Calculate correct electrical cable and wire gauge for any circuit. Account for voltage drop, ampacity, distance, and NEC requirements.
The Cable Size Calculator determines the correct wire gauge (AWG) for electrical circuits based on current, voltage, distance, and allowable voltage drop. Proper wire sizing ensures safety, code compliance, and efficient power delivery.
Undersized wires create fire hazards, trip breakers, and waste energy through excessive resistance heating. Oversized wires waste money on unnecessary copper or aluminum. This calculator finds the optimal gauge by checking both ampacity (current-carrying capacity per NEC Table 310.16) and voltage drop limits (typically 3% for branch circuits, 5% total including feeder).
Enter your circuit parameters — current, voltage, one-way distance, conductor material, and installation conditions — to get the minimum wire size that satisfies both ampacity and voltage drop requirements. It gives you a practical starting point for choosing a safe conductor before you buy the wire. That is especially useful when you are comparing a short run to a long one, or deciding whether a larger gauge is worth it.
Use this calculator when you need a wire size that satisfies both ampacity and voltage-drop limits instead of checking only one side of the problem. It is useful for branch circuits, feeders, and longer runs where the code-minimum gauge may not be the performance-minimum gauge. That matters most when distance starts to erode usable voltage at the load.
Wire Size (cmil) = (2 × K × I × D) / VD. Where K = resistivity (copper: 12.9, aluminum: 21.2 Ω·cmil/ft), I = current (A), D = one-way distance (ft), VD = allowable voltage drop (V). Voltage Drop % = VD / System Voltage × 100.
Result: AWG 8 copper wire
VD allowed = 240 × 0.03 = 7.2V. cmil = 2 × 12.9 × 30 × 100 / 7.2 = 10,750. AWG 10 = 10,380 cmil (too small). AWG 8 = 16,510 cmil (sufficient). AWG 8 ampacity = 40A (>30A). Use 8 AWG.
NEC Table 310.16 provides the foundation for wire sizing. For 75°C-rated copper wire in normal conditions: 14 AWG = 20A, 12 AWG = 25A, 10 AWG = 35A, 8 AWG = 50A, 6 AWG = 65A, 4 AWG = 85A, 3 AWG = 100A, 2 AWG = 115A, 1 AWG = 130A, 1/0 = 150A, 2/0 = 175A, 3/0 = 200A, 4/0 = 230A.
These values assume no more than 3 current-carrying conductors, 30°C ambient temperature, and a single conduit or cable. Derating is required for higher temperatures, more conductors, or bundled cables.
For single-phase circuits: VD = 2 × I × R × D / 1000 (where R is resistance per 1000ft). For three-phase circuits: VD = √3 × I × R × D / 1000. Using circular mils directly: VD = 2 × K × I × D / cmil for single-phase.
The factor of 2 accounts for the round trip (hot and neutral). In three-phase balanced circuits, √3 replaces 2 because neutral current is zero under balanced conditions.
15A circuit (typical lighting): 14 AWG copper, up to 50ft. 20A circuit (general receptacles): 12 AWG, up to 50ft. 30A circuit (dryer, water heater): 10 AWG, up to 50ft. 40A circuit (range, large appliance): 8 AWG. 50A circuit (ranges, subpanels): 6 AWG. 100A subpanel: 3 AWG copper or 1 AWG aluminum. 200A service: 2/0 copper or 4/0 aluminum.
NEC requires minimum 10 AWG copper (or 8 AWG aluminum) for 30-amp circuits. However, voltage drop over long runs may require larger wire. A 30A circuit at 240V running 100ft may need 8 AWG copper for acceptable voltage drop.
NEC recommends (not requires) that voltage drop on branch circuits not exceed 3%, and total drop (feeder + branch) not exceed 5%. For a 120V circuit, 3% = 3.6V. Exceeding this causes dim lights, slow motors, and equipment problems.
Copper has lower resistance (smaller wire for same ampacity), better connections, and is standard for branch circuits. Aluminum is cheaper and lighter, used mainly for large feeders and service entrance cables (4 AWG and larger).
Conduit fill affects the number of conductors, which requires ampacity derating per NEC 310.15(C). More than 3 current-carrying conductors in a conduit reduces allowable ampacity by 20-50%. The wire gauge itself doesn't change, but you may need a larger size to compensate.
Motor circuits require wire sized for 125% of full-load current (NEC 430.22). Use the NEC motor FLA tables (not the nameplate) for sizing. A 10HP 240V motor has 50A FLA, so wire for 62.5A — typically 6 AWG copper.
Wire ampacity decreases in high-temperature environments. Standard NEC tables assume 30°C (86°F) ambient. Above 30°C, apply correction factors from NEC Table 310.15(B)(1). In a 45°C attic, 90°C-rated wire capacity drops by about 13%.