3.6
V
6,000
CM
-22
AWG
1.8
Ω/kft
3.6
V
6,000
CM
-22
AWG
1.8
Ω/kft
The wire size calculator determines the minimum conductor size required to carry a given current over a specified distance while keeping voltage drop within acceptable limits. Proper wire sizing is one of the most critical aspects of electrical installation safety and efficiency — undersized wires overheat, waste energy, and create fire hazards, while oversized wires are unnecessarily expensive.
Wire sizing in the United States is governed primarily by two criteria: ampacity (current-carrying capacity) per NEC Article 310, and voltage drop per NEC Section 210.19(A) and 215.2(A) (informational notes recommend maximum 3% voltage drop for branch circuits and 5% total). This calculator focuses on the voltage drop criterion, which often determines the minimum wire size for longer runs.
The American Wire Gauge (AWG) system sizes conductors in a counterintuitive manner: larger AWG numbers correspond to smaller wire diameters. AWG 14 (the minimum for 15A branch circuits) has a diameter of 1.628 mm, while AWG 0000 (4/0) measures 11.68 mm. Above 4/0 AWG, wire sizes are expressed in kcmil (thousands of circular mils), a cross-sectional area measurement.
The resistivity of the conductor material fundamentally limits how thin a wire can be for a given application. Copper (resistivity ≈ 10.8 Ω·CM/ft at 75°C) is the standard conductor material due to its low resistance and excellent mechanical properties. Aluminum (resistivity ≈ 17.0 Ω·CM/ft) has higher resistance per unit area, requiring a larger conductor cross-section. However, aluminum is significantly lighter and less expensive, making it preferred for long-distance transmission lines and large service entrance conductors.
Voltage drop is particularly important in long branch circuit runs (over 50 feet), motor feeders, outdoor landscape lighting, low-voltage systems, and EV charging installations. Even within NEC limits, excessive voltage drop causes motors to run hotter, reduces LED light output, and slows charging times.
This calculator accepts current in amperes, supply voltage, one-way wire length in feet, maximum acceptable voltage drop percentage, and conductor material (copper or aluminum). It outputs the maximum allowable voltage drop in volts, the minimum required circular mil area, and the recommended AWG gauge.
Voltage drop formula: Vd = (2 × ρ × L × I) / CM, where ρ is resistivity (10.8 for copper, 17.0 for aluminum in Ω·CM/ft), L is one-way length, I is current, and CM is cross-sectional area in circular mils. Rearranging: CM = (2 × ρ × L × I) / Vd_max. The AWG size is derived from the CM value using the AWG/CM relationship: AWG = -4.312 × log₂(CM/211600), rounded up to the next standard gauge.
Lower AWG numbers mean larger, heavier, more expensive wire but lower resistance and voltage drop. If the calculated AWG is negative (or 0000), you may need multiple conductors in parallel. Always check the selected AWG against NEC ampacity tables to ensure the wire can safely carry the required current — voltage drop sizing and ampacity sizing both apply, and the larger (lower AWG number) result governs.
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Results
For a 20A circuit at 120V over 50 ft (copper), AWG 12 (6530 CM) satisfies the 3% voltage drop limit.
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Results
A 48A EV charger at 240V over 100 ft requires at least AWG 4 copper (41740 CM) for 2% voltage drop — use AWG 3 or 4 and verify ampacity.
A circular mil is a unit of wire cross-sectional area equal to the area of a circle with a diameter of 1 mil (0.001 inch). 1 CM = π/4 sq. mils = 5.067 × 10⁻⁴ mm². AWG 12 = 6530 CM, AWG 10 = 10380 CM.
NEC informational notes recommend maximum 3% voltage drop for branch circuits and feeders separately, and 5% total. Many designers use 2% for sensitive loads or long runs. These are recommendations, not mandatory requirements.
The voltage drop formula uses 2 × L because current travels from source to load and back through two conductors. Total conductor length in the circuit is twice the one-way distance.
Both. Calculate the minimum wire size for each criterion separately, then use the larger (lower AWG number) result. For short runs at high current, ampacity often governs. For long runs at moderate current, voltage drop usually governs.
Aluminum is appropriate for service entrance conductors, feeders, and large branch circuits (typically 4 AWG and larger). It must not be connected directly to most devices rated for copper only. Always use anti-oxidant compound and AL-rated terminals with aluminum wire.
For 15A branch circuits: AWG 14 copper or AWG 12 aluminum. For 20A: AWG 12 Cu or AWG 10 Al. For 30A: AWG 10 Cu or AWG 8 Al. Always verify with NEC Article 310 and local codes.
Common conversions: AWG 14 = 2.08 mm², AWG 12 = 3.31 mm², AWG 10 = 5.26 mm², AWG 8 = 8.37 mm², AWG 6 = 13.3 mm², AWG 4 = 21.2 mm². European wire sizing uses mm² directly.
When multiple conductors share a conduit, NEC requires derating the ampacity. Three or fewer current-carrying conductors: no derating. 4-6 conductors: derate to 80%. 7-9: 70%. Also, conductors rated at 60°C vs. 90°C have different ampacities — use the lower applicable rating.
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