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Wire Voltage Drop Calculator

Computes the resistance of a copper or aluminium conductor from its AWG gauge or cross-section and one-way length, then the voltage drop, percentage drop, voltage at the load and power lost for DC, single-phase or three-phase circuits.

Source voltage (V)Supply voltage (line-to-line for three-phase). Load current (A)Current carried by the conductor in amperes. One-way length (m)Distance from source to load in metres (1 ft = 0.3048 m); the return conductor is accounted for automatically. Wire gauge (AWG) (optional)American Wire Gauge number. Use 0 for 1/0, −1 for 2/0, −2 for 3/0, −3 for 4/0. Give either awg or cross_section_mm2. Cross-section (mm²) (optional)Conductor cross-sectional area in mm² (metric cable sizes 1.5, 2.5, 4, 6, 10 …). Give either awg or cross_section_mm2. Conductor material (optional)Conductor material; resistivity at 20 °C. Circuit type (optional)DC and single-phase count the outgoing and return conductor (×2); balanced three-phase uses ×√3. Conductor temperature (°C) (optional)Operating temperature of the conductor; resistance rises about 0.4 % per °C above 20 °C (use 75 for a fully loaded cable).

When to use

You are sizing a cable run (solar, automotive, 12 V LED, branch circuit) and need to know whether the voltage drop stays within a limit such as 3 %.

Do not use when: You need ampacity (heating limits, which depend on insulation and installation method, not covered here), or the conductor is not copper or aluminium.

Formula

d_mm = 0.127 × 92^((36 − awg) / 39); A = π d² / 4; R = ρ × (1 + α (T − 20)) × length_m / A; voltage_drop_v = k × current_a × R with k = 2 (dc, single_phase) or √3 (three_phase); ρ_copper = 1.724e-8 Ω·m, ρ_aluminium = 2.65e-8 Ω·m

Resistive (DC) drop only; conductor reactance and power factor are ignored, which is accurate for cables up to about 25 mm² / 4 AWG at 50–60 Hz. The 3 %/5 % limits follow NEC 210.19(A) and 215.2(A) informational notes and similar rules in IEC 60364-5-52.

Inputs

Parameter	Type	Unit	Required	Description
`voltage_v`	number	V	yes	Supply voltage (line-to-line for three-phase). Range: > 0, ≤ 100000
`current_a`	number	A	yes	Current carried by the conductor in amperes. Range: ≥ 0, ≤ 100000
`length_m`	number	m	yes	Distance from source to load in metres (1 ft = 0.3048 m); the return conductor is accounted for automatically. Range: > 0, ≤ 100000
`awg`	integer		no	American Wire Gauge number. Use 0 for 1/0, −1 for 2/0, −2 for 3/0, −3 for 4/0. Give either awg or cross_section_mm2. Range: ≥ -3, ≤ 40
`cross_section_mm2`	number	mm²	no	Conductor cross-sectional area in mm² (metric cable sizes 1.5, 2.5, 4, 6, 10 …). Give either awg or cross_section_mm2. Range: > 0, ≤ 2000
`material`	enum: copper \| aluminium		default copper	Conductor material; resistivity at 20 °C.
`phase`	enum: dc \| single_phase \| three_phase		default dc	DC and single-phase count the outgoing and return conductor (×2); balanced three-phase uses ×√3.
`conductor_temperature_c`	number	°C	default 20	Operating temperature of the conductor; resistance rises about 0.4 % per °C above 20 °C (use 75 for a fully loaded cable). Range: ≥ -50, ≤ 200

Outputs

Output	Type	Unit	Description
`conductor_diameter_mm`	number	mm	Bare conductor diameter (from AWG or derived from the area).
`cross_section_mm2`	number	mm²	Conductor area.
`resistance_per_km_ohm`	number	Ω/km	ρ / A at the given temperature, per conductor.
`conductor_resistance_ohm`	number	Ω	Resistance of one conductor over length_m.
`voltage_drop_v`	number	V	2 × I × R (DC, single-phase) or √3 × I × R (three-phase).
`voltage_drop_percent`	number	%	voltage_drop_v / voltage_v × 100.
`voltage_at_load_v`	number	V	voltage_v − voltage_drop_v.
`power_loss_w`	number	W	2 × I² × R (DC, single-phase) or 3 × I² × R (three-phase).
`assessment`	string		Comparison with the common 3 % (branch circuit) and 5 % (total) voltage-drop recommendations (NEC informational note).

Example

12 V, 10 A, 5 m of 12 AWG copper: {"voltage_v":12,"current_a":10,"length_m":5,"awg":12} → {"conductor_diameter_mm":2.053,"cross_section_mm2":3.309,"resistance_per_km_ohm":5.2104,"conductor_resistance_ohm":0.02605,"voltage_drop_v":0.521,"voltage_drop_percent":4.34,"voltage_at_load_v":11.479,"power_loss_w":5.21}

230 V single-phase, 16 A, 30 m of 2.5 mm² copper: {"voltage_v":230,"current_a":16,"length_m":30,"cross_section_mm2":2.5,"phase":"single_phase"} → {"resistance_per_km_ohm":6.896,"conductor_resistance_ohm":0.20688,"voltage_drop_v":6.62,"voltage_drop_percent":2.88,"voltage_at_load_v":223.38,"power_loss_w":105.923}

GET https://tttkmbb.com/api/v1/calculate/wire-voltage-drop?voltage_v=12&current_a=10&length_m=5&awg=12

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/wire-voltage-drop (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/wire-voltage-drop · Markdown: https://tttkmbb.com/engineering/wire-voltage-drop.md · JSON definition: https://tttkmbb.com/engineering/wire-voltage-drop.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="wire-voltage-drop"
OpenAPI operationId: calculate_wire_voltage_drop
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – American wire gauge (reference)
Wikipedia – Electrical resistivity and conductivity (reference)
NFPA 70 – National Electrical Code (voltage drop informational notes 210.19(A), 215.2(A)) (standard)

FAQ

Is the length one-way or round trip?

Enter the one-way distance from source to load; the calculator doubles it for DC and single-phase circuits and uses √3 for balanced three-phase.

How much drop is acceptable?

The NEC recommends at most 3 % on a branch or feeder and 5 % overall for efficiency; low-voltage systems (12 V solar or automotive) often need thicker cable than the current alone suggests to stay under 3 %.

Related calculators

Ohm's Law Calculator — V = I·R for the conductor resistance.
Electric Power Calculator — Load current from power and voltage.
LED Resistor Calculator — Low-voltage LED circuits where drop matters.