Wire Size Calculator — Free Online Calculator

How to Use This Calculator

Enter the circuit current in amps, select your system voltage, and provide the one-way wire run distance in feet. Choose between copper and aluminum conductors, select single or three-phase, and set your maximum allowable voltage drop percentage (NEC recommends 3% for branch circuits, 5% total including feeder). The calculator determines the minimum wire gauge needed based on both ampacity requirements per NEC Table 310.16 and voltage drop calculations using the circular mil method.

The Formula Explained

Wire sizing involves two critical checks. First, the wire must safely carry the required current without overheating — this is the ampacity rating from NEC Table 310.16, based on insulation type and ambient temperature. Second, the voltage drop across the wire run must stay within acceptable limits, typically 3% for branch circuits per NEC 210.19 Informational Note No. 4.

The voltage drop formula uses the circular mil method: CM = (K × I × D × 2) / VD, where K is the resistivity constant (10.37 for copper, 17.02 for aluminum), I is current in amps, D is one-way distance in feet, and VD is the maximum allowable voltage drop in volts. For three-phase circuits, the factor of 2 is replaced by 1.732 (√3).

The calculator selects the larger wire gauge that satisfies both the ampacity and voltage drop requirements, ensuring a safe and code-compliant installation.

NEC Wire Ampacity Table (75°C Insulation, NEC 310.16)

When Wire Sizing Decisions Really Matter

Most residential branch circuits are short enough that the NEC ampacity table alone gives you the right answer. A 20-amp kitchen circuit with a 30-foot run? Calibre 12 AWG copper, done. Where wire sizing gets interesting — and where most mistakes happen — is on longer runs and continuous loads. A detached garage 150 feet from the main panel, a submersible well pump 200 feet down and 80 feet from the house, a level-2 EV charger in a workshop at the property line, a subpanel feeding a pool house. In each case the voltage drop constraint dominates, and the wire needed is one to three gauges larger than the ampacity table would suggest.

Professional electricians treat every wire run longer than 50 feet as a voltage drop problem first, ampacity problem second. The rule of thumb: for every 100 feet of run at 120V, add one wire gauge to whatever the ampacity table says. For 240V circuits you get roughly double that distance before needing to upsize. Three-phase 208V/480V commercial circuits are even more forgiving because the math uses √3 (1.732) instead of 2 in the denominator — one reason commercial work favors higher voltage distribution.

Worked Example: 40-Amp EV Charger at 75 Feet

You're installing a Tesla Wall Connector, a Wallbox Pulsar Plus, or similar Level 2 charger rated 40 amps continuous. NEC 625.42 and 210.19(A)(1) require sizing the circuit at 125% of the continuous current, so the circuit must be rated for 50 amps. Location: garage, 75 feet of wire run from the main panel, 240V single-phase.

Ampacity check: 50 amps at 75°C copper per NEC 310.16 requires 8 AWG minimum. Voltage drop check: using the formula CM = (2 × 10.37 × 40 × 75) / (240 × 0.03) = 62,210 / 7.2 = 8,640 circular mils. Calibre 10 AWG has 10,380 CM, so it passes voltage drop. But ampacity requires 8 AWG (which has 16,510 CM), so the ampacity constraint wins. Final answer: 8 AWG THHN copper in conduit, with a 50-amp double-pole breaker.

What if the run were 150 feet instead? CM = (2 × 10.37 × 40 × 150) / 7.2 = 17,280 CM. Now 8 AWG (16,510 CM) is marginally insufficient on voltage drop — you need 6 AWG (26,240 CM). The breaker stays at 50A but the wire jumps a size. The extra 75 feet costs about $200 more in copper but prevents the charger from throttling on hot days.

Worked Example: 100-Amp Subpanel Feeder to Detached Shop

A common homeowner project: run a 100-amp subpanel to a detached shop 125 feet from the main house. The load might include a welder, compressor, lighting, and small HVAC. At 240V single-phase with 100A continuous capacity, voltage drop dominates the calculation.

Copper calculation: CM = (2 × 10.37 × 100 × 125) / (240 × 0.03) = 259,250 / 7.2 = 36,007 CM. That requires 4 AWG copper (41,740 CM). Ampacity: 4 AWG copper at 75°C handles 70A — not enough for a 100A feeder. So the ampacity constraint forces you up to 1 AWG copper (110A at 75°C, 83,690 CM — passes both checks).

Aluminum alternative: CM = (2 × 17.02 × 100 × 125) / 7.2 = 59,097 CM. That's calibre 2 AWG aluminum (66,360 CM) for voltage drop, but ampacity at 75°C requires 1/0 aluminum (100A). So the final answer for aluminum is 1/0 aluminum USE-2 or XHHW-2, which costs roughly 40% of the copper option and is standard practice for detached building feeders. Remember to bury at proper depth (NEC 300.5) and include an equipment grounding conductor plus a separate ground electrode at the detached structure per NEC 250.32.

Five Common Wire Sizing Mistakes

1. Using 90°C ampacity with 75°C terminations. Your THHN wire is rated 90°C, but nearly every breaker, lug, and device terminal is listed at 75°C. NEC 110.14(C) requires you to use the lower rating of the weakest link. You can buy 90°C wire but you must size it per the 75°C column unless every connection is verified 90°C rated. This mistake turns 60A-rated 6 AWG into 55A-rated 6 AWG overnight.

2. Ignoring voltage drop on runs over 50 feet. The NEC ampacity table doesn't care about length, but your equipment does. A motor starting at 85% of rated voltage draws dramatically more current, heats up, and eventually fails. Well pumps are especially sensitive — undersized wire is the leading cause of premature pump motor burnout.

3. Forgetting the 125% rule for continuous loads. A 30-amp continuous load needs a circuit rated for 37.5 amps minimum, which rounds up to a 40-amp breaker and 8 AWG wire. Lighting circuits over 3 hours, EV chargers, and commercial refrigeration all trigger this rule.

4. Stuffing too many conductors in a conduit. NEC 310.15(C)(1) requires ampacity derating when you exceed 3 current-carrying conductors in a raceway. Four to six conductors means 80% of listed ampacity; seven to nine means 70%. Miss this and your "20-amp" circuit is actually a 14-amp circuit.

5. Mixing copper and aluminum without AL/CU-rated connectors. The expansion coefficients differ, the oxidation is incompatible, and the result is loose connections and fires. Every splice and termination between dissimilar metals must use listed AL/CU connectors and anti-oxidant compound. This was the root cause of the 1970s aluminum wire fires that gave aluminum a bad reputation.

Professional Installation Tips

Size for tomorrow, not just today. The incremental cost of going from 6 AWG to 4 AWG on a 100-foot feeder is maybe $80. The cost of pulling new wire later when you add a hot tub, workshop, or EV charger is thousands. On any feeder install, size one step up from the current calculation.

Use XHHW-2 or THHN/THWN-2 dual-rated wire. You gain 90°C dry/wet capability at virtually no price premium. Even if your terminations limit you to 75°C, the 90°C rating gives you margin against derating factors.

Torque every connection to spec. Loose connections are the number one cause of electrical fires in modern installations. Every lug, terminal, and splice has a torque specification in inch-pounds. Invest in a calibrated torque screwdriver and use it religiously — especially on aluminum connections where creep under load will loosen improperly torqued joints.

Label both ends of every wire run. Future electricians (including future you) will thank you. A three-dollar label maker saves hours of troubleshooting later.

Pull an extra conductor in every conduit run. The additional cost of one more 12 AWG pull wire is trivial; the cost of opening walls later to add a circuit is enormous. If your conduit fill allows, always pull an extra.

Key NEC Code References

The wire sizing decisions in this calculator are based on specific NEC articles. Table 310.16 provides the core ampacity values for insulated conductors in raceway or cable, at three temperature ratings (60°C, 75°C, 90°C). 310.15(B) covers ambient temperature correction factors — critical for installations in hot attics, industrial environments, or direct sun. 310.15(C)(1) covers adjustment factors for more than three current-carrying conductors in a raceway.

240.4(D) establishes the small conductor rule: 14 AWG copper gets 15A max, 12 AWG gets 20A max, 10 AWG gets 30A max — regardless of what the ampacity table says, because of short-circuit considerations. 210.19(A)(1) requires branch circuit conductors to be sized at 125% of continuous load plus 100% of non-continuous load. 215.2(A)(1) applies the same rule to feeders. Voltage drop recommendations (3% branch, 5% total) appear as an informational note in 210.19 — not mandatory but universally followed by professionals.

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