Wire size calculator

Use the calculator

Pick an application preset (branch, feeder, solar, marine, low-voltage DC), enter load and length, and the calculator returns the smallest standard size that passes both ampacity and voltage drop, plus the recommended overcurrent device and equipment grounding conductor.

The wire size decision rule

There is no single closed-form formula for wire size — sizing is a decision rule that takes the smaller-of-two-evils answer between two independent tests: ampacity and voltage drop. Both must pass.

The decision rule: pick the smallest standard size from NEC Table 310.16 (or IEC 60228 for metric) where both Eq. 01 and Eq. 02/03 pass. The calculator iterates from smallest to largest and returns the first size that satisfies both — this is exactly the manual procedure formalised.

Worked example, 240 V branch circuit

A 20 A continuous load (e.g. a 240 V baseboard heater) on a branch circuit, 30 m one-way run, copper, 75°C THWN insulation, 30°C ambient, 3 conductors in conduit.

The 240.4(D) small-conductor cap, not raw ampacity, decides 10 AWG over 12 AWG here. On a 12 V DC system with the same 30 m and 20 A, voltage drop would jump to 33% — far above the 3% limit — and the calculator would push you to 4 AWG or larger. Length and voltage matter as much as load.

How to size a wire, step by step

1. Determine the load current. From nameplate full-load amps (FLA) for equipment, or compute from power: I = P / V (DC), I = P / (V·cos φ) (1-phase), I = P / (√3·V·cos φ) (3-phase).
2. Apply the continuous-load factor. If the load runs 3 hours or more at full current, multiply by 1.25 per NEC 210.19(A). The conductor must carry this adjusted current, not just the nominal.
3. Pick the insulation rating that matches your wire and terminations. Most modern installations use 75°C or 90°C insulation (THWN, THHN, XHHW). Note: NEC 110.14(C) caps your effective rating to 60°C for circuits ≤100 A unless the equipment is listed for higher.
4. Apply derating for ambient and conductor count. NEC 310.15(B)(1): correction factor for ambient ≠ 30°C. NEC 310.15(B)(3): adjustment factor for >3 current-carrying conductors in the same raceway. Multiply both into the base ampacity.
5. Pick the smallest size whose derated ampacity ≥ adjusted load. Look up NEC Table 310.16 for the chosen material, insulation, and class. Verify against the small-conductor rule (NEC 240.4(D)): 14/12/10 AWG copper cannot be protected above 15/20/30 A regardless of insulation class.
6. Check voltage drop on that size. Compute V_drop = 2·L·I·ρ/A (DC, 1-ph) or √3·L·I·ρ/A (3-ph). If it exceeds your limit (NEC informational note: 3% branch, 5% combined), step up one or more sizes until V_drop ≤ limit.
7. Size the EGC and the breaker. Round the load up to the next standard breaker size per NEC 240.6(A). Look up minimum equipment grounding conductor (EGC) per Table 250.122 based on that breaker rating.

NEC Table 310.16 — copper and aluminium ampacity

Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

Wire sizing: copper vs aluminium

Variants: solar PV and marine wiring

Two common installations have stricter rules than ordinary residential branches. The calculator above includes presets for each.

Solar / photovoltaic (NEC Article 690)

NEC 690.8(A): conductors must be sized for 1.25× short-circuit current (Isc) of the PV array, then a second 1.25× factor is applied for continuous operation — total 1.56× the rated Isc. Voltage drop target on PV strings is typically 2% to preserve power; conduit on a hot roof can reach 60–75°C ambient, derating the ampacity to roughly 0.71–0.82× of base. Use 90°C XHHW-2 or similar high-temp insulation; never use 60°C insulation on a roof run.

Marine (ABYC E-11)

The American Boat & Yacht Council E-11 standard is more aggressive than NEC for marine DC: 3% maximum drop on critical loads (navigation, bilge pumps) and 10% maximum on non-critical loads (cabin lighting, accessories). All conductors must be tin-plated, finely-stranded marine-grade copper with thermoplastic or thermoset insulation. The very low DC bus voltage (12 V or 24 V) makes voltage drop the dominant constraint — wire is usually two AWG sizes larger than the ampacity table alone would suggest.

Low-voltage DC: 12 V, 24 V and 48 V systems

The same engine handles 12 V, 24 V and 48 V DC sizing — RVs, off-grid solar, golf carts, automotive aux loads, and battery banks all share the same physics. The takeaway: at 12 V the drop coefficient is 20× harsher than at 240 V single-phase for equal current and length, so a 12 volt DC wire-size calculator that ignores length will mislead you. Worked rule of thumb at 12 V: a 20 A draw on 14 AWG copper drops about 1 V (8%) per 5 m one-way; jump to 8 AWG (8.37 mm²) and the same run drops to ~3%. For 24 V double the acceptable run; for 48 V quadruple it. Pick the AS/NZS or NEC preset and switch the calculator to DC mode for accurate sizing on solar-PV combiner output, RV chassis runs, and trolling-motor circuits.

Cu, 75°C insulation, 30°C ambient, ≤3 conductors. Drop budget is 3% of the nominal bus voltage. Doubling the voltage roughly doubles the acceptable run for the same wire size, which is why 48 V battery banks in solar / telecom installs almost always beat 12 V on conductor cost beyond ~15 m.

Related concepts on this site

Frequently asked questions

How do I calculate wire size for amps?

Two checks must both pass. Ampacity: the wire must carry the load × 1.25 (if continuous) after derating for ambient temperature and conductor count. Voltage drop: the drop along the run must not exceed 3% (branch, NEC informational note 210.19(A)) or your project limit. Pick the smallest standard AWG/mm² that satisfies both — that is what the calculator above does automatically against NEC 310.16.

What size wire do I need for a 30 amp circuit?

10 AWG copper is the standard minimum for a 30 A branch circuit at 75°C insulation in a typical residential ambient (30°C, ≤3 conductors). NEC 240.4(D) caps 10 AWG Cu at 30 A regardless of insulation rating, so you cannot protect smaller wire with a 30 A breaker. Aluminium 30 A circuits use 8 AWG Al as the minimum. For long runs, voltage drop may push you to 8 AWG Cu — use the calculator with your actual length.

What size wire for 25, 50, 70, 100, 125, 200 amps?

At 75°C copper, 30°C ambient, ≤3 ccc: 25 A → 10 AWG, 50 A → 8 AWG, 70 A → 4 AWG, 100 A → 3 AWG (often 2 AWG for service), 125 A → 1 AWG, 200 A → 2/0 AWG (or 4/0 Al for 200 A service). For a 200 A sub-panel feeder, NEC 310.12 dwelling-unit reduction also permits 2/0 Cu. These are NEC 310.16 minima for ampacity only — real installations almost always need a larger size to keep voltage drop within 3% on typical run lengths, especially over 30 m at 240 V. The calculator on this page checks both ampacity and drop at once.

What size wire for an AC unit or 25 amp breaker?

Central AC and heat pumps are sized from the nameplate MCA (minimum circuit ampacity), not the running amps — see the calculator's HVAC preset. A 3-ton residential AC unit typically lists MCA 18–24 A and uses a 25 A or 30 A circuit on 10 AWG copper; larger 4–5 ton units jump to 35–40 A on 8 AWG. For a generic 25 A breaker (welder, motor, baseboard heat) NEC 240.4(D) caps 10 AWG at 30 A, so 10 AWG copper is the minimum, with 8 AWG used when the run length pushes voltage drop past 3%.

Should I use copper or aluminium?

Copper for branch circuits, panel terminations, and anything where space is tight: better conductivity, smaller size for the same ampacity, more forgiving of poor terminations. Aluminium for service entrance and large feeders: 30–40% cheaper for the same ampacity, lower weight on long runs. Aluminium is one or two AWG sizes larger for the same load. NEC 110.14(B) requires anti-oxidant compound on Al terminations and torque-checked connections.

How does ambient temperature affect wire size?

Hot ambient lowers ampacity. NEC Table 310.15(B)(1) correction factors: at 75°C insulation, going from 30°C to 41–45°C ambient drops ampacity to 0.82× of base; at 51–55°C ambient, it drops to 0.67×. Engine rooms, attics, rooftop conduit, and industrial settings routinely sit above 30°C — always derate. For 90°C insulation, the factors are slightly less punishing (0.87× and 0.76× at the same ambients).

When do I need parallel conductors?

NEC 310.10(H) allows parallel conductors per phase for sizes 1/0 AWG and larger. Common reasons: (1) the single-conductor option is too large to bend or pull; (2) two smaller cables fit existing conduit; (3) reducing voltage drop without buying a single huge cable. All parallel conductors must be same length, material, size and insulation. Each carries its share of the current — divide the per-phase load by the number of parallel sets when sizing each.

Why does my measured voltage drop differ from the calculator?

Common causes: (1) actual conductor temperature is higher than the formula assumes — apply temperature correction to ρ; (2) length used in the calc is shorter than the real cable route; (3) splices and terminations add 0.5–2 V on long runs; (4) loose connections are the most frequent culprit and easy to miss. Verify connection torque to manufacturer spec before suspecting the calculation.

What is the difference between NEC and AS/NZS wire sizing?

NEC (USA) uses American Wire Gauge (AWG) and Table 310.16 ampacity; voltage-drop guidance is informational (3% branch / 5% total). AS/NZS 3008.1.1 (Australia / New Zealand) uses metric mm² per IEC 60228, has a different ampacity table format, and treats 5% from point of supply as a mandatory limit. The calculator above supports both materials and both AWG / mm² standards; use the AS/NZS preset if you are sizing under that code.

Sources and methodology

1. NFPA. National Electrical Code (NEC) NFPA 70, 2023 Edition. Articles 210.19(A), 215.2(A), 240.4(D), 240.6(A), 250.122, 310.15, 310.16, 690.8(A).
2. Standards Australia / Standards New Zealand. AS/NZS 3008.1.1:2017 Electrical installations — Selection of cables.
3. IEC. IEC 60364-5-52:2009 Low-voltage electrical installations — Wiring systems.
4. IEC. IEC 60228:2004 Conductors of insulated cables (mm² standard sizes).
5. ASTM. ASTM B258 — Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes.
6. ABYC. E-11: AC and DC Electrical Systems on Boats, 2018 Edition.
7. IEEE. IEEE Std 141-1993 (R1999) Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book).