Free Tool

Cable Size & Wire Gauge Calculator

Enter amps, distance and voltage — get the right cable size, wire gauge and voltage drop instantly. Copper or aluminium. Single-phase, 3-phase and 12V/24V/48V DC. AWG and mm². NEC, BS 7671 and IEC references.

Calculate Cable Size See Features
Cable size calculator showing recommended wire gauge, ampacity and voltage drop results

One Tool for AC, DC, AWG and mm²

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Voltage Drop Calculator

Single-phase, 3-phase and DC voltage drop. Stay within the 3 % branch and 5 % total limits used by NEC, BS 7671 and IEC.

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Cable Size in Seconds

Get a recommended cable size and wire gauge from load amps, run length and voltage system — with ampacity and derating already applied.

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AWG ↔ mm² Built In

Convert between American AWG / kcmil and metric IEC 60228 mm² sizes. Cross-section, diameter and approximate ampacity side by side.

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DC: Solar, Marine, RV, 12V/24V/48V

Dedicated 12V, 24V and 48V DC voltage systems for solar, marine, RV and low-voltage lighting — where voltage drop matters most.

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Conduit Fill & Derating

NEC conduit fill for EMT, IMC and rigid raceway. Temperature correction and bundling adjustment factors applied automatically.

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Private & Offline

Runs entirely in your browser. No account, no upload, no tracking. Works offline after the first load.

Built for Real Cable Sizing Work

Cable & Wire Size Calculator With Ampacity and Derating

Pick load current, distance and voltage system — the calculator returns the minimum cable size that satisfies ampacity, temperature derating, conductor bundling and the 125 % continuous-load factor. Works for copper and aluminium.

  • NEC Table 310.16 ampacity at 60 / 75 / 90 °C
  • Temperature correction (Table 310.15(B)(1))
  • Bundling adjustment for 4+ conductors (Table 310.15(C)(1))
  • Continuous-load 125 % factor per 210.20(A) / 215.3
  • Cross-reference to IEC 60228 mm² standard sizes
Wire size calculator showing recommended cable size and ampacity result

Voltage Drop Calculator — AC, 3-Phase and DC

Calculate voltage drop in volts and percent for single-phase, three-phase and DC circuits. Compare conductor sizes to find the most economical cable that stays inside the 3 % branch / 5 % total recommendation.

  • Single-phase: VD = 2 × I × R × L ÷ 1000
  • Three-phase: VD = √3 × I × R × L ÷ 1000
  • DC: same as single-phase, with proper return-conductor factor
  • Copper and aluminium resistance per NEC Chapter 9, Table 8
  • Distance in feet or metres, °C or °F ambient
Voltage drop calculator showing percentage and end voltage for selected wire gauge

AWG ↔ mm² Reference & Conduit Fill

Built-in AWG to mm² converter with the full reference table — area, diameter and the nearest IEC 60228 standard cable size. Conduit fill calculator for EMT, IMC, rigid and PVC raceways.

  • AWG / kcmil ↔ mm² conversion (IEC 60228)
  • Conductor diameter in mm and inches
  • Approximate ampacity for copper and aluminium
  • EMT, IMC, RMC and PVC conduit fill per NEC Chapter 9
  • Neutral conductor sizing notes for harmonic-rich loads
Conduit fill calculator showing conductor count and fill percentage

Cable Sizing FAQ

A cable size calculator picks the conductor cross-section that can carry the load current safely without exceeding the allowed voltage drop. You need three things: load current in amps, one-way run length, and the voltage system (e.g. 120 V single-phase, 240 V three-phase, 12 V DC). The calculator above returns AWG and the nearest metric mm² size, plus the voltage drop.
They refer to the same idea — the cross-section of the conductor. North America says wire size and uses AWG / kcmil with NEC tables. The UK, EU, Australia, NZ and most of the world say cable size and use mm² with BS 7671 / IEC 60364 / AS/NZS 3008 tables. This calculator gives you both: AWG plus the nearest IEC 60228 mm² standard size.
A 30 A circuit typically needs 10 AWG copper (≈ 6 mm²) or 8 AWG aluminium (≈ 10 mm²) for ampacity per NEC Table 310.16 (60 °C column). On long runs you may have to upsize for voltage drop. Use the calculator with your actual length and voltage to confirm.
A 50 A circuit at 100 ft (30 m) one-way usually wants 6 AWG copper (≈ 16 mm²) for ampacity. At 240 V single-phase this gives roughly 2.5 % drop at 100 ft. At 120 V the drop doubles to ~5 %, so you would upsize to 4 AWG (~25 mm²). Run the calculator for your exact voltage to verify.
A 200 A residential service entrance typically uses 2/0 AWG copper or 4/0 AWG aluminium per NEC Table 310.12 (services rated 100–400 A from a single dwelling). For long underground runs from the meter to the panel, voltage drop calculations may push you to a larger size. Aluminium is common because of the cost savings at this gauge.
The standard formula uses conductor resistance per 1000 ft (or per km):

Single-phase / DC: VD = 2 × I × R × L ÷ 1000    Three-phase: VD = √3 × I × R × L ÷ 1000

Where I = amps, R = Ω/kft (NEC Ch. 9 Table 8), L = one-way length in feet. Industry rule of thumb is 3 % maximum for branch circuits and 5 % total (feeder + branch). The calculator above does this automatically for AC and DC systems.
DC systems live and die by voltage drop because the supply voltage is so low — a 0.6 V drop on 12 V is already 5 %. Pick 3 % maximum drop for critical circuits (electronics, fridges, pumps) and 10 % for non-critical lights. Use the DC option in the voltage system dropdown to size 12 V / 24 V / 48 V circuits correctly. Marine installations follow ABYC E-11 (which is stricter than NEC for vibration and corrosion); solar PV in North America follows NEC Article 690.
For balanced three-phase loads, the voltage drop formula uses √3 instead of 2 because the current returns through the other two conductors instead of a single neutral. Select a 3-phase option (208 V, 240 V, 480 V or 600 V) in the voltage system dropdown — the calculator switches to the √3 formula automatically. Ampacity sizing still uses NEC Table 310.16; bundling derating applies when more than three current-carrying conductors share a raceway.
Common conversions: 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm², 8 AWG ≈ 10 mm², 6 AWG ≈ 16 mm², 4 AWG ≈ 25 mm², 2 AWG ≈ 35 mm², 1/0 AWG ≈ 50 mm², 4/0 AWG ≈ 120 mm². Exact area in mm² is calculated from the AWG formula; the calculator above shows both the exact value and the nearest IEC 60228 standard size.
For most circuits the neutral is sized the same as the phase conductors. For 3-phase 4-wire systems feeding non-linear loads (LED drivers, computers, VFDs) triplen (3rd, 9th, 15th…) harmonics add up on the neutral and can push it above the phase current — sometimes requiring a neutral up to 200 % of the phase conductor size. NEC 220.61 and BS 7671 Section 524 cover this.
The widely-quoted limits are 3 % maximum for branch circuits and 5 % total (feeder + branch combined). In the NEC these are informational notes in 210.19(A) and 215.2(A) — not enforceable code, but standard engineering practice. Motor circuits, sensitive electronics and DC circuits often use tighter limits.
The ampacity engine uses NEC Table 310.16 (US National Electrical Code, 2020 edition) — that is the most heavily-searched standard. The AWG / mm² conversion table also shows the nearest IEC 60228 standard cable size and a conservative reference ampacity. For UK BS 7671 (Appendix 4), Australian / NZ AS/NZS 3008 or full IEC 60364-5-52 sizing you should still verify against your local tables — values can differ by up to 20 % depending on installation method.
No. Use it for estimation, planning and sanity checks. Final designs must follow your local code (which can be stricter than NEC, BS 7671 or IEC), project specifications and professional review. Always have installations checked by a licensed electrician.
No. The calculator runs entirely in your browser. No inputs, calculations or project details are sent to any server. It works offline after the first page load.

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