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Results are for guidance only. Always verify with a qualified professional. Disclaimer

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Wire reference

AWG — American Wire Gauge

AWG is the US standard for wire sizing. Counterintuitively, larger AWG numbers mean thinner wire. 14 AWG (thin, 15A) vs 4 AWG (thick, 70A). The gauge number refers to the number of drawing dies the wire was pulled through — more pulls = thinner wire.

Copper vs aluminum

Aluminum wire requires 2 AWG sizes larger than copper for the same ampacity. 12 AWG copper (20A) ≈ 10 AWG aluminum. Aluminum is cheaper and lighter but requires anti-oxidant compound at connections and aluminum-rated terminals. Never use aluminum for branch circuits under 60A.

Voltage drop rules

NEC recommends max 3% drop on branch circuits, 5% total system. Motors need under 2% for reliable starting. Longer runs always need larger wire. Double the run length = double the drop. Use the Voltage Drop tab to check any existing circuit.

Temperature derating

Wire ampacity is rated at 60°C or 75°C ambient. In hot locations (attics, near heat sources), derate: above 30°C (86°F) derate by ~9%; above 40°C (104°F) derate by ~18%. Multiple conductors in conduit also require derating.

Quick reference — copper wire

14 AWG → 15A · 12 AWG → 20A · 10 AWG → 30A · 8 AWG → 40A · 6 AWG → 55A · 4 AWG → 70A · 2 AWG → 95A · 1/0 AWG → 125A · 2/0 AWG → 145A · 4/0 AWG → 195A

Wire Gauge Calculator — AWG Size & Voltage Drop

Free tool · Works offline · Copper & aluminum · Metric & imperial

What gauge wire do I need?

Wire gauge selection is determined by two criteria — ampacity (the wire must handle the current without overheating) and voltage drop (the wire must not lose too much voltage over the run length). This calculator sizes by both and picks the more conservative result. For short runs under 30m (100ft), ampacity usually governs. For long runs, voltage drop takes over and you may need to go up one or two sizes beyond the ampacity minimum.

What wire size for 20 amps?

12 AWG copper (2.5mm²) is the minimum for a 20A circuit — this is code in both the US (NEC) and UK/EU (IEE 18th Edition). For runs under 30m (100ft) at 120V or under 50m at 230V, 12 AWG keeps voltage drop under 3%. For longer runs, step up to 10 AWG (4mm²). In aluminum, use 10 AWG minimum for 20A — aluminum requires two sizes larger than copper for the same ampacity.

What is voltage drop and why does it matter?

Every wire has electrical resistance. That resistance causes voltage to drop along the run — the longer the wire, the more voltage is lost before it reaches the load. A 3% drop at 230V means your appliance receives only 223V. For lighting this is barely noticeable. For motors and sensitive electronics, low voltage causes overheating, reduced torque and shortened equipment life. The NEC recommends keeping branch circuit voltage drop under 3%, with a maximum of 5% for the total system (feeder + branch combined).

Copper vs aluminum wire — which should you use?

For branch circuits under 60A, always use copper. Aluminum is cheaper and lighter but requires anti-oxidant compound at every connection, aluminum-rated terminals and connectors, and proper torque on terminations. Loose aluminum connections are a fire hazard — they were responsible for widespread house fires in the 1960s–70s when aluminum branch wiring was common. Aluminum is perfectly safe and widely used for service entrance cables (100A+) and feeder runs, where the larger wire sizes make the cost saving significant.

How to read the AWG table

AWG (American Wire Gauge) is counterintuitive — larger numbers mean thinner wire. 14 AWG is thinner than 10 AWG. The scale runs from 40 AWG (ultra-fine) down to 4/0 AWG (very heavy). For reference: 14 AWG — 15A circuits, lighting · 12 AWG — 20A circuits, kitchen · 10 AWG — 30A, dryers, A/C · 8 AWG — 40A, cooktops · 6 AWG — 55A · 4 AWG — 70A · 2 AWG — 95A. The AWG tab in this calculator shows the full table with metric equivalents.

Frequently asked questions

What AWG wire for 30 amps over 100 feet?

At 120V, 30A over 100ft: 10 AWG gives 3.6% voltage drop — slightly over 3%. Use 8 AWG to stay comfortably under 3%. At 240V, 10 AWG keeps drop well under 3% for the same run.

Can I use 12 AWG wire on a 20 amp breaker?

Yes — 12 AWG is rated for 20A and is the correct wire for a 20A circuit. Never use 14 AWG on a 20A breaker — the wire would overheat before the breaker trips.

What is the difference between solid and stranded wire?

Solid wire is stiffer and makes better contact in push-in connectors and terminal blocks. Stranded wire is more flexible — used in appliance cords, panel wiring and anywhere the wire needs to flex. For fixed in-wall wiring, solid is standard in sizes 14 AWG–10 AWG.

How do I calculate voltage drop?

Voltage drop = Current (A) × Wire resistance (Ω/1000ft) × Run length (ft, round trip) ÷ 1000. Enter your values in the voltage drop tab and it calculates instantly. The result is shown as both volts and percentage of supply voltage.