100A Aluminum Wire Size: Sizing Guide for Subpanels and Feeders

The 100a aluminum wire size refers to the minimum cross-sectional gauge of an aluminum conductor required to safely carry 100 amps of current without exceeding its thermal insulation limits. For a standard 100-amp circuit, the absolute code minimum aluminum wire size is 1 AWG for non-continuous loads, but 1/0 AWG (pronounced "one-aught") is the standard real-world size used for 100A subpanel feeders based on the 75°C column of NEC Table 310.16.

Selecting the correct gauge dictates your conduit diameter, breaker lug compatibility, voltage drop over long runs, and whether you need to apply anti-oxidant compound at the terminations. DIYers frequently confuse aluminum sizing with copper (which only requires 3 AWG for 100A) or mistakenly look at the 60°C ampacity column, which does not support 100A for standard aluminum building wire.

The Physics and Code Behind 100A Aluminum Sizing

Aluminum has a higher electrical resistance than copper—roughly 1.6 times higher for the same cross-sectional area. To carry the same 100-amp current without generating excess heat, an aluminum wire must be physically thicker. This is why aluminum gauge numbers are always larger (meaning a physically thicker wire) than their copper equivalents for the same ampacity.

The National Electrical Code (NEC) governs wire sizing primarily through Table 310.16, which lists ampacities based on the wire's insulation temperature rating. Here is where most mistakes happen:

• The 60°C Column: Older equipment and certain cable types (like NM-B / Romex) are limited to this column. 1 AWG aluminum is only rated for 83A here, meaning you cannot use it for a 100A circuit.
• The 75°C Column: Almost all modern 100A breakers, panel lugs, and THHN/XHHW/SER cables are rated for 75°C terminations. In this column, 1 AWG aluminum is rated for exactly 100 amps.
• The 90°C Column: While the wire insulation (like XHHW-2) might be rated for 90°C, you can only use this column for derating calculations (like bundling multiple wires in a conduit). The final termination ampacity is still capped at the 75°C rating of the breaker lug.

Worked Numeric Example: Sizing a 100A Subpanel Feeder

Let's look at a real-world scenario: You are running a 100A feeder from your main house panel to a detached garage subpanel located 80 feet away. You are using individual XHHW-2 aluminum conductors in PVC conduit.

Step 1: Base Ampacity Check
We select 1/0 AWG Aluminum. According to the 75°C column, 1/0 AWG is rated for 120A. This safely covers our 100A breaker and provides headroom for minor continuous loads.

Step 2: Voltage Drop Calculation
The NEC recommends keeping feeder voltage drop under 3%. We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM.

• K (Specific resistance for aluminum at operating temp) ≈ 21.2
• I (Current) = 100 Amps
• D (One-way distance) = 80 feet
• CM (Circular Mils for 1/0 AWG) = 105,600

VD = (2 × 21.2 × 100 × 80) / 105,600 = 339,200 / 105,600 = 3.21 Volts

On a 240V system, a 3.21V drop is 1.34%. This is well under the 3% threshold. If we had used the bare-minimum 1 AWG aluminum (83,690 CM), the drop would be 4.05V (1.68%). While still under 3%, the 1/0 AWG provides a safer thermal margin and better performance for heavy motor starts in the garage.

Step 3: Equipment Grounding Conductor (EGC)
For a 100A feeder, NEC Table 250.122 requires a minimum 8 AWG copper or 6 AWG aluminum equipment ground. Do not rely on the conduit alone as a ground for a detached structure; pull a dedicated wire.

Where You Meet This in Practice

You will encounter 100A aluminum sizing primarily in three scenarios: main service entrance upgrades, detached structure subpanels, and high-draw dedicated appliances like Level 2 EV chargers or tankless electric water heaters.

When pulling aluminum, the physical installation differs significantly from copper. Aluminum is softer, more prone to "creep" (expanding and contracting under heat cycles, which loosens terminations), and highly reactive to oxygen. To prevent a melted breaker lug or a fire, follow these jobsite rules:

1. Anti-Oxidant Paste is Mandatory: Aluminum oxidizes within minutes of being stripped, forming a high-resistance surface layer. Brush the stripped strands with a UL-listed anti-oxidant compound (like Noalox or Penetrox) before inserting them into the lug.
2. Use a Torque Wrench: Because aluminum creeps, hand-tightening is a fire hazard. Check the breaker or panel datasheet for the exact torque specification (typically between 40 and 50 in-lbs for 100A lugs) and use a calibrated torque screwdriver or inch-pound wrench. NEC 110.14(D) now strictly requires terminations to be torqued to manufacturer specs.
3. Watch Your Bending Radius: 1/0 AWG aluminum SER cable is stiff. When routing it into a panel, leave enough slack to meet the NEC bending radius requirements (typically 5 times the cable diameter for shielded, but check 312.6 for cabinet wiring space). Forcing a tight bend can crack the insulation or damage the internal neutral strands.