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Voltage Drop Calculator

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Use this calculator to estimate cable voltage drop for sizing conductors.  The calculation assumes uncoated copper or aluminum conductors operating at the temperature selected and is based on the ac/dc resistance or impedance from NEC 2005 Tables 8 and 9 for stranded conductors operating on a DC or AC 60Hz system.  The ampacity of each conductor size in the dropdown menu below is based on NEC 2005 Table 310.16 for 60C 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 very close to those shown here.

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

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Click here for alternate calculator that also includes a transformer and motor load.

This should be the line-to-line voltage for multi-voltage and 3 phase systems.  For a 120/240V single phase system select 240V.  For other single phase systems select the line-to-neutral voltage.

These are uncoated copper or aluminum conductor sizes with CU/AL 60C Ampacity listed at 30C ambient for reference.

NEC tables assume 75C operating temperature. If conductor is oversized a lower temperature can be used but this temperature should always be higher than ambient. This value only used for adjusting conductor resistance.

See NOTE 1 below

Length of cable in feet (one-way distance)

Load current in amperes


Estimated voltage drop
See NOTE 2 below



  1. Examples for parallel runs: 120/240V single phase system with single black-red-white conductors (installed in single conduit) select "single set of conductors", 120/208V 3phase system with 2 conductors per phase and neutral (installed in 2 parallel conduits) select "2 conductors per phase in parallel", dc system with 3 positive and 3 negative conductors select "3 conductors per phase in parallel". 

  2. Voltage drop for ac systems should total no more than 5% under full load conditions.  This is consistent with the NEC which allows a total of 5% drop for feeders plus branch circuits.  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%.

  3. 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.  A common rule of thumb is to check the voltage drop when the one-way circuit length in feet exceeds the system voltage number.  Therefore, using this rule one would check the drop for a 240V system if the circuit length exceeds 240 feet.

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

UPDATE: On 11/4/2009 the 3-phase % calculation was adjusted by a factor of 1.732
UPDATE: On  9/25/2013 added #16 AWG; ac values extrapolated



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