What Size is 100 Amp Wire? AWG Sizing, NEC Rules, and Feeder Examples

For a standard 100-amp circuit using modern 75°C-rated breakers and THHN wire in conduit, you need 3 AWG copper or 1 AWG aluminum. Wire size, measured in American Wire Gauge (AWG), defines the physical cross-sectional area of a conductor, which directly dictates its ampacity—the maximum continuous current it can carry without exceeding its insulation's thermal limits. Choosing the correct 100-amp wire size changes the circuit's thermal safety margin, its voltage drop over distance, and whether the conductor will physically fit into the breaker's mechanical lugs.

However, simply memorizing '3 AWG' is dangerous if you ignore the installation environment. The National Electrical Code (NEC) requires you to size wire based on the lowest temperature rating of any connected component, not just the wire's insulation. Below is the exact breakdown of how to size 100-amp wire, the math behind voltage drop, and the code traps that fail inspections.

The NEC Ampacity Columns: Why 3 AWG Copper is the Standard

When you look at a spool of Southwire SIMpull THHN, the insulation is rated for 90°C. A common and dangerous mistake is using the 90°C column in NEC Table 310.16 to size your wire. Under NEC 110.14(C), you must size your conductors based on the temperature rating of the terminations (the breaker lugs and busbars), which are almost universally rated for 75°C in modern residential and commercial equipment.

If you are using older equipment limited to 60°C, or running specific cable assemblies that mandate the 60°C column, you must step up to 1 AWG copper or 1/0 AWG aluminum.

Worked Numeric Example: Sizing a 100A Subpanel Feeder

Let's apply this to a real-world installation. You are running a 100-amp feeder from a main service panel to a detached garage subpanel. The one-way distance is 80 feet, and the load is 240V split-phase. You plan to pull individual THHN conductors through 1.5-inch PVC conduit.

Step 1: Base Conductor Sizing
Using the 75°C column, we select 3 AWG Copper for the two hot legs and the neutral (if a 4-wire feeder is required, which it is for a detached building per NEC 250.32). For the Equipment Grounding Conductor (EGC), NEC 250.122 dictates 8 AWG copper for a 100A breaker.

Step 2: Voltage Drop Calculation
While the NEC recommends a maximum 3% voltage drop for feeders, it is not strictly enforceable unless specified by local AHJ. Still, good practice demands we check it. The resistance of 3 AWG copper at 75°C is approximately 0.245 ohms per 1,000 feet.

• Formula: V_drop = (2 × Length × Current × Resistance_per_ft)
• Math: 2 × 80 ft × 100A × (0.245 / 1000) = 3.92 Volts
• Percentage: (3.92V / 240V) × 100 = 1.63%

A 1.63% drop is excellent and well under the 3% guideline. If you opted for 1 AWG Aluminum to save on material costs (resistance ~0.403 ohms/kft), the drop would be 6.44V (2.68%), which is also acceptable but leaves less margin for future load expansion.

Where You Meet 100-Amp Wire in Practice

You will rarely see a 100-amp circuit powering a single standard outlet. This wire size is reserved for heavy-duty infrastructure and high-draw distribution:

• Subpanel Feeders: The most common use case. Supplying power to a detached garage, workshop, or barn where a 100A subpanel provides enough capacity for lighting, receptacles, and a mid-sized welder or air compressor.
• EV Charging Infrastructure: While a single Level 2 EV charger usually requires a 40A to 60A circuit (using 6 AWG or 4 AWG), installers often pull 100-amp wire to a junction box or subpanel to future-proof for dual-charger setups.
• Tiny Homes and Cabins: Modern suburban homes require 200A or 400A services, but off-grid cabins, ADUs (Accessory Dwelling Units), and tiny homes frequently use a 100A main service entrance, requiring 1/0 AWG aluminum SER (Service Entrance Rated) cable.
• Commercial HVAC: Large rooftop package units or commercial walk-in freezer compressors often require 100A fused disconnects, fed by 3 AWG copper in EMT conduit.

Common Sizing Mistakes and Code Traps

When working with 100-amp conductors, the physical installation is just as critical as the math. Here is what people commonly confuse or get wrong:

1. Ignoring Aluminum Torque Specifications
If you choose 1 AWG aluminum to save money, you must use an anti-oxidant compound (like Noalox) on the stripped conductor and torque the breaker lugs to the manufacturer's exact specification using a calibrated torque screwdriver. Aluminum expands and contracts more than copper under thermal cycling; under-torqued lugs will loosen, arc, and cause a fire.

2. The Neutral Sizing Misconception
For a 100A subpanel feeder, some DIYers assume the neutral can be smaller than the hot legs. For a standard 120/240V single-phase feeder, the neutral must be the same size as the ungrounded (hot) conductors (3 AWG Cu or 1 AWG Al) because it must be capable of carrying the maximum unbalanced current, and it also serves as the bonded neutral at the main disconnect.

3. Lug Fitment Issues
While 3 AWG copper fits perfectly into standard 100A breaker lugs, if you decide to oversize to 1 AWG copper for a long run to mitigate voltage drop, the wire may be too thick to bend into the panel or fit into the lug barrel. Always check the breaker datasheet for 'Maximum Wire Size' before purchasing oversized conductors.