Quick answer: For most household circuits: 14 AWG for 15A circuits, 12 AWG for 20A, 10 AWG for 30A. Voltage drop over long runs is the main reason to go up a gauge — keep drop under 3% for lighting and 5% for power circuits.
Related Calculators
What AWG wire do I need for a 240V air conditioner?
A typical 240V window AC (10,000–12,000 BTU, 8–12A) needs 12 AWG wire on a 20A circuit. Larger central AC units (20–40A) need 10 AWG or 8 AWG. Always check the unit's nameplate for MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection).
What is the difference between AWG and mm²?
AWG (American Wire Gauge) is used in the US and Canada — higher numbers mean thinner wire (14 AWG ≈ 2.5mm², 12 AWG ≈ 4mm², 10 AWG ≈ 6mm²). mm² is the European standard measuring actual cross-sectional area. This calculator converts between both systems.
How do I calculate voltage drop over long cable runs?
Voltage drop = (2 × length × current × resistance per metre) ÷ 1000. Keep drop under 3% for lighting, 5% for power. For a 30m run at 16A in 2.5mm² cable on 230V: drop = 2 × 30 × 16 × 7.41 ÷ 1000 = 7.1V = 3.1% — borderline, use 4mm² instead.
Can I use aluminium wire instead of copper?
Aluminium wire is used for large services and feeders because it's cheaper and lighter. For the same current capacity, aluminium needs to be 2 sizes larger than copper (use 4mm² aluminium where 2.5mm² copper would work). Never use aluminium for branch circuits in the US — fire risk from loose connections.
What wire gauge for a EV charger?
A Level 2 home EV charger (32A, 240V) needs 6 AWG (10mm²) wire on a dedicated 40A circuit. For a 48A charger, use 4 AWG (16mm²) on a 60A circuit. Run conduit if possible for future upgradeability. Add 20% spare capacity for future higher-power vehicles.
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.