Wire Size Calculator

To size wire, multiply your load current by 1.25 for continuous loads, then pick the smallest AWG whose ampacity meets that number. A 50-amp continuous load needs 50 × 1.25 = 62.5 A of ampacity, which at the 75°C column of NEC Table 310.16 is satisfied by 6 AWG copper (65 A). Then confirm the run is not too long: voltage drop should stay under 3% for branch circuits.

This wire size calculator handles both checks at once. It returns the recommended AWG, the required ampacity after the 125% rule, and the voltage drop across your actual one-way run length so you never undersize a conductor or overheat it. Aluminum is supported too — it carries less current per gauge, so a 50-amp aluminum feeder typically steps up to 4 AWG.

Two independent limits govern conductor selection: ampacity (heat) and voltage drop (performance). The wire must pass both.

The National Electrical Code defines a continuous load as one expected to run at maximum current for three hours or more. NEC 210.20(A) and 215.3 require the overcurrent device and conductor to be rated at 125% of that continuous load — equivalently, the conductor may be loaded to only 80% of its ampacity. That is why a 65-amp wire is the correct pick for a 50-amp continuous circuit, not a 50-amp wire.

• Ampacity — the current a conductor carries continuously without exceeding its insulation temperature rating. Read it from the 60°C, 75°C, or 90°C column of Table 310.16.
• The 80% / 125% rule — the same idea from two directions: size for 125% of the load, or load a wire to no more than 80% of its rating.
• Termination temperature — most breakers and lugs are listed for 75°C, so even with 90°C insulation you must size from the 75°C column for circuits over 100 A (and typically at or below).

For the second limit, this tool computes one-way voltage drop with Vdrop = 2 × length × current × resistance ÷ 1000, then divides by system voltage. See the dedicated voltage drop calculator for long-run design.

Example 1 — 50 A EV charger, 240 V, 40 ft copper: continuous load, so required = 50 × 1.25 = 62.5 A. Smallest 75°C copper conductor is 6 AWG (65 A). Voltage drop = 2 × 40 × 50 × 0.491 ÷ 1000 = 1.96 V, or 0.82% of 240 V — well under 3%. Answer: 6 AWG copper.

Example 2 — 30 A well pump, 240 V, 220 ft copper: required ampacity = 37.5 A, met by 8 AWG (50 A). But voltage drop = 2 × 220 × 30 × 0.778 ÷ 1000 = 10.3 V = 4.3% — over the limit. Step up to 6 AWG: drop = 2 × 220 × 30 × 0.491 ÷ 1000 = 6.5 V = 2.7%. On long runs, voltage drop, not ampacity, sets the size.

Example 3 — 100 A subpanel feeder, 240 V, 60 ft copper: required = 125 A, met by 1 AWG (130 A) at 75°C. Drop = 2 × 60 × 100 × 0.154 ÷ 1000 = 1.85 V = 0.77%. Answer: 1 AWG copper (many installers use 2/0 to gain margin and reuse for future upgrades). Confirm the breaker with the breaker size calculator.

Example 4 — 20 A aluminum branch, 120 V, 100 ft: required = 25 A. Aluminum 10 AWG carries 30 A. Drop = 2 × 100 × 20 × 2.04 ÷ 1000 = 8.2 V = 6.8% — too high. Aluminum's higher resistance forces an upsize to 8 AWG (drop ≈ 5.1 V, 4.3%) or a shorter run.

Table 310.16 ampacities assume an ambient of 30°C (86°F) and no more than three current-carrying conductors in a raceway. Real installations often violate both, and the code requires you to derate — reduce the allowable ampacity.

Ambient temperature correction. In a hot attic (say 50°C), a 75°C conductor's ampacity is multiplied by roughly 0.75. An 8 AWG copper wire rated 50 A becomes about 37 A of usable ampacity. This is where 90°C insulation (THHN/THWN-2) helps: you size the terminations at 75°C but apply derating from the higher 90°C ampacity, preserving margin.

Conductor bundling. When 4–6 current-carrying conductors share a raceway, ampacity is multiplied by 0.80; for 7–9 conductors, 0.70. A conduit stuffed with circuits must use larger wire than a single-circuit run. Neutral conductors that carry only unbalanced current usually do not count, but shared neutrals on nonlinear loads may.

Because derating stacks multiplicatively (ambient × bundling), a wire that looks adequate on the bare table can fall short in practice. When in doubt, upsize one gauge — the cost difference is small compared with a failed inspection or an overheated cable.

Ampacity keeps wire from overheating; voltage drop keeps equipment working correctly. NEC 210.19 and 215.2 recommend limiting branch-circuit drop to 3% and total (feeder + branch) to 5%. These are informational notes, not hard mandates, but ignoring them causes dimming lights, motors that run hot, and electronics that misbehave.

The factor of 2 accounts for current flowing out and back on both conductors. Resistance is per 1,000 feet: about 0.491 Ω for 6 AWG copper, 0.194 Ω for 2 AWG, and roughly 1.6× those values for aluminum. Longer runs and higher currents raise drop linearly, so a circuit that is fine at 40 feet can fail at 200 feet.

The practical takeaway: on short runs, ampacity decides the size; on long runs, voltage drop decides. This calculator reports both so you never overlook the one that governs. For a deeper treatment including 12 V DC systems, use the voltage drop calculator and the 12V wire size calculator.

Sizing conductors is not only a safety exercise — it is a budget decision, and the two pull in opposite directions. A larger gauge costs more copper per foot but wastes less energy as heat and leaves room for future load, while a minimum-legal size saves money today at the expense of margin.

Copper price scales roughly with cross-sectional area, and each step up the AWG ladder increases area about 26%. Going from 6 AWG to 4 AWG on a 40-foot feeder might add only a modest amount to a project, yet it can cut voltage drop by a third and reclaim capacity for an EV charger or shop tools added later. On long runs the math tilts further toward upsizing: the wire that a voltage-drop check forces you to buy also runs cooler and lasts longer.

Aluminum flips the calculation on large feeders. Even though aluminum needs one to two sizes larger, it costs far less per pound than copper, which is why 200-amp services routinely use 4/0 aluminum instead of 2/0 copper. The rule of thumb: use copper where space is tight and runs are short, and consider aluminum where the run is long, the gauge is large, and terminations are rated for it. Either way, size for the load first, then let voltage drop and future-proofing decide whether to step up.

1. Enter your load current in amps. Use the appliance nameplate rating or the branch circuit breaker size. If you only know watts, divide by voltage first, or use the wire gauge calculator.
2. Select system voltage. 120 V for standard receptacles and lighting, 240 V for ranges, dryers, EV chargers, and subpanels, or 12 V for DC/automotive work.
3. Enter one-way run length. Measure from the panel to the load along the actual cable path, not straight-line distance. The tool doubles it internally for the return path.
4. Choose copper or aluminum. Copper is the default and carries more current per gauge; aluminum is common on large feeders and service entrances but needs to step up a size or two.

The recommended AWG updates instantly, along with the required ampacity and voltage drop. If the note warns that drop exceeds 3%, increase the gauge until it clears. Always verify the final choice against your local amendments and have work inspected.

• Sizing to load instead of 125% of load. Continuous loads (EV chargers, electric heat, lighting) require the wire and breaker rated at 125% of the current.
• Using the 90°C column at terminations. Breakers and lugs are usually listed for 75°C; you may use 90°C only for derating math, not final termination sizing.
• Ignoring voltage drop on long runs. A 200-foot circuit can need a wire two sizes larger than ampacity alone suggests.
• Forgetting to derate for heat or bundling. Attics, conduit fill, and rooftop exposure all cut ampacity.
• Mixing aluminum and copper at terminals. Only use connectors rated CO/ALR or AL-CU, with antioxidant paste, to avoid corrosion and loose joints.

When adding a subpanel or a large appliance, pair this tool with the breaker size calculator so the conductor and overcurrent device are a matched, code-compliant set.

Household circuits. 15 A lighting uses 14 AWG, 20 A receptacles use 12 AWG, 30 A dryers and water heaters use 10 AWG, and 50 A ranges use 6 AWG copper. These are the everyday defaults this calculator confirms.

EV charging. A 48 A continuous charger (typical Level 2) requires 60 A of breaker and 6 AWG copper minimum, often 4 AWG on longer garage-to-panel runs to hold voltage drop under 3%.

Solar and battery systems. DC circuits are voltage-drop sensitive because system voltage is low. A 30 A array at 48 V behaves very differently from the same current at 240 V — the 12V wire size calculator and voltage drop calculator are essential here.

Subpanels and feeders. A 100 A feeder uses 1 AWG copper or 2/0 aluminum at 75°C; a 200 A service uses 4/0 copper or 250–350 kcmil aluminum. Long feeders almost always upsize for voltage drop.