Calculate the correct wire gauge for 100-amp circuits. NEC ampacity tables for copper and aluminum conductors with voltage drop analysis.
Selecting the correct wire size for a 100-amp circuit is one of the most important decisions in residential and commercial electrical work. An undersized conductor can overheat, trip breakers, and create fire hazards, while an oversized wire wastes money and complicates installation.
Per the National Electrical Code (NEC), the minimum wire size for a 100-amp circuit depends on conductor material, insulation temperature rating, and the length of the run. For copper conductors at 75°C, a 3 AWG wire is typically required, while aluminum conductors need a 1 AWG wire. However, long runs may require upsizing to keep voltage drop within the recommended 3% limit.
This calculator uses NEC Table 310.16 ampacity ratings and calculates the actual voltage drop for your specific installation. It recommends the smallest wire that satisfies both ampacity and voltage-drop requirements, helping you choose the right conductor for sub-panels, EV chargers, workshops, and main service feeders. Check the example with realistic values before reporting.
Incorrectly sized wiring for a 100-amp circuit is a leading cause of electrical fires and code violations. Manually looking up NEC tables and calculating voltage drop is error-prone, especially when balancing ampacity, distance, and conductor material.
This calculator instantly cross-references NEC ampacity ratings with voltage-drop calculations to recommend the smallest safe wire gauge. It saves time during planning, prevents costly mistakes, and helps electricians, homeowners, and inspectors verify that wiring meets code requirements.
Voltage Drop = 2 × I × R × D / 1000, where I = current in amps, R = resistance in Ω per 1000 ft, D = one-way distance in feet. Drop % = (Vdrop / V) × 100. The recommended wire is the larger of the ampacity-based and voltage-drop-based requirements.
Result: 3 AWG copper
At 100A, 240V, and 100 ft one-way distance, a 3 AWG copper conductor provides 100A ampacity at 75°C and produces a voltage drop of about 4.9V (2.0%), which is within the 3% limit.
Wire sizing involves two independent requirements: ampacity and voltage drop. Ampacity is the maximum current a conductor can carry without overheating, determined by NEC Table 310.16 based on conductor material, insulation type, and ambient temperature. Voltage drop is the loss of voltage along the length of the conductor due to its inherent resistance.
For a 100-amp circuit, ampacity alone may dictate a 3 AWG copper or 1 AWG aluminum wire. However, if the run is long — say 150 feet or more — the voltage drop may exceed the recommended 3%, requiring you to upsize the conductor. This calculator identifies which constraint is limiting and recommends the appropriate gauge.
Copper has about 61% of the resistance of aluminum, making it a better conductor per unit area. However, aluminum is significantly lighter and cheaper, making it popular for long feeder runs and service entrance cables. When using aluminum, you typically need a wire one or two sizes larger than copper for the same ampacity. Proper installation techniques — including anti-oxidant compound and AL-CU rated connectors — are essential to prevent oxidation and loose connections.
Beyond ampacity and voltage drop, consider conduit fill, bending radius, and ambient temperature. NEC requires derating when more than three current-carrying conductors share a conduit, which can push you to a larger wire size. Ambient temperatures above 86°F (30°C) also require derating. For underground installations, use UF or USE-rated cable and account for the thermal resistance of the soil.
For copper at 75°C, use 3 AWG minimum. For aluminum, use 1 AWG minimum per NEC Table 310.16. Long runs may require larger wire to limit voltage drop.
Yes, 1 AWG aluminum is rated for 100A at 75°C. Aluminum is cheaper but requires anti-oxidant compound and proper connectors rated for aluminum.
At 200 ft, voltage drop becomes significant. You may need 1 AWG or 1/0 copper or 2/0 aluminum to stay within the 3% voltage drop recommendation.
Longer runs have more resistance, causing greater voltage drop. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total for feeders plus branch circuits.
These are insulation temperature ratings. Higher ratings allow more current, but most connections are rated for 75°C, so the 75°C column is typically used even with 90°C wire.
Yes, when multiple conductors share a conduit, the NEC requires derating (reducing) ampacity. Three current-carrying conductors in a conduit typically require an 80% derating factor.