Voltage drop calculator

Find the voltage drop, percentage and power lost in a cable run of copper or aluminium.

Your inputs

A resistive estimate for short low-voltage runs. It is not a wiring design: follow your local electrical code and use a qualified electrician for installations.

Circuit

Load current (A)

Cable length, one way (from supply to load)

Length unit

Conductor size

Conductor material

Supply voltage (V)

Line-to-line for three-phase.

Conductor temperature (°C)

Resistance rises with temperature; 20 °C is the reference.

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Your result

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A clear answer. Here’s how.

The conductor resistance per metre is ρ ÷ area, with the cross-section from AWG or the metric size and ρ from the International Annealed Copper Standard (or 61% of its conductivity for aluminium), corrected for temperature. A single-phase or DC circuit drops 2 × I × R × length because current flows out and back; a balanced three-phase circuit drops √3 × I × R × length between lines. The percentage divides by the supply voltage, and power lost is I² × the circuit resistance (three times the single-conductor loss for three-phase). Only resistance is modelled, so it suits short low-voltage runs.

How this is checked

3 reference checks passed These 3 cases were run when this page was built, on 20 October 2026, and all reproduce within the stated tolerance. The same cases are part of the automated tests that run on every change to Toolmoz. Engine version 1.

See the checks and where each expected answer comes from

Case 1
Inputs
Circuit singleLoad current (A) 15Cable length, one way (from supply to load) 50Length unit ftConductor size awg:10Conductor material copperSupply voltage (V) 120Conductor temperature (°C) 20
Expected, and what Toolmoz gave
Voltage drop (V) expected 1.4983, Toolmoz 1.4982943267868947Voltage drop (% of supply) expected 1.2486, Toolmoz 1.2485786056557455Power lost in the cable (W) expected 22.4744, Toolmoz 22.474414901803424Total circuit resistance (Ω) expected 0.099886, Toolmoz 0.09988628845245966Within 3% (a common design target) expected Yes, Toolmoz Yes
Where the expected answer comes from
Hand calculation in 40-digit decimal arithmetic: R = 3.2771 mΩ/m × 15.24 m × 2 = 0.099886 Ω, times 15 A = 1.4983 V, which is 1.2486% of 120 V
Allowed difference
±0.0001 absolute
Case 2
Inputs
Circuit threeLoad current (A) 20Cable length, one way (from supply to load) 100Length unit mConductor size mm2:6Conductor material copperSupply voltage (V) 400Conductor temperature (°C) 20
Expected, and what Toolmoz gave
Voltage drop (V) expected 9.9543, Toolmoz 9.954314986028029Voltage drop (% of supply) expected 2.4886, Toolmoz 2.4885787465070073Power lost in the cable (W) expected 344.8276, Toolmoz 344.8275862068965
Where the expected answer comes from
R = 1 ÷ 58 MS/m ÷ 6 mm² = 2.8736 mΩ/m, so √3 × 20 A × 2.8736 mΩ/m × 100 m = 9.9543 V and the loss is 3 × 20² × 0.28736 Ω = 344.83 W
Allowed difference
±0.001 absolute
Case 3
Inputs
Circuit singleLoad current (A) 16Cable length, one way (from supply to load) 20Length unit mConductor size mm2:2.5Conductor material copperSupply voltage (V) 230Conductor temperature (°C) 20
Expected, and what Toolmoz gave
Voltage drop (V) expected 4.4138, Toolmoz 4.413793103448277Voltage drop (% of supply) expected 1.919, Toolmoz 1.9190404797601202Within 3% (a common design target) expected Yes, Toolmoz Yes
Where the expected answer comes from
R = 1 ÷ 58 MS/m ÷ 2.5 mm² = 6.8966 mΩ/m, so 2 × 16 A × 6.8966 mΩ/m × 20 m = 4.4138 V
Allowed difference
±0.001 absolute

What this shows: the engine reproduces independently worked values for these cases. It does not prove every possible input is right. To test an answer you were given, see how to check a number; to see what is and is not checked across Toolmoz, see the accuracy ledger.

See a worked example

Worked example

A fixed demonstration, separate from the values you enter above.

Circuit: Single-phase or DC (out and back)
Load current (A): 15
Cable length, one way (from supply to load): 50
Length unit: Feet
Conductor size: 10 AWG
Conductor material: Copper (annealed, 100% IACS)
Supply voltage (V): 120
Conductor temperature (°C): 20

Example result

Voltage drop (V): 1.498
Voltage drop (% of supply): 1.25
Voltage at the load (V): 118.50
Power lost in the cable (W): 22.47
Conductor resistance (Ω per km): 3.277
Total circuit resistance (Ω): 0.0999
Within 3% (a common design target): Yes
Conductor: 10 AWG copper

Resistive drop only: single-phase or DC, 2 × I × R × length for the out-and-back path. Reactance, power factor, voltage-dependent loads, parallel runs and starting current are ignored, so use it for short low-voltage runs and check your local wiring rules, which set their own limits. Length is one way, from supply to load.