Wire Gauge Calculator: AWG Ampacity and Wire Size Guide

Undersized wire is a fire hazard. Oversized wire wastes money and is harder to work with. Getting the gauge right matters whether you're running a new circuit in your house, wiring up a solar charge controller, or adding accessories to a vehicle. The CalcHub Wire Gauge Calculator takes your amperage, run length, and acceptable voltage drop and recommends the correct AWG size.

How AWG Works

American Wire Gauge is counterintuitive — lower numbers mean thicker wire. AWG 14 is thicker than AWG 18, and AWG 2 is much thicker than both. Solid wire and stranded wire of the same gauge have similar current capacity, but stranded wire is more flexible and better for runs that flex or vibrate (vehicles, appliances).

Ampacity Reference Table

Ampacity is the maximum continuous current a wire can carry without overheating. These are National Electrical Code (NEC) values for copper wire in conduit at 60°C:

AWG	Ampacity (Copper)	Resistance (Ω/1000 ft)	Common Use
14	15A	2.525	Lighting circuits (15A breaker)
12	20A	1.588	Outlets, kitchen (20A breaker)
10	30A	0.999	Dryers, AC units
8	40A	0.628	EV chargers (Level 2)
6	55A	0.395	Sub-panels, large HVAC
4	70A	0.249	Large sub-panels
2	95A	0.156	Service entrance
1/0	125A	0.098	Main service panels

Note: Free-air ratings are higher; conduit ratings are more conservative. Always follow local code.

Voltage Drop Matters on Long Runs

For short runs (under 10 feet), just matching ampacity is usually fine. But for long runs — landscape lighting, solar systems, barn circuits, marine wiring — voltage drop becomes the limiting factor.

Voltage Drop Formula:

Vdrop = (2 × Length × Current × Resistance per foot) / 1000

The factor of 2 accounts for both the hot and return conductors.

Example: Running 10A over 50 feet with 12 AWG wire:

• Resistance: 1.588 Ω/1000 ft = 0.001588 Ω/ft
• Vdrop = 2 × 50 × 10 × 0.001588 = 1.59V drop

On a 120V circuit that's 1.3% — well under the 3% maximum recommended by NEC. On a 12V DC system it would be 13% — way too much, and you'd need to upsize to 10 AWG or 8 AWG.

DC vs. AC: Different Rules

For 12V/24V DC systems (solar, automotive, marine), voltage drop is far more critical because you're starting from a lower voltage. A 1V drop on a 12V system is an 8% loss. The general rule for DC:

• Under 3% drop: use standard sizing
• Under 1% drop target for sensitive electronics

For automotive wiring, ampacity ratings also differ slightly because wire runs in a vehicle bundle and heat builds up. Use automotive-specific ampacity charts when wiring accessories.

Using the Calculator

1. Enter your circuit amperage (use continuous load, not peak)
2. Enter the one-way run length in feet or meters
3. Set your voltage drop tolerance (3% for AC, 1–2% for 12V DC)
4. Choose copper or aluminum wire
5. The calculator returns the minimum AWG and the actual voltage drop

The calculator also shows the next size up, which is often worth using for future-proofing or if you're close to the limit.

Should I size for the breaker rating or the actual load?

Size the wire to match the breaker rating, not the load. The breaker protects the wire. If you're installing a 20A circuit, use 12 AWG even if your load only draws 8A. This allows future devices to be added without rewiring.

Can I use aluminum wire?

Aluminum wire is commonly used for larger residential circuits (100A+ panels, dryer circuits) but requires special techniques — anti-oxidant compound at connections, aluminum-rated terminals, and sizing up one gauge versus copper. For branch circuits and most DIY work, stick to copper.

What gauge wire for trailer wiring?

Most trailer lighting circuits use 14 AWG or 16 AWG for short runs. For trailers with electric brakes or heavy loads, use 12 AWG for the brake circuit. Always verify with your trailer's wiring diagram and the truck manufacturer's tow package specs.

Related Tools

• Voltage Drop Calculator — dedicated long-run voltage drop tool
• Voltage Divider Calculator — signal-level resistor circuits
• Ohm's Law Calculator — current, voltage, resistance relationships