Article — Wire Size Calculator
Wire Size Calculator: AWG sizing for any electrical circuit
A wire size calculator returns the minimum AWG conductor that satisfies both voltage drop (typically 3% per NEC 210.19) and NEC 310.16 ampacity. The voltage drop formula is circular mils equals K times 2I times length, divided by allowable voltage drop. For a 20-amp 120V circuit running 50 feet with 3% drop, #12 AWG copper meets both criteria. Use aluminum and you typically need two AWG sizes larger for the same circuit.
The NEC (National Electrical Code, NFPA 70) is the governing standard in the US. Most calculations use Table 310.16 for ampacity, with derating factors for high ambient temperature, more than three current-carrying conductors per raceway, and continuous loads. The calculator above implements the basic case — consult a licensed electrician for installations with derating concerns.
What is a wire size calculator
A wire size calculator translates electrical circuit parameters — current, voltage, run length, allowable voltage drop, and conductor material — into a minimum AWG (American Wire Gauge) recommendation. The result is the smallest standard conductor that won't overheat at the rated current and won't drop more voltage than the allowable threshold.
The two failure modes a wire size calculator prevents are different: ampacity failure (overheating that can melt insulation and start fires) and voltage drop failure (motors stalling, lights dimming, electronics losing their hold-up voltage). Code addresses both. Voltage drop is technically an Informational Note in NEC 210.19, not a strict code requirement, but many local jurisdictions enforce it as code.
The AWG system was created by Joseph R. Brown in 1857 to standardize wire sizes for telegraph circuits. Brown chose a logarithmic scale: each AWG step changes diameter by a factor of about 1.1229 (the 39th root of 92), which means area doubles every three sizes. The system was adopted internationally for any wire-based product and is still used worldwide today.
Wire size formula (voltage drop)
The core formula relates wire cross-section (circular mils) to the circuit's voltage drop:
CM = (K × 2 × I × L) ÷ VDK copper = 12.9 at 75°CK aluminum = 21.2 at 75°CVD allowed = voltage × 0.03 (3%)CM → AWG round up to standard sizeWorked example: 20 amps at 120V, 50-foot one-way run, copper, 3% voltage drop. Allowable VD = 120 × 0.03 = 3.6 V. CM = (12.9 × 2 × 20 × 50) ÷ 3.6 = 7,167. The next standard AWG is #12 (6,530 CM), which is slightly below. Round up to #10 (10,380 CM). Cross-check ampacity: #12 copper at 75°C carries 25 A, so ampacity is fine for 20 A. Voltage drop wins; the answer is #10.
NEC wire size ampacity table
NEC Table 310.16 lists ampacity at three temperature ratings (60, 75, and 90°C) for each AWG. Most modern residential wiring is rated 75°C, so that column drives the calculator. The 90°C column is referenced only for derating, not for the final ampacity decision.
- #14 AWG copper: 20 A. Standard for 15-amp circuits with margin.
- #12 AWG copper: 25 A. Standard for 20-amp circuits.
- #10 AWG copper: 35 A. Standard for 30-amp circuits and electric water heaters.
- #8 AWG copper: 50 A. Range / oven branch circuits.
- #6 AWG copper: 65 A. Subpanel feeders, large appliances.
- #4 AWG copper: 85 A. Larger subpanels.
- #2 AWG copper: 115 A. Common service entrance for older 100-amp panels.
- 2/0 AWG copper: 175 A. Service entrance for 200-amp residential panels (sometimes 4/0 aluminum).
Copper vs. aluminum wire size
Copper has about 35% lower resistivity than aluminum (1.72 vs. 2.65 microhm-cm). To carry the same current at the same temperature rise, aluminum needs about 1.6 times the cross-section, which is roughly 2 AWG sizes larger.
Aluminum is lighter and cheaper than copper, which is why utility service drops and large feeders are usually aluminum. Aluminum branch circuits inside the home were common from the late 1960s to mid-1970s and have been associated with elevated fire risk due to connection oxidation; modern code requires aluminum-rated connectors and anti-oxidant compound.
For a quick mental conversion: aluminum needs two AWG sizes larger than copper for the same current. 30 A: #10 copper or #8 aluminum. 50 A: #8 copper or #6 aluminum. 100 A: #3 copper or #1 aluminum. Always verify against NEC 310.16 for your installation.
Wire size for long runs
Wire size is usually decided by ampacity for short runs (under 50 feet) and by voltage drop for longer runs. The crossover depends on current and voltage — higher current and lower voltage make voltage drop dominate earlier.
Practical thresholds: a 15-amp branch circuit at 120V hits 3% voltage drop with #14 at about 50 ft. Past that, upsize to #12 by 75 ft or #10 by 120 ft. A 30-amp 240V circuit hits 3% drop with #10 at about 100 ft; upsize to #8 by 150 ft. The exact crossover depends on the formula above.
Voltage drop and wire size
NEC voltage drop guidance is in Informational Notes to 210.19 (branch circuits) and 215.2 (feeders). The recommended limits are 3% on a branch circuit, 2% on a feeder, and 5% combined system. These are recommendations from the code committee, made enforceable by many state and local amendments.
Voltage drop is wasted power dissipated as heat in the wire. Beyond 5%, motors compensate by drawing more current, which makes them run hotter and shortens their service life. Lights dim noticeably. Electronics may drop below their hold-up voltage and reset unpredictably. Long-term, the wire itself runs at elevated temperature, which degrades insulation over years.
Common residential wire sizes
US residential wiring follows a handful of standard combinations:
- 15-amp lighting and outlets: #14 AWG copper, 14/2 with ground in cable form. 120 V.
- 20-amp kitchen and bath outlets: #12 AWG copper, 12/2 with ground. 120 V.
- 30-amp dryer: #10 AWG copper, 10/3 with ground. 240 V.
- 40-amp electric range: #8 AWG copper, 8/3 with ground. 240 V.
- 50-amp electric range or sub-panel: #6 AWG copper or #4 aluminum.
- 100-amp sub-panel feeder: #3 AWG copper or #1 aluminum.
- 200-amp service entrance: 2/0 AWG copper or 4/0 aluminum, typical for modern residential service.
Common wire size mistakes
The most common mistake is sizing by current alone and ignoring voltage drop. The result is a circuit that meets code on ampacity but performs poorly under load. This is especially common in detached garages, sheds, and outbuilding circuits with run lengths over 100 feet.
The second mistake is forgetting derating. The published 75°C ampacity assumes 30°C (86°F) ambient and a maximum of three current-carrying conductors in a raceway. Adding conductors (a four-wire range circuit, or multi-circuit run) or running through hot attics requires multiplying the ampacity by a derating factor — sometimes as low as 0.7. The conductor that "carries 50 amps" by the table might actually only carry 35 in your installation.
The third mistake is using the breaker size to choose the wire. The breaker protects the wire, not the load. Wire size is determined by load and run; breaker is sized to wire. A 30-amp breaker on #14 wire is a fire hazard, regardless of whether the load draws only 10 amps. Always: wire first, then breaker.