Estimate DC, single-phase, or three-phase voltage drop by conductor size, material, current, voltage, and one-way run length.
Calculator
Voltage Drop Results
Review the main result, supporting measurements, and calculation details.
Voltage and Run-Length Analysis
Calculation Details
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How to calculate voltage drop
Select DC, single-phase AC, or three-phase AC, then enter conductor material and size, one-way run length, current, and system voltage. The calculator uses approximate conductor resistance to estimate voltage lost and voltage remaining at the load.
DC or single phase: Vdrop = 2 x length x current x resistance / 1,000; three phase: Vdrop = √3 x length x current x resistance / 1,000
Voltage-drop example
A 100 ft one-way, 120 V single-phase circuit carrying 15 A on 12 AWG copper at an approximate 1.93 ohms per 1,000 ft loses 5.79 V, or 4.83%. The calculator also estimates a larger conductor and the maximum run length for the selected drop target.
Why conductor size matters
Longer runs, higher current, and higher resistance increase drop. Larger conductors have lower resistance. Aluminum has higher resistance than copper at the same gauge, so equivalent installations may require different sizes and terminations.
Planning limits
A common design target is 3% for a branch circuit, but the correct limit depends on the complete system and applicable requirements. Southwire's voltage drop calculator also considers ampacity and installation details. This page uses a simplified resistive model and does not account for reactance, temperature, harmonics, parallel conductors, or every installation method.
Frequently asked questions
Is one-way length or round-trip length entered?
Enter one-way length. The DC and single-phase formula doubles it for the outgoing and returning path.
Does the recommended gauge prove code compliance?
No. It addresses the entered voltage-drop target only. Check ampacity, insulation, terminals, temperature, bundling, conduit fill, equipment instructions, and local code with a qualified electrician.
Where can I check ampacity too?
Use the wire size calculator for a combined reference ampacity and voltage-drop estimate.