Wire Size Calculator — NEC AWG & kcmil Sizing

TL;DR — What this calculator returns. Enter load amps (or use a preset), system voltage, one-way run length, wire material (copper / aluminum), ambient temperature, and number of current-carrying conductors. The tool returns the smallest NEC Table 310.16 (75 °C column) compliant AWG or kcmil, the corrected ampacity after NEC 310.15(B)(1) temperature and NEC 310.15(C)(1) conduit-fill adjustment, the recommended breaker (NEC 240.6(A) standard OCPD), and the voltage-drop % against the NEC 3 % branch / 5 % combined limits.

What size wire do I need?

Wire Size Calculator — sized right, NEC right. Pick the correct copper or aluminum conductor for any residential, commercial, or industrial branch circuit in seconds. This NEC-aware wire size calculator uses the 75 °C ampacity column from NEC Table 310.16 and applies temperature-correction and conduit-fill adjustment factors per NEC 310.15(B) — the same numbers your inspector checks. Enter load amps (or watts + volts), system voltage, one-way run length, and wire material; the tool returns the smallest compliant AWG or kcmil, the temperature-corrected ampacity, and a NEC 240.4(B) round-up flag if the next-larger size is required for overcurrent coordination. Built for licensed electricians, EV-charger installers, solar-PV designers, and DIY homeowners pulling a permit. No signup, no ads, no email gate.

Why this wire size calculator?

• NEC 310.16 compliant — calculates copper & aluminum conductor sizes using the 75 °C ampacity column (the inspector-favored terminal rating per NEC 110.14(C)).
• Covers 14 AWG to 750 kcmil — from a single 15-amp lighting branch circuit to a 600-amp service entrance, in one calculator.
• EV-charger, subpanel & solar presets — one-click sizing for 30/40/50/60/100 amp services, 11 kW/22 kW Level-2 EV chargers, and 12V/24V/48V PV DC runs.
• Temperature & conduit-fill correction — applies NEC 310.15(B) ambient-temperature adjustment factors (30 °C / 35 °C / 40 °C / 45 °C) and conduit-fill derating (more than 3 current-carrying conductors).
• Voltage-drop check built-in — instantly shows the % drop at the recommended size and warns if you cross NEC 3 % branch / 5 % feeder limits; one-click link to our full voltage-drop calculator.

How to use this wire size calculator

1. Choose a preset or enter your load manually. Start with the “Quick Presets” chips at the top for instant EV-charger, subpanel, service, dryer, or range sizing — or enter load amps directly. For three-phase motors, enter kW first then let the calculator convert to FLA via our three-phase power calculator.

2. Set the system conditions. Pick system voltage (120 / 208 / 220 / 240 / 277 / 347 / 480 / 600 V), phase (1Φ or 3Φ), and one-way run length in feet. The round-trip voltage drop is computed as 2× the one-way length.

3. Pick the wire material and environment. Toggle between copper and aluminum. Set ambient temperature (default 30 °C) and the number of current-carrying conductors in the conduit — both feed the NEC 310.15(B)(1) temperature correction and NEC 310.15(C)(1) conduit-fill adjustment.

4. Flag continuous loads. EV chargers, solar PV inverters, and some motor loads are continuous (NEC 210.20(A) / 215.3 / 625) and require the conductor + breaker to be sized to 125 % of the load. Check the “Continuous load” box or pick an EV / solar preset to auto-enable it.

5. Read the result card. The 3-column card shows the recommended AWG / kcmil, the corrected ampacity, the recommended OCPD size, and the voltage-drop percentage. The compliance badge at the top goes 🟢 GREEN (fully compliant), 🟡 YELLOW (compliant but voltage drop marginal), or 🔴 RED (does not meet NEC — upsize).

6. Cross-check with the voltage-drop calculator. If the V-drop card flags a yellow or red, click through to our full voltage drop calculator for length-by-gauge and ambient temperature sensitivity analysis. Then pair the conductor with the breaker using our circuit breaker sizing calculator.

NEC wire sizing basics

NEC wire sizing rests on three pillars. First, ampacity — the maximum current a conductor can carry continuously without exceeding its insulation temperature rating — is defined per NEC Table 310.16 for each AWG / kcmil size, with separate columns for 60 °C, 75 °C, and 90 °C insulation. The 75 °C column is the inspector-favored reference for circuits rated 100 A or less (NEC 110.14(C)).

Second, derating. Base ampacity assumes 30 °C ambient and at most three current-carrying conductors in the conduit. Higher ambient temperatures apply NEC 310.15(B)(1) correction factors; more than three current-carrying conductors apply NEC 310.15(C)(1) adjustment factors (0.80 for 4–6 conductors, 0.70 for 7–9, and so on).

Third, overcurrent protection coordination. The breaker must not exceed the conductor’s corrected ampacity — except where NEC 240.4(B) permits the next-larger standard OCPD when no standard OCPD matches the conductor ampacity.

NEC ampacity reference (Table 310.16, 75 °C column)

This table is the inspector-favored reference for ampacity in the US — every AWG / kcmil from 14 AWG through 750 kcmil, copper and aluminum, 75 °C terminal-rating column per NEC 110.14(C). Cross-check the recommended conductor against this table, or look up an existing conductor’s corrected ampacity with our cable ampacity lookup.

Source: NFPA 70 — National Electrical Code 2023, Table 310.16 (75°C column). Use this table to verify the calculator’s output or look up an existing conductor’s corrected ampacity.

Wire size for common services (presets)

The calculator ships with one-click presets for the most common residential, commercial, and light-industrial circuits. Each preset auto-fills the inputs and returns the smallest NEC-compliant conductor. For service-entrance sizing (100 A / 200 A), pair the recommended conductor with the breaker per NEC 240.4(B) using our circuit breaker sizing calculator.

30 A dryer circuit

NEC 220.54 — standard 30 A dryer receptacle (10-30R) on a 30 A breaker. Typical run 35 ft at 120/240 V → 10 AWG copper + 30 A breaker. Larger gauge is rarely needed; verify with your local AHJ.

40 A electric range circuit

NEC 220.55 demand Table — 40 A range receptacle (14-50R). Typical run 35 ft at 120/240 V → 8 AWG copper + 40 A breaker.

50 A RV / subpanel feeder

NEC 215.2 / 551.71 — 50 A subpanel feeder (detached garage, workshop) or RV pedestal. Typical run 75 ft at 120/240 V → 6 AWG copper + 50 A breaker (or 4 AWG aluminum).

60 A EV charger (Level 2, 11 kW)

NEC 625.41 + 210.20(A) — 48 A continuous-load Level-2 EV charger. 125 % rule → conductor rated 60 A minimum → 4 AWG copper + 60 A breaker. Verify the charger’s actual demand with the EV charging power calculator.

100 A subpanel feeder

NEC 215.2(A)(1) — detached-building subpanel fed at 100 A. Typical run 100 ft at 120/240 V → 3 AWG copper + 100 A breaker.

200 A service entrance

NEC 230.42 — modern residential service entrance. Typical run 75 ft at 120/240 V → 3/0 AWG copper + 200 A breaker (or 4/0 AWG aluminum).

Solar / PV (12V / 24V / 48V DC)

NEC 690.8 + 210.20(A) — off-grid PV battery circuit, 30 A continuous load → 8 AWG copper with 40 A OCPD. For grid-tied PV inverters with step-up transformers, size the upstream transformer with our transformer capacity selection calculator.

Copper vs aluminum wire sizing

Aluminum has approximately 61 % the conductivity of copper by cross-sectional area, so to carry the same ampacity you must step up roughly two AWG sizes. A 6 AWG copper conductor (65 A) is roughly equivalent to a 4 AWG aluminum conductor (65 A). NEC 310.6 permits aluminum building wire only at 8 AWG and larger; 12 AWG and 10 AWG aluminum are restricted to specific applications like transformer secondaries.

Aluminum is widely used for service entrance and large feeders because it costs 30–50 % less per amp than copper, but it requires antioxidant joint compound (per NEC 110.14) and CU/AL-rated terminals to prevent galvanic corrosion at the termination. For EV-charger and subpanel branch circuits, copper is the de-facto standard — see our EV charging time calculator for EV-specific scenarios, or estimate the operating cost of the load with our electricity cost calculator.

Frequently asked questions

What size wire do I need for a 100 amp service?

For a 100 A residential service entrance (NEC 230), the typical minimum is 3 AWG copper or 1 AWG aluminum for a 120/240 V single-phase run under 100 ft, after NEC 310.15(B) temperature- correction. For runs over 150 ft, bump up one AWG to stay under 3 % voltage drop. Always verify with your local AHJ.

What size wire for a 50 amp subpanel?

A 50 A subpanel feeder (NEC 215) typically requires 6 AWG copper or 4 AWG aluminum, with an 8 AWG copper equipment grounding conductor. Use the calculator above to confirm for your exact run length and conduit-fill conditions.

What wire size for a Level-2 (40-50 A) EV charger?

A 40 A continuous-load EV charger (NEC 625) requires a conductor rated 125 % of the load → 50 A minimum → typically 6 AWG copper with a 50 A breaker. For a 48 A / 11 kW charger: 4 AWG copper with a 60 A breaker. For a 50 A / 12 kW charger: 4 AWG copper with a 50 A breaker — verify the terminal rating (75 °C).

How do I size aluminum wire vs copper?

Aluminum has about 61 % the conductivity of copper, so for the same ampacity you typically go two AWG sizes larger (e.g. 6 AWG copper ≈ 4 AWG aluminum). Aluminum is common for service entrance and large feeders (above 6 AWG equivalent) but is restricted for branch circuits in many jurisdictions (NEC 310.6).

What is the NEC 240.4(B) round-up rule?

NEC 240.4(B) says the next-larger standard overcurrent device (OCPD) is permitted only if the ampacity of the conductor does not correspond to a standard OCPD size AND the next-larger OCPD is no more than 800 A. In practice this means: if your load calculation lands on a non-standard ampacity, the conductor must be sized to the next standard OCPD — the calculator flags this automatically.

When does voltage drop drive the sizing instead of ampacity?

Voltage drop is usually the second constraint — but for long runs it becomes the first. Rule of thumb: if the one-way length in feet is greater than the system voltage in volts × 1.5 (150 ft @ 120 V, 300 ft @ 240 V, 720 ft @ 480 V), the smallest ampacity-compliant conductor will likely exceed the 3 % branch limit. How to upsize: bump one or two AWG sizes until the calculator’s V-drop card turns 🟢 green. For very long runs (e.g. detached outbuildings, solar arrays, well pumps), combine this calculator with the voltage drop calculator for a full length-vs-gauge sensitivity sweep.

Do I need to derate for solar PV DC string wiring?

Yes — but the 125 % continuous-load rule (NEC 690.8) is the bigger factor. Solar PV inverter output circuits are treated as continuous, so the conductor ampacity must be ≥ 1.25 × the inverter’s max output current. PV wire (typically USE-2 / PV wire, 90 °C wet-rated) still uses NEC Table 310.16 for ampacity. For 12V / 24V / 48V off-grid battery banks, the calculator’s “12 V / 24 V / 48 V” preset chains the same logic with low-voltage high-current arithmetic — expect 4/0 AWG or larger on a 48 V bank pushing 100 A.