Wire Size Calculator
Find the minimum wire gauge (AWG) for a given load based on NEC 310.16 ampacity tables and voltage drop limits.
How It Works
- 1
Enter load current, length, and voltage
Put in the continuous load in amps, the one-way run length in feet, and the nominal circuit voltage. Set the maximum voltage drop you'll accept (3% is the NEC informational recommendation for branch circuits).
- 2
Pick phase, material, and insulation rating
Choose DC, single-phase, or three-phase. Select copper or aluminum. Pick the insulation temperature rating from 60, 75, or 90 °C based on the terminations and cable type you're using.
- 3
Read the recommended AWG and the limiting factor
The calculator returns the smallest standard size that meets both ampacity and voltage drop, along with the actual drop and whether ampacity or voltage drop set the final size.
Wire Sizing With NEC 310.16: AWG, Ampacity, and Voltage Drop
A 12 AWG copper wire and a 12 AWG aluminum wire look nearly identical, but one can safely carry 20 A on a typical branch circuit and the other cannot. Pick the wrong one and the termination overheats. American Wire Gauge has been in use since 1857 and is standardized by ASTM B258. The numbering runs backwards: smaller AWG numbers mean thicker wire, with 4/0 ("four-aught") the largest practical size and 40 the smallest. Adjacent sizes differ by a factor of 92^(1/39), so dropping three gauge numbers roughly doubles the cross-sectional area and halves the resistance. In the United States, sizing is governed by the National Electrical Code (NFPA 70), first published in 1897 and updated on a three-year cycle. It is not federal law, but most states and municipalities adopt it with local amendments. Table 310.16 lists base ampacities assuming 30 °C ambient temperature and no more than three current-carrying conductors in a raceway. Real installations almost always need derating: higher ambient temperatures, bundled conductors, and continuous loads running three or more hours all reduce the usable rating. NEC 210.19 requires that conductors be sized to 125% of a continuous load, so a 16 A continuous draw needs a conductor rated for at least 20 A. Aluminum has about 61% the conductivity of copper, which typically pushes the required size one or two AWG steps larger. Aluminum also demands listed AL-rated terminations and anti-oxidation compound. Historically, ignoring those requirements caused house fires, and building codes reflect that history.
Common pitfalls
Confusing ampacity with voltage drop. Ampacity (NEC 310.16) caps the current a conductor can safely carry without overheating the insulation. Voltage drop is a separate performance check. A wire can be ampacity-legal and still drop 6 V on a long run, browning out motors and dimming lights.
Skipping bundled-conductor derating. NEC 310.15(C)(1) derates when more than three current-carrying conductors share a raceway: 80% for 4-6, 70% for 7-9, 50% for 10-20. Four 12 AWG THHN in one conduit drops from 30 A to 24 A before any other adjustment.
Missing the 125% continuous-load rule. NEC 210.19(A)(1) and 210.20(A) require branch-circuit conductors and overcurrent devices to be sized for 125% of a continuous load (3+ hours). A 16 A continuous draw needs a 20 A breaker and 12 AWG, not 14 AWG at 15 A.
Ignoring terminal temperature ratings. NEC 110.14(C) limits most 100 A or smaller breakers to the 60 °C column regardless of cable insulation rating. Running THHN-90 does not let you use 90 °C ampacity on a standard breaker; you still sit in the 60 °C column.
Forgetting aluminum AL-rated terminations. Pre-1970s aluminum branch wiring used CU-rated screws and caused house fires. NEC now requires AL-listed (or CU/AL) devices with anti-oxidant paste at every termination point.
Frequently Asked Questions
What is AWG and how does it relate to wire size?
American Wire Gauge (AWG) is the standard for conductor diameters in North America. Lower numbers are thicker wires — 14 AWG is thinner than 12 AWG, and 4/0 ("four-aught") is thicker than 1 AWG. Each 3 AWG steps roughly doubles the cross-sectional area, halving the resistance.
How do I size wire for a given load?
Two constraints must be satisfied. First, ampacity — the gauge must be rated to carry the current without overheating (NEC 310.16). Second, voltage drop — the IR loss across the run must stay under 3% for branch circuits (recommended) or 5% total feeder + branch. This tool picks the smallest standard AWG that meets both.
Why is voltage drop important for long runs?
Wire resistance × current creates a voltage drop that reduces voltage at the load. Over long distances the drop can become significant: motors run hot, LED drivers flicker, heaters underperform. For a 20A 120V branch circuit at 100 ft, 12 AWG copper drops about 3.86V (3.2%) — right at the recommended limit.
Copper vs. aluminum — which should I use?
Copper has lower resistance for a given gauge and is standard for branch circuits under 100A. Aluminum is significantly cheaper for large feeders and service entrances but requires specific terminations and anti-oxidation compound. Aluminum also needs roughly one size larger than copper for equivalent ampacity. Never mix copper and aluminum at a termination — this causes fires.
Does this account for temperature derating?
No. This tool uses the base NEC 310.16 ampacity for 60, 75, or 90°C insulation. Real-world sizing must also apply ambient-temperature correction factors (NEC 310.15(B)(2)(a)) and bundling derating when more than 3 current-carrying conductors share a raceway. For continuous loads (3+ hours), the conductor must also be sized to 125% of the load per NEC 210.19.
What does 'limited by voltage drop' mean?
When the wire needs to be upsized beyond what ampacity alone requires, the voltage drop constraint is what drives the choice. Long runs are typically voltage-drop-limited; short runs with high current are ampacity-limited. This tool reports which constraint set the final size.
Related Tools
Voltage Drop Calculator
Calculate voltage drop over a wire run for single-phase or three-phase circuits, based on wire gauge, distance, and current.
Ohm's Law Calculator
Calculate voltage, current, resistance, or power from any two known values using Ohm's Law and power formulas.
Power Calculator
Calculate electrical power. Convert watts to amps, amps to watts, and volts to watts for DC, single-phase, and three-phase AC circuits.