Wire Size Calculator

How to Use This Wire Size Calculator

This wire size calculator finds the correct AWG for any circuit, balancing NEC ampacity and voltage drop — choose your load input mode (enter amps directly, or enter watts and voltage to have the calculator derive the current). Select your system voltage using the preset buttons (120V, 240V, 208V, or 480V) or type a custom value. Set the phase (single or three-phase), conductor material (copper or aluminum), one-way circuit length, and your maximum allowable voltage drop (default 3%). The calculator returns the smallest AWG that satisfies both the NEC ampacity requirement and your voltage drop limit, along with the next size up for additional headroom.

What Is AWG (American Wire Gauge)?

AWG — American Wire Gauge — is the standardized wire sizing system used in the United States and Canada for copper and aluminum electrical conductors. The gauge number runs counterintuitively: a smaller AWG number means a thicker, heavier wire. 4/0 AWG (pronounced "four-aught") is approximately 11.7 mm in diameter with a cross-sectional area of about 107 mm², while 14 AWG is only 1.63 mm in diameter. Each step down in gauge number increases the conductor cross-section by roughly 26%.

Why Does Gauge Matter?

Thicker wire has lower resistance per foot, which means two things: (1) it can carry more current before overheating — higher ampacity — and (2) it produces less voltage drop over a long run. Undersized wire is a fire hazard and degrades performance of everything connected to the circuit. The NEC establishes minimum wire sizes matched to standard breaker ratings (14 AWG for 15A, 12 AWG for 20A, and so on) specifically to keep heat generation within safe limits.

NEC Ampacity Requirements

Ampacity — the maximum current a conductor can carry continuously — is defined by the National Electrical Code in Article 310 and Table 310.16. The values depend on conductor material (copper or aluminum), insulation temperature rating (60°C, 75°C, or 90°C), and the number of current-carrying conductors sharing a conduit or cable assembly. This calculator uses the 75°C column, which applies to most modern insulation types (THHN, THWN-2) and is the column most residential and commercial panel terminations are rated for under NEC 110.14(C).

When more than three current-carrying conductors share a conduit, NEC Table 310.15(C)(1) requires ampacity derating — 80% for 4–6 conductors, 70% for 7–9, and so on. This calculator uses the standard conduit values for simplicity; consult the NEC or a licensed electrician for bundled or high-temperature installations.

Voltage Drop and Wire Sizing

Ampacity tells you whether a wire is safe. Voltage drop tells you whether the wire will actually work well. The NEC recommends (in informational notes to sections 210.19(A)(1) and 215.2(A)(1)) that conductors be sized so voltage drop on any branch circuit does not exceed 3%, and the total drop across the feeder and branch circuit together does not exceed 5%. These are recommendations, not enforceable requirements — but most AHJs, equipment manufacturers, and utility companies cite the same thresholds.

Voltage Drop Formula

For single-phase circuits: V_drop = 2 × L × I × R/1000, where L is the one-way length in feet, I is current in amps, and R is the resistance per 1,000 ft from NEC Chapter 9 Table 9. The factor of 2 accounts for the return conductor. For three-phase: V_drop = √3 × L × I × R/1000. Drop percentage = (V_drop / V_source) × 100. Our voltage drop calculator lets you check drop for any specific wire gauge you already have.

When to Upsize for Voltage Drop

Any run longer than about 75 ft at typical residential loads (15–20A) will require upsizing beyond the ampacity minimum to stay under 3%. Long runs to detached garages, barns, or workshops almost always need wire two or three gauges larger than the ampacity minimum — or a subpanel fed by large feeder conductors. Motors and sensitive electronics are especially intolerant of voltage drop.

Copper vs. Aluminum Wire

Copper has lower resistance per foot and higher ampacity per gauge than aluminum, but aluminum costs significantly less and is lighter — which matters for long feeder runs. Aluminum wire is standard for utility service entrance conductors (the wires from the meter to your main panel) and for large feeders to subpanels. For branch circuits in conduit, copper is the common choice because it is easier to terminate and does not require anti-oxidant compound at connections.

Sizing Aluminum Wire

To match copper ampacity with aluminum, upsize by one or two AWG. For example, 6 AWG copper (55A) is equivalent in ampacity to 4 AWG aluminum (55A). For voltage drop, aluminum's higher resistance means even larger upsizing may be needed on long runs. The NEC does not permit 14 AWG aluminum for branch circuits — minimum aluminum branch circuit wire is 12 AWG (15A ampacity). Always use CO/ALR-rated devices and listed anti-oxidant compound at all aluminum terminations.

Common Circuit Wire Sizes

The following standard circuit/wire pairings apply to most US residential and light commercial installations using copper conductors in conduit (NEC Table 310.16, 75°C):

• 15-amp circuits — 14 AWG copper. Typical use: general-purpose receptacle and lighting circuits.
• 20-amp circuits — 12 AWG copper. Typical use: kitchen small appliance circuits, bathroom outlets, garage outlets. NEC requires 20A circuits in kitchens, bathrooms, and garages.
• 30-amp circuits — 10 AWG copper. Typical use: clothes dryers, water heaters, EV chargers (Level 1 EVSE at 24A continuous).
• 40-amp circuits — 8 AWG copper. Typical use: electric ranges, large water heaters.
• 50-amp circuits — 6 AWG copper. Typical use: electric ranges with larger elements, hot tubs, Level 2 EV charging (40A continuous).

These are minimum wire sizes for short runs. For longer runs, check voltage drop with this calculator or our electrical calculators and upsize as needed. Always consult the appliance nameplate and local code for specific installation requirements.

Electrical Safety Disclaimer

This calculator provides estimates for informational and planning purposes only. It is not a substitute for the judgment of a licensed electrician or electrical engineer. Wire sizing depends on many factors not captured here — ambient temperature, conduit fill, bundled conductors, continuous vs. non-continuous loads, local code amendments, and AHJ requirements. Always have electrical work designed, installed, and inspected by a qualified professional. The National Electrical Code is a model code; your jurisdiction may adopt amendments that differ from the values used here. Never perform electrical work without first verifying requirements with your local Authority Having Jurisdiction (AHJ).