MasterTool

Wire Size Calculator

Find a copper gauge that meets an entered voltage-drop target.

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What this tool does

Find a copper gauge that meets an entered voltage-drop target. This checks voltage drop ONLY, not safe current capacity or code compliance.

How to use the Wire Size 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 maximum voltage drop (%).
  6. Read the calculated result; change any measurement to compare alternatives.

Formula

choose smallest listed copper gauge where phaseFactor×current×length×resistance <= voltage×allowed/100
current
Current (amps)
length
One-way conductor length (ft, before metric conversion)
voltage
Voltage
phase
Supply
allowed
Maximum voltage drop (%)

This checks voltage drop ONLY, not safe current capacity or code compliance.

Worked example

For wire size calculator, the following measurements illustrate the exact method: Current (amps): 20; One-way conductor length: 100; Voltage: 240; Supply: 1; Maximum voltage drop (%): 3.

Inputs

  • Current (amps)20
  • One-way conductor length100 ft
  • Voltage240
  • SupplySingle phase
  • Maximum voltage drop (%)3

Result

  • Gauge for voltage drop ONLY1/0 AWG
  • Drop at selected gauge (%)0.2

Results explained

Gauge for voltage drop ONLY
Gauge for voltage drop ONLY from the formula above. This checks voltage drop ONLY, not safe current capacity or code compliance.
Drop at selected gauge (%)
Drop at selected gauge (%) from the formula above. This checks voltage drop ONLY, not safe current capacity or code compliance.

Frequently asked questions

choose smallest listed copper gauge where phaseFactor×current×length×resistance <= voltage×allowed/100

This checks voltage drop ONLY, not safe current capacity or code compliance.

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.