ac systems should total no more than 5% under full load conditions.  Drop may be significantly larger during surge or motor starting conditions -- sometimes in the 15% to 25% range if other devices on the system can withstand this momentary dip.  Voltage drop for dc systems should be designed as low as possible or less than 2%.

For most 120/240V systems using cables of adequate ampacity voltage drop is not a concern unless cable lengths are well over a hundred feet.

More information about voltage drop based on IEC standards is available in the Schneider Electric Electrical Installation Guide.