Wire Gauge Calculator
Enter the load current, voltage, one-way run length and a voltage-drop limit to get the smallest wire gauge that carries the load safely, in copper or aluminium.
This wire size calculator picks the smallest AWG conductor that both carries your load and keeps voltage drop inside your limit. Type the amps, the system voltage, how far the run travels one way, and the material, and it checks each gauge against its ampacity and its resistance. It doubles as an AWG calculator and an amp wire gauge chart, since it lists the drop and current rating for every size next to the pick. Everything runs in your browser, nothing is sent anywhere.
Circuit
Recommended wire gauge
12 AWG
Sized by voltage drop
- Ampacity (75 °C)
- 25 A
- Actual voltage drop
- 2.65%
- Drop in volts
- 3.18 V
- Next size up
- 10 AWG · 1.67%
Reference chart
| AWG | Ampacity | Drop |
|---|---|---|
| 14 AWG | 20 A | 4.21% |
| 12 AWG ← | 25 A | 2.65% |
| 10 AWG | 35 A | 1.67% |
| 8 AWG | 50 A | 1.05% |
| 6 AWG | 65 A | 0.66% |
| 4 AWG | 85 A | 0.42% |
| 3 AWG | 100 A | 0.33% |
| 2 AWG | 115 A | 0.26% |
| 1 AWG | 130 A | 0.21% |
| 1/0 AWG | 150 A | 0.16% |
| 2/0 AWG | 175 A | 0.13% |
| 3/0 AWG | 200 A | 0.1% |
| 4/0 AWG | 230 A | 0.08% |
A planning estimate for single-phase copper or aluminium runs, using 75 °C ampacities and standard conductor resistances. It does not replace the National Electrical Code or local rules — derate for conduit fill, ambient heat and continuous loads (125%), and have an electrician confirm the final size.
How it works
- 1
Enter the load and voltage
Add the current the circuit draws in amps and the system voltage, such as 12, 120 or 240 V.
- 2
Set the run and material
Give the one-way distance from source to load in feet or metres, then pick copper or aluminium.
- 3
Read the gauge
The recommended AWG shows with its ampacity, the actual voltage drop and the next size up in the chart.
Instant & 100% private — nothing is uploaded
Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.
Frequently asked questions
- How does the tool choose the wire size?
- It walks the AWG chart from thinnest to thickest and returns the first gauge that clears two tests: its ampacity is at least your load current, and its resistance keeps voltage drop at or under the percent you set. Whichever test needs a bigger wire wins.
- Why does a longer run need a thicker wire?
- Voltage drop grows with distance because the current travels out and back through the conductor's resistance. Double the run and you double the drop, so a long circuit often needs a fatter wire than its amp rating alone would suggest.
- What voltage-drop limit should I use?
- 3% is the common target for a branch circuit and is the default here. Some designers hold the whole path from panel to load to 5%, and low-voltage DC runs like solar or lighting often use tighter numbers to save power. Set the slider to match your spec.
- Why does aluminium call for a bigger gauge than copper?
- Aluminium has about 1.6 times the resistance of copper for the same size, so it drops more voltage and carries less current per gauge. The calculator uses aluminium ampacities and resistance, which usually bumps the pick up one or two sizes versus copper.
- Is this a substitute for the electrical code?
- No. It is a planning estimate using 75 °C ampacities and standard conductor resistances for a single-phase run. It does not derate for conduit fill, high ambient temperature, bundling or the 125% continuous-load rule, so confirm the final size against the NEC or your local code and an electrician.
- What is AWG and how do the sizes run?
- AWG is the American Wire Gauge scale. A smaller number means a thicker wire, so 10 AWG is heavier than 14 AWG. Above 1 AWG the sizes continue as 1/0, 2/0, 3/0 and 4/0, each thicker than the last.
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