Wire Size Selection for Long Runs

  • Voltage drop becomes important when the length of a run of wire or cable becomes very long. Usually this is not a problem in circuits within a house, but may become an issue when running wire to an outbuilding, well pump, etc.
  • Excessive voltage drop can cause loss of efficiency in operation of light, motors and appliances. This could result in lights that are dim and motors or appliances with a shortened life.
  • To avoid excessive voltage drop, select a size wire that will minimize voltage drop. You need to know the length of the wire run and the load (current) that will be on the circuit. To determine the load, add up the wattage of all electrical devices that will be on the circuit and divide this total by the voltage of the circuit, usually 120 volts or 240 volts.
120 Volt, Conductor size (AWG or kcmil)
Single Phase, Max 3% Voltage Drop*
Length of Run
Copper1412108615 AMP**
Copper121286420 AMP**
Copper101064430 AMP**
Copper1112/04/0100 AMP**
Aluminum1/01/02/04/0300 MCM100 AMP**
Copper3/03/03/0300 MCM500 MCM200 AMP***
Aluminum250 MCM250 MCM300 MCM600 MCM900 MCM200 AMP***
240 Volt, Conductor Size (AWG or kcmil)
Single Phase, Max 3% Voltage Drop*
Length of Run
Copper141412101015 AMP**
Copper12121210820 AMP**
Copper1010108630 AMP**
Aluminum8886430 AMP**
Copper8886440 AMP**
Aluminum6664340 AMP**
Copper6666450 AMP**
Aluminum4444250 AMP**

*The tables assume steel conduit, a power factor of 0.9, an ambient temperature of 86°F, and no more than 3 current-carrying conductors. We have taken reasonable care to establish the accuracy of these values; however, this tool is intended only as a guide, consult a professional engineer to determine suitability for your application.
**Based on 60°C per NEC 110.14(c)(1)(a).
***Based on 75°C per NEC 110.14(c)(1)(b).

WARNING! Installation of electrical wire can be hazardous if done improperly, and can result in personal injury or property damage. For safe wiring practices, consult the National Electrical Code®, your local building inspector, or a qualified electrician.


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