Wire Size & Ampacity
Convert conductor sizes between AWG/kcmil and mm², apply ambient-temperature and grouping derating to a cable’s current-carrying capacity, and find the smallest conductor that will carry a given design current. Typical NEC 310.16 and BS 7671 base figures included and fully editable, offline.
Open Wire Size & Ampacity →What is the wire size & ampacity calculator?
A free conductor sizing tool that runs entirely in your browser. It does three jobs. The size converter turns any AWG or kcmil size into square millimetres, circular mils, conductor diameter and resistance per kilometre and per thousand feet — and back again from a metric area. The capacity and derating tab takes a cable's base current-carrying capacity and applies the two corrections installers most often forget: the ambient-temperature factor and the adjustment for bundling several current-carrying conductors together. The third tab works backwards, naming the smallest conductor that still carries your design current once both corrections are applied. Typical base figures from NEC Table 310.16 and the BS 7671 reference methods are built in, and every one of them is an editable field so you can type the value from your own edition and still get the derating maths right. Nothing is uploaded.
How to use Wire Size & Ampacity
- Choose your units — Switch between Metric (mm², °C, reference methods A–E) and Imperial (AWG/kcmil, °F, 60/75/90 °C insulation ratings).
- Convert a size — On the Size converter tab, pick an AWG or kcmil size to see its metric equivalent, or type an area in mm² to find the nearest AWG.
- Apply the derating — On the Capacity & derating tab, pick the conductor, material and rating. The base capacity fills in automatically — overwrite it with your own table value if you prefer — then enter the ambient temperature and how many current-carrying conductors share the route.
- Or size for a load — On the Size for a load tab, enter the design current and the same conditions, and the smallest conductor that carries it is named along with its derated capacity.
Frequently asked questions
How are the derating factors calculated?
The ambient correction uses the NEC formula √((Tc − Ta) ÷ (Tc − 30)), where Tc is the insulation rating and Ta the ambient temperature; it reproduces the published correction table exactly. The grouping adjustment uses the standard steps: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20, then 45%, 40% and 35%.
Why can I edit the base capacity?
Because current-carrying capacity depends on the cable type, the installation method and the edition of the wiring rules you work to. The built-in figures are typical reference values; typing your own keeps the derating arithmetic correct whatever table you are using.
How do AWG and mm² compare?
They are different systems, so sizes rarely match exactly. 12 AWG is about 3.31 mm², 6 AWG about 13.3 mm² and 4/0 about 107 mm². The converter names the nearest standard size in the other system for you.
Does this replace the wiring regulations?
No. It is a planning and estimating aid. Final conductor selection must follow the current edition of the rules that apply where the work is done, including protection, earthing and installation method.
Tips
- Derating is multiplicative — a hot roof space and a full conduit together can easily halve a cable's usable capacity.
- Aluminium needs roughly one or two sizes more than copper for the same current, and the tool's tables reflect that.
- Always check voltage drop as well as capacity; on a long run the drop usually decides the size.