Voltage drop calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- AWG diameter = 0.005 × 92^((36 − n) ÷ 39) inch = 0.08081 in; area A = π d² ÷ 4.
- Resistance R = resistivity × length ÷ area (resistivity 1.7241 × 10⁻⁸ Ω·m at 20 °C) = 0.1588 Ω for one conductor.
- Drop = 2 × R × I = 4.765 V. Real wire is warmer than 20 °C and codes set their own limits; use this as a first estimate.
| Inputs | Answer |
|---|---|
| Circuit: Single phase or DC (there and back); Conductor: Copper; Wire size: 12 AWG; One-way length: 100; Current: 15; Source voltage: 120 | Over 100 ft of 12 AWG copper at 15 A the voltage drops about 4.76 V, 3.97% of 120 V. |
| Circuit: Single phase or DC (there and back); Conductor: Copper; Wire size: 10 AWG; One-way length: 150; Current: 20; Source voltage: 240 | Over 150 ft of 10 AWG copper at 20 A the voltage drops about 5.99 V, 2.5% of 240 V. |
| Circuit: Three phase; Conductor: Aluminium; Wire size: 4 AWG; One-way length: 200; Current: 60; Source voltage: 400 | Over 200 ft of 4 AWG aluminium at 60 A the voltage drops about 8.45 V, 2.11% of 400 V. |
How to use it
Voltage drop is the loss of voltage along a wire because of its resistance. The calculator finds the wire's cross-section from its AWG number (diameter = 0.005 × 92^((36 − n) ÷ 39) inch), computes the resistance from the material's resistivity (copper 1.7241 × 10⁻⁸ Ω·m, aluminium about 2.82 × 10⁻⁸ Ω·m, at 20 °C), and multiplies by the current, using the round trip length for single-phase and DC circuits.
100 feet of 12 AWG copper carrying 15 A on 120 V drops about 4.8 V (4%). Warm wire has higher resistance and electrical codes set their own limits, so treat the result as an estimate and follow the code that applies to you.
Frequently asked questions
How do I calculate voltage drop?
Resistance of the wire × current, using twice the length for a single-phase or DC circuit.
What voltage drop is acceptable?
Rules of thumb often use about 3% for a branch circuit; check your local code.
Does a bigger wire reduce the drop?
Yes, a thicker wire (lower AWG number) has less resistance.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.