Wire Gauge Ampacity Calculator — Free Online Calculator & Guide

How Wire Gauge Works

Required Ampacity = Load Current ÷ Derating Factor | Select next AWG at or above this ampacity

Wire gauge selection uses NEC Table 310.15 ampacity ratings, adjusted for temperature rating and conduit fill derating factors. The required ampacity must equal or exceed the load current divided by the derating factor.

AWG

American Wire Gauge — Smaller AWG number = thicker wire = higher ampacity. 14 AWG is the smallest common household wire. Each gauge step (e.g., 14 to 12) approximately doubles the cross-sectional area.

Derating

Conduit Fill Derating — Multiple conductors in conduit generate mutual heat, requiring derating: 80% for 4-6 conductors, 70% for 7-9, 50% for 10+. This means a 20A circuit in a conduit with 4 conductors must use wire rated for 25A (20 ÷ 0.8).

Ampacity

Current-Carrying Capacity — The maximum continuous current a wire can safely carry without exceeding its insulation temperature rating. Depends on gauge, material, and insulation type.

How to Use

1. Enter the circuit current in amperes.
2. Select copper or aluminum conductor.
3. Select the insulation temperature rating.
4. Select how many conductors share the conduit.
5. The calculator recommends the correct AWG gauge per NEC tables.
6. Always consult a licensed electrician for actual installations — local codes may differ.

Quick Reference

Common Uses

• •Home electrical wiring — select the correct wire gauge for new circuits, whether running lighting in 14 AWG or installing a 50A EV charger circuit in 6 AWG copper
• •Industrial panel building — size power and control wiring for motor control centers, PLC cabinets, and distribution panels per NEC code requirements
• •Solar panel installation — calculate wire gauge between panels, inverter, and battery bank to minimize voltage drop over long DC runs at high current
• •Automotive and marine wiring — choose wire sizes for 12V and 24V systems where voltage drop is more critical due to the lower voltage baseline
• •Audio system installation — select appropriate speaker wire gauge based on run length and impedance to minimize signal loss and maintain audio quality

Understanding the Result

Selecting the correct wire gauge is critical for electrical safety. Undersized wire overheats, degrades insulation, and can cause fires. The NEC (National Electrical Code) publishes ampacity tables based on wire gauge, material, and insulation type. When multiple conductors share a conduit, they generate mutual heat and must be derated. Always consult a licensed electrician for actual installations. Real-world scenario: you are running a 20A kitchen circuit with 4 conductors in a conduit (two hots sharing a neutral, plus ground). At 80% derating, the required ampacity is 20 ÷ 0.8 equals 25A. With 75°C rated copper wire, 12 AWG is rated for 25A — just barely sufficient. If the ambient temperature is high (for example an attic), additional derating may push the requirement to 10 AWG. This is why professional electricians always verify ampacity for the specific installation conditions. Another common scenario: running a 50A EV charger circuit. At 75°C copper, 6 AWG is rated for 65A and would work. But if the run is in conduit with 4 other circuits (requiring 80% derating), the effective capacity of 6 AWG drops to 52A — still sufficient for 50A. If there are 7 conductors total, derating to 70% drops capacity to 45.5A, requiring an upgrade to 4 AWG.

Worked Examples

Frequently Asked Questions

Why is aluminum wiring a concern?

Aluminum wire expands and contracts more than copper with temperature changes, which can loosen connections over time. It requires special connectors, outlets, and switches rated for aluminum (AL-Cu rated) to prevent fire hazards. Aluminum also has about 40% higher resistance than copper for the same gauge, requiring larger wire sizes.

What AWG is used for common household circuits?

14 AWG for 15A circuits (lighting, general outlets), 12 AWG for 20A circuits (kitchen, bathroom, garage), 10 AWG for 30A circuits (dryers, water heaters, AC units), 8 AWG for 40A circuits (ranges, EV chargers), 6 AWG for 55A circuits (large appliances, subpanels).

What happens if I use wire that is too small for the circuit?

Undersized wire overheats under load, which degrades the insulation over time and can eventually cause a short circuit or electrical fire. The circuit breaker is sized to protect the wire — never put a 20A breaker on 14 AWG wire (rated for 15A), as the wire would overheat before the breaker trips.

What is voltage drop and when should I worry about it?

Voltage drop is the reduction in voltage along a wire due to its resistance. Long runs of wire can cause significant voltage drop, which makes motors run hot, lights dim, and equipment malfunction. The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total for the feeder plus branch circuit. For a 120V, 20A circuit at 100 feet with 12 AWG copper, voltage drop is approximately 3.6%, which exceeds the 3% recommendation — you would need to upgrade to 10 AWG for that run length.

What does "derating" mean and why is it necessary?

Derating means reducing the ampacity of wire because of unfavorable installation conditions. The most common derating factor is conduit fill: when multiple current-carrying conductors share a conduit, they generate mutual heat that cannot dissipate as effectively. NEC Table 310.15(B)(3)(a) requires derating to 80% for 4-6 conductors, 70% for 7-9, and 50% for 10+. Ambient temperature above 86°F (30°C) also requires derating. This is why a 20A circuit might need 10 AWG wire if it runs through a hot attic with multiple other circuits in the same conduit.

Can I mix copper and aluminum wire in the same circuit?

Copper and aluminum should never be directly connected without an approved AL-CU rated connector or anti-oxidation compound. Direct contact causes galvanic corrosion — the two dissimilar metals react electrochemically in the presence of moisture, causing the connection to degrade and overheat over time. When transitioning from aluminum to copper (common in older homes with aluminum branch circuits), use a special connector approved for both metals. This is one of the most common fire hazards found in homes built between 1965 and 1973.

Pro Tips

• →Always treat continuous loads (running 3+ hours) at 125% of the nameplate rating. A 40A EV charger on a 50A breaker is actually a 50A continuous load per NEC 625 — size wire for 62.5A, not 50A. This is the most common mistake in residential EV charger installations.
• →Voltage drop becomes the governing factor — not ampacity — on runs longer than 100 feet. For 120V circuits, a 100-foot run with 12 AWG at 20A loses about 3.6V (3%), which is right at the NEC recommendation. For 240V circuits at the same length, the voltage drop percentage is half (since the voltage is double), giving you more headroom. Always check voltage drop separately for long runs — this calculator handles ampacity only.
• →The ground wire does NOT count toward the conductor count for derating purposes per NEC 310.15(B)(6). If you have 2 hots, 1 neutral, and 1 ground in a conduit, that counts as 3 conductors for derating (ground excluded), keeping you in the 80% derating tier rather than the 70% tier. The neutral only counts if it carries current under normal conditions (not just for fault conditions).
• →Aluminum wire requires anti-oxidation compound on ALL connections, even those rated AL-CU. The oxide layer on aluminum is non-conductive and forms within minutes of exposure to air. Wire-brush the stripped conductor, apply anti-oxidant immediately, and torque connections to manufacturer specification. Re-torque after thermal cycling if accessible. Most aluminum wiring failures trace back to improper connection technique, not the wire itself.
• →When in doubt between two gauges, choose the larger wire (smaller AWG number). The marginal cost of upsizing one gauge step is usually minimal for residential runs under 100 feet, but the safety margin is substantial. The cost difference between 14 AWG and 12 AWG Romex is roughly $0.15/foot — on a 50-foot circuit, that is $7.50 for significantly more safety margin and future-proofing.

Limitations to Know

• •This calculator uses NEC Table 310.15 ampacity values at 30C (86F) ambient with standard derating factors for conduit fill. Ambient temperature correction factors above 30C (attics, engine compartments, direct sunlight exposure) are not applied — a separate temperature correction may be required.
• •Voltage drop over distance is NOT calculated. For runs over 100 feet, voltage drop may govern wire size rather than ampacity. A separate voltage drop calculation is required for long wire runs.
• •Continuous vs. non-continuous load classification per NEC Article 100 is not handled. Continuous loads (3+ hours) require 125% sizing — the user must apply this multiplier to the input current before using the calculator.
• •Special occupancies with modified ampacity rules (healthcare facilities per NEC 517, hazardous locations per NEC 500-516, theaters, agricultural buildings, marinas) are not covered. These environments have stricter wiring requirements.
• •Wire gauge selection is a life-safety decision. Always consult a licensed electrician and your local Authority Having Jurisdiction (AHJ) for actual installations. Local code amendments may be more restrictive than the NEC.

Sources

• American Wire Gauge
• Wire Gauge