How do you calculate voltage drop?

In DC and single-phase circuits current flows out and back, so ΔU = 2 × I × L × R′ (for AC multiply by power factor and add the reactance term). Three-phase uses the line voltage: ΔU = √3 × I × L × (R′cosφ + X′sinφ). R′ = ρ/S, with copper at 0.0172 Ω·mm²/m at 20 °C; at 70 °C resistance is about 20% higher.

What voltage drop is acceptable?

A common reference from IEC 60364 is 3% for lighting and 5% for other uses when supplied from the public low-voltage network; the US NEC recommends about 3% for a branch circuit and 5% overall. Solar DC wiring is usually kept to 1–3%. Excess drop wastes energy, heats motors, dims LEDs and slows charging.

If the ampacity is enough, why check voltage drop?

Ampacity only keeps the cable from overheating and does not depend on length, while voltage drop grows with length. Short runs are usually sized by ampacity; long runs — a pump 100 m away, an EV charger across the yard, a 12 V solar array — are often sized by voltage drop. The tool computes both and tells you which governs.

Why do 12 V systems need such thick wire?

For the same power, current scales as 1/V and percentage drop as 1/V². A 120 W load draws 10 A at 12 V but only 0.5 A at 230 V, so on the same cable the 12 V percentage drop is about 370 times larger. That is why RVs and solar systems move to 24 V or 48 V and LED strips need power injected every 5 m or so.

Can I use this ampacity table for an installation?

Not directly. It summarises typical copper values from IEC 60364-5-52 / GB/T 16895.6 Annex B, with aluminium estimated by a factor, and real ratings depend on cable construction, soil resistivity, harmonics and fault protection. Follow your local code — NEC, BS 7671, AS/NZS 3008 or similar — and have a licensed electrician confirm.

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🔌Wire Size & Voltage Drop Calculator - Cable Sizing, Ampacity, AWG to mm²

A cable that is too thin causes two problems: too much voltage drop, leaving the load under-voltage, and overheating beyond what the insulation allows. This tool calculates voltage drop and losses from resistivity with temperature correction, applies a simplified ampacity reference with installation-method, ambient and grouping factors, and recommends the smallest size that satisfies both, with a breaker hint. It also covers 12 V / 24 V long runs, LED strip voltage and an AWG conversion table.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Wire Size & Voltage Drop Calculator

1Pick a preset or set the system (DC, single or three phase), voltage, current or power and one-way length
2Choose copper or aluminium, insulation, installation method, ambient temperature and number of bunched circuits
3Set the allowed drop (e.g. 3% lighting, 5% other) and read the recommended size, breaker, drop, voltage at the load and losses
4In Low-voltage runs compare 12, 24 and 48 V and see the voltage and brightness along an LED strip

FAQ

How do you calculate voltage drop?: In DC and single-phase circuits current flows out and back, so ΔU = 2 × I × L × R′ (for AC multiply by power factor and add the reactance term). Three-phase uses the line voltage: ΔU = √3 × I × L × (R′cosφ + X′sinφ). R′ = ρ/S, with copper at 0.0172 Ω·mm²/m at 20 °C; at 70 °C resistance is about 20% higher.
What voltage drop is acceptable?: A common reference from IEC 60364 is 3% for lighting and 5% for other uses when supplied from the public low-voltage network; the US NEC recommends about 3% for a branch circuit and 5% overall. Solar DC wiring is usually kept to 1–3%. Excess drop wastes energy, heats motors, dims LEDs and slows charging.
If the ampacity is enough, why check voltage drop?: Ampacity only keeps the cable from overheating and does not depend on length, while voltage drop grows with length. Short runs are usually sized by ampacity; long runs — a pump 100 m away, an EV charger across the yard, a 12 V solar array — are often sized by voltage drop. The tool computes both and tells you which governs.
Why do 12 V systems need such thick wire?: For the same power, current scales as 1/V and percentage drop as 1/V². A 120 W load draws 10 A at 12 V but only 0.5 A at 230 V, so on the same cable the 12 V percentage drop is about 370 times larger. That is why RVs and solar systems move to 24 V or 48 V and LED strips need power injected every 5 m or so.
Can I use this ampacity table for an installation?: Not directly. It summarises typical copper values from IEC 60364-5-52 / GB/T 16895.6 Annex B, with aluminium estimated by a factor, and real ratings depend on cable construction, soil resistivity, harmonics and fault protection. Follow your local code — NEC, BS 7671, AS/NZS 3008 or similar — and have a licensed electrician confirm.

Features

DC, single-phase and three-phase drop formulas with conductor temperature and AC reactance
Sizing by both voltage drop and derated ampacity, showing which governs, with an I_B ≤ I_n ≤ I_z breaker check
Cable loss in watts, as a share of load power, and its yearly energy cost
Drop-versus-length chart for neighbouring sizes so you see when to step up
LED strip voltage profile for single- and double-ended feeds, with a 12 / 24 / 48 V comparison
Two-way AWG ↔ mm² conversion table with diameter and resistance per km

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