About — Voltage Drop Calculator

This calculator was built by a lighting and electrical designer with 18 years of industry experience. Over that time, the same problem came up repeatedly: existing online voltage drop calculators were either too simple (single-phase only, no cable comparison, no ampacity check), based on rough approximations rather than the actual copper resistivity formula, or designed for one country with no way to switch standards.

This tool was built to fix that. It covers five calculation modes — single phase, three phase, DC, series cable runs, and motor starting — across four regions. Every result shows the formula used, the working, and a plain-English pass/fail against the relevant standard. The cable comparison table lets you see the voltage drop for every common cable size at once, so you can make a quick engineering decision without running the calculation eight times.

It's completely free and always will be. No login, no paywall, no app to download — just open it on your phone on site and use it.

Which standards does this calculator follow?

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AS/NZS 3000 + AS 3008

Australian/NZ Wiring Rules & Cable Selection. 5% max voltage drop, copper resistivity, mm² sizing.

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NEC / AWG

US National Electrical Code. 3% preferred / 5% max, AWG wire sizing, circular mils.

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IS 732

Indian Standard for electrical wiring installations. mm² sizing, 5% drop limit.

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SANS 10142

South African wiring standard. mm² cable sizing, 5% voltage drop limit.

How accurate are the calculations?

The voltage drop formula used is the standard engineering method based on copper resistivity: VD = 2 × L × I × ρ / A (single phase and DC) and VD = √3 × L × I × ρ / A (three phase), where ρ = 0.0175 Ω·mm²/m for copper and 0.028 Ω·mm²/m for aluminium. This is the same formula used in AS 3008 and the other regional standards.

The calculations do not account for power factor correction, cable bundling derating, elevated ambient temperature, or installation method — all of which affect real-world cable sizing. For a final engineering sign-off, the full AS 3008 cable selection process should be followed. This tool gives you fast, accurate working calculations for design and checking purposes.

What are the five calculation modes?

Single Phase AC — standard 230V/120V circuits. Current travels down the active and returns via neutral; formula uses ×2 for total conductor length.
Three Phase AC — 400V/208V/415V three-phase circuits. The √3 factor accounts for the phase relationship between conductors, giving about 13% less drop than single phase for the same parameters.
DC Circuits — for solar, battery, EV charging, and industrial DC applications. Uses the same ×2 formula as single phase but without the AC power factor considerations.
Series Cable Runs — for circuits where cable size changes along the run, or where you have multiple different-length sections. Each run segment is calculated separately and the drops are summed.
Motor Starting — calculates the momentary voltage drop during motor starting, based on the starting current multiplier (typically 5–7× FLA for DOL starters). Important for checking that motor starting won't affect other equipment on the same supply.

Why is it free?

Because a tool this useful shouldn't cost anything. The site is supported by Google AdSense advertising and Amazon affiliate links on product recommendation pages. These cover hosting costs — the calculator itself will never be paywalled or put behind a login.

What other tools are available?

This site focuses specifically on voltage drop calculation. For other lighting and electrical tools — LED strip driver sizing, lux calculation, garden lighting design, and LED strip selection — visit the sister site at lighttools.com.au.

About This Calculator

A proper voltage drop calculator — built the way electricians actually need it

Which standards does this calculator follow?

How accurate are the calculations?

What are the five calculation modes?

Why is it free?

What other tools are available?

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