DC Voltage Drop Calculator
Estimate loss in an existing two-wire DC copper run.
Uses two equal-length standard annealed copper conductors: one supply and one return.
Calculated voltage drop
- Voltage loss
- V
- Percent drop
- %
- Estimated load voltage
- V
The calculated loss meets or exceeds the source voltage. Do not treat the assumed current as a validated load operating point.
- AWG resistance
- Ω/1,000 ft
- One-way length
- ft
- Round-trip length
- ft
- Loop resistance
- Ω
Substitution
This is a resistance-only estimate for standard annealed copper at 20 °C. Actual wire temperature, construction, connectors and installation can change the voltage at the load. Voltage drop alone does not establish ampacity or code compliance; check equipment requirements and applicable rules with a qualified electrician.
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What this DC voltage drop calculator measures
This page estimates resistive voltage loss for an existing American Wire Gauge size. It assumes a supply conductor and a return conductor of the same gauge and one-way length, made from standard annealed copper at 20 °C. It does not select a wire size or approve an installation.
Formula and round-trip length
For resistance r in ohms per 1,000 ft and one-way length L in feet, the copper loop is 2 × L. Loop resistance is r × (2L) ÷ 1,000. Voltage drop is current × loop resistance; percent drop is 100 × voltage drop ÷ source voltage.
For example, 50 ft one way uses 100 ft in the loop. With 14 AWG copper at 2.52 Ω/1,000 ft, the loop resistance is 0.252 Ω. At 5 A, that is a 1.26 V loss.
Source and model boundary
The eight resistance presets are transcribed from the U.S. National Bureau of Standards Copper Wire Tables, Handbook 100 (1966), Table 5, for American Wire Gauge standard annealed copper at 20 °C. The source notes that copper with a different resistivity needs correction; later tables separately cover stranded construction.
Actual wire temperature, construction, connectors, installation, and return path can change the voltage at the load. This page does not model AC impedance or power factor, aluminum, temperature correction, contact loss, conductor ampacity, breaker sizing, or electrical-code compliance.
Feet and meters
Feet are the default because the source table reports resistance per 1,000 ft. Choosing meters converts the entered physical length using the exact relation 1 ft = 0.3048 m. Switching units changes the visible number but not the run being measured.
Frequently asked questions
How do I calculate DC voltage drop?
Multiply the load current by the total resistance of the supply-and-return loop. This calculator takes the NBS resistance for the selected AWG, doubles the one-way length exactly once, calculates loop resistance, then reports voltage loss, percent drop, and estimated load voltage.
Why does one-way length double?
A two-wire DC circuit has an outbound supply conductor and an equal-length return conductor. A 50 ft one-way run therefore has 100 ft of copper in the resistance loop. Do not use this model for a single conductor with an unknown return path.
Does a low voltage drop mean my wire is safe?
No. This resistance-only result does not establish conductor ampacity, breaker size, temperature rise, connector suitability, or code compliance. Check the equipment requirements and applicable rules with a qualified electrician.
Can I use this calculator for AC?
No. The model is deliberately limited to DC resistance. AC circuits may require impedance, reactance, power factor, phase, conductor construction, and installation assumptions that are not included here.