Enter the current your circuit will carry to get the recommended copper wire gauge — with the ampacity table explained.
What Is a Wire Size Calculator?
A wire size calculator maps current to copper gauge: 20 A → 12 AWG. Thicker wire (lower AWG number) carries more current without overheating — the ampacity table is about heat, not just voltage drop. This is a planning guide: local electrical code (NEC and equivalents) has the final word, especially for mains wiring.
How to Use
- Enter the current in amps.
- Click “Recommend Wire”.
Formula
Ampacity table (copper): ≤15 A → 14 AWG; ≤20 A → 12 AWG; ≤30 A → 10 AWG; above → 8 AWG.
Worked Example
20 A → 12 AWG copper.
How It Works, In Plain Words
Wire sizing is really about heat. Every wire resists the current flowing through it a little, and that resistance turns electrical energy into heat. A thin wire has more resistance, so the same current heats it more; push too much current through it and the insulation suffers long before the copper melts. Ampacity is simply the most current a wire can carry continuously without getting too hot, and this calculator looks your current up in a standard copper ampacity table: up to 15 A needs 14 AWG, up to 20 A needs 12 AWG, up to 30 A needs 10 AWG, and anything above that lands on 8 AWG.
The only input is the current your circuit will actually carry, in amps — usually the rating of the load or the breaker protecting the circuit. Notice the gauge numbers run backwards: the lower the AWG number, the thicker the wire and the more current it handles. The table here is deliberately simple. It assumes copper wire in ordinary conditions, which is why the result always comes with the reminder to check local code and derate for long runs, heat or bundled cables — real installations have all three.
Real-World Uses
A car enthusiast fitting accessories — extra lighting or an upgraded audio amplifier — uses the tool before ordering cable, because automotive wiring is low-voltage and DIY-friendly, and the gauge decides whether the new run stays cool. A home owner planning a solar add-on uses it the same way for the DC side, checking what the panels’ current calls for before buying reels of copper. The third user is not touching a wire at all: a householder getting quotes for a new circuit runs the expected load through the calculator so the electrician’s specification makes sense — why one circuit is quoted in a thicker gauge than another — before agreeing to the work.
Benefits
- Instant gauge recommendation
- The ampacity logic explained
- Safety-first: code reminder built in
- Useful for automotive, solar and DIY wiring
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes
- Treating it as code-compliant design — always check local code
- Forgetting derating (heat, bundling, long runs)
- Using aluminium ampacities for copper wire
Common Mistakes to Avoid
One mistake matters more than all the others: treating any online calculator — including this one — as permission to do household wiring yourself. Mains wiring is dangerous and regulated; household electrical work belongs to a qualified electrician, and this tool is a planning guide only. Within that limit, the common errors are: sizing for the appliance’s running current while forgetting the breaker and start-up surges it must live with; ignoring derating, so a cable bundled with others in a hot roof space is asked to carry its full open-air rating; copying aluminium figures for a copper run, or the other way round — this table is copper only; and choosing a thinner wire because it is cheaper or easier to bend. Thinner is the unsafe direction: when in doubt, the thicker gauge is the cautious answer, confirmed by your electrician.
FAQ
What wire for 20 amps?
12 AWG copper.
What does AWG mean?
American Wire Gauge — lower numbers are thicker wire.
Can I use thinner wire for short runs?
Sometimes, but heat is the limit — check code and derate properly.
Copper or aluminium?
Copper carries more per size; this tool’s table is for copper.
Does length matter?
For heating, no — but long runs add voltage drop (see the Voltage Drop Calculator).
Why does a lower AWG number mean thicker wire?
The scale is historical: wire was made by drawing it through a series of dies, and the gauge number counted the drawing passes — the more passes, the thinner the finished wire. So 8 AWG is a thick cable and 14 AWG is much thinner, and within this calculator’s table the thicker, lower-numbered gauges are the ones matched to the higher currents.
Can I use this calculator for household wiring?
Only as a planning guide — to understand sizes, compare quotes and ask better questions. It is not a design service and it does not know your local code, installation method or cable runs. Actual household wiring work should be designed and carried out by a qualified electrician.