Wire Size for 100A Service: Copper vs Aluminum & NEC Rules

The correct wire size for 100a service is #3 AWG copper or #1 AWG aluminum when using individual conductors in conduit, governed strictly by the 75°C ampacity column of the National Electrical Code (NEC). To define it in one sentence: the wire size for a 100-amp service is the minimum conductor cross-section that can safely carry 100 amps continuously without exceeding the temperature rating of the panel's termination lugs. Getting this right dictates your physical conduit diameter, whether the wires physically fit into the main breaker lugs, your voltage drop over distance, and the overall fire safety of your installation.

The Core Theory: Ampacity vs. Termination Temperature

The most critical concept in feeder sizing is understanding the difference between wire insulation rating and termination rating. Modern THHN/THWN-2 wire is manufactured with insulation rated for 90°C. However, under NEC Article 110.14(C), the ampacity of your wire is limited by the lowest temperature rating of any connected component in the circuit.

Almost all standard 100-amp main breakers, subpanel lugs, and meter sockets are only tested and rated for 75°C terminations. Think of the wire as a high-capacity water pipe and the panel lug as a narrower brass fitting; the system's safe capacity is bottlenecked by the lowest-rated fitting. Because of this rule, you must look at the 75°C column in NEC Table 310.16, not the 90°C column.

Where You Meet This in Practice

You will typically encounter the 100A wire sizing requirement in three specific jobsite scenarios:

• Main Service Entrance: The run from the utility meter base to your main 100A interior or exterior load center. This is usually handled by the utility or a licensed electrician using specialized Service Entrance (SE) cable.
• Detached Subpanels: Running a 100A feeder from your main 200A panel to a detached garage, workshop, or barn. This is the most common DIY and contractor scenario, usually requiring individual wires pulled through PVC conduit.
• Heavy Equipment Branch Circuits: Feeding a large 100A welder receptacle, a massive HVAC condenser, or a commercial-grade electric vehicle (EV) fast charger.

The SE Cable Exception: If you are using Service Entrance cable (specifically SE-R or SE-U style) rather than individual THHN wires in conduit, you will frequently see 2-2-2-4 Aluminum SE cable used for 100A services. While #2 AWG aluminum is only rated 90A in the standard 75°C table, UL specifically lists 2-2-2-4 SE cable for 100A residential service entrance use due to the cable's specific assembly and heat dissipation properties. However, if you are pulling individual aluminum wires in a pipe, you must step up to #1 AWG.

Decision Tree: Picking Your Exact 100A Conductor

Use this decision path to select the exact wire you need to buy at the supply house. Do not deviate from these picks unless a specific voltage drop calculation (shown below) demands an upsize.

Worked Example: Voltage Drop on a 150-Foot Garage Run

Ampacity tables assume a standard distance. When your run gets long, resistance increases, and voltage drops. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders to ensure equipment operates efficiently.

Let's calculate the voltage drop for a 100A subpanel located 150 feet from the main panel, using our default pick of #1 AWG Aluminum.

1. Identify the Variables:
   • Distance (D) = 150 feet (one way)
   • Material Constant (K) for Aluminum = 21.2 (Ohms-cmil/ft at 75°C)
   • Circular Mils (CM) for #1 AWG = 83,690 cmil
   • Current (I) = While the breaker is 100A, a residential subpanel rarely pulls a continuous 100A. We will calculate at a realistic heavy load of 80 Amps.
2. Apply the Single-Phase Voltage Drop Formula:
   VD = (2 × K × I × D) / CM
3. Run the Math:
   VD = (2 × 21.2 × 80 × 150) / 83,690
VD = 508,800 / 83,690 = 6.08 Volts
4. Calculate Percentage:
   (6.08V / 240V) × 100 = 2.53%

A 2.53% drop is well under the 3% threshold. You can verify these figures using the Southwire Voltage Drop Calculator. If your run was 250 feet, the drop would exceed 4%, and you would need to upsize to #1/0 AWG Aluminum to compensate.

Common Confusions and Installation Traps

Even when you buy the right wire size for 100a service, poor installation practices can cause the connection to fail. Watch out for these common errors:

1. Skipping Anti-Oxidant Paste on Aluminum

Aluminum oxidizes rapidly when exposed to air, creating a highly resistive layer that generates intense heat at the lug. If you are using #1 AWG Aluminum THHN, you must strip the insulation, brush the bare conductor with a wire brush, and apply a UL-listed anti-oxidant compound (like Noalox or Penetrox) before inserting it into the lug. Never skip this step.

2. Ignoring Torque Specifications

The days of tightening lugs "until they feel tight" are over. NEC 110.14(D) requires terminations to be tightened to the manufacturer's specified torque. A 100A breaker lug typically requires between 25 and 40 lb-in (pound-inches) of torque, depending on the brand (e.g., Eaton, Square D, Siemens). Use a calibrated torque screwdriver or torque wrench. Under-torquing causes arcing and heat; over-torquing strips the threads or snaps the aluminum conductor strands.

3. Confusing the Neutral and Ground in Subpanels

If your 100A service is feeding a subpanel (not the main service disconnect), the neutral and ground buses must be physically separated. Your #1 AWG Aluminum neutral wire goes to the isolated neutral bar, and your #6 AWG copper ground wire goes to the bonded grounding bar. If you bond the neutral to the ground in a subpanel, you create a parallel neutral path, which can electrify the grounding system and trip upstream GFCI/AFCI breakers.