MasterTool

Voltage Drop Calculator

Estimate resistive voltage drop for copper conductors.

Loading calculator…

What this tool does

Estimate resistive voltage drop for copper conductors. Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.

How to use the Voltage Drop Calculator

  1. Enter or select current (amps).
  2. Enter or select one-way conductor length in ft.
  3. Enter or select voltage.
  4. Enter or select supply.
  5. Enter or select copper wire gauge.
  6. Read the calculated result; change any measurement to compare alternatives.

Formula

drop = (2 single phase or √3 three phase)×current×one-way length×resistance per foot; percent=100×drop/voltage
current
Current (amps)
length
One-way conductor length (ft, before metric conversion)
voltage
Voltage
phase
Supply
awg
Copper wire gauge

Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.

Worked example

For voltage drop calculator, the following measurements illustrate the exact method: Current (amps): 20; One-way conductor length: 100; Voltage: 240; Supply: 1; Copper wire gauge: 12.

Inputs

  • Current (amps)20
  • One-way conductor length100 ft
  • Voltage240
  • SupplySingle phase
  • Copper wire gauge12 AWG

Result

  • Voltage drop (V)7.72
  • Voltage drop (%)3.22
  • Estimated load voltage232.28

Results explained

Voltage drop (V)
Voltage drop (V) from the formula above. Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.
Voltage drop (%)
Voltage drop (%) from the formula above. Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.
Estimated load voltage
Estimated load voltage from the formula above. Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.

Frequently asked questions

drop = (2 single phase or √3 three phase)×current×one-way length×resistance per foot; percent=100×drop/voltage

Uses typical copper resistance around 75°C; reactance, terminals and unbalanced loading are excluded.

No. Electrical and HVAC results are estimates. Installation must comply with local codes and be checked by a qualified licensed professional.

Nameplate values, conductor temperature, power factor, efficiency, weather and actual installation conditions may differ from your entered assumptions.

Confirm units, manufacturer data and the formula's assumptions, then verify sizing with the equipment instructions and applicable local standards.