電線サイズ計算機（AWG・mm²）

How to use this calculator

1. Enter your load current in amps.
2. Choose the conductor metal — copper or aluminum.
3. Pick the insulation temperature rating (60/75/90°C) marked on the cable.
4. Click Find wire size to get the smallest AWG whose ampacity covers your load.

How wire sizing works

Each conductor size has a maximum continuous current (its ampacity) that depends on the metal and the temperature rating of the insulation. This calculator returns the smallest standard size whose ampacity is at least your load current, using the widely used NEC Table 310.16 values for conductors in a raceway at an ambient temperature of 30°C.

AWG size, mm² and copper ampacity

Quick reference for copper conductors (75°C column):

Important factors not included here

• Small-conductor rule (NEC 240.4(D)): overcurrent protection is generally limited to 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper.
• Continuous loads must be sized at 125% of the load current.
• Voltage drop, ambient temperature and conductor bundling can require a larger size.

Always confirm with the current code edition and a licensed electrician.

よくある質問

What size wire do I need for 20 amps?

12 AWG copper is the usual choice for a 20 A circuit; use 14 AWG for 15 A and 10 AWG for 30 A, following the NEC 240.4(D) small-conductor limits.

Is a higher AWG number thicker or thinner?

Thinner. AWG runs backwards — a smaller number is a thicker wire (6 AWG is much thicker than 14 AWG).

Does aluminum need a bigger size than copper?

Yes. Aluminum carries less current for the same size, so it usually needs to be one to two AWG sizes larger than copper for the same load.

What does the temperature rating mean?

It is the maximum temperature the insulation can withstand (60/75/90°C). A higher rating allows slightly more ampacity, but terminations are often limited to the 60°C or 75°C column.