A 100A wire size refers to the specific American Wire Gauge (AWG) or circular mil (kcmil) cross-section required to safely carry 100 amps of current without exceeding the thermal limits of the conductor's insulation. Unlike a simple on/off switch, wire sizing is a multi-variable equation: the correct gauge changes based on whether you are pulling copper or aluminum, the temperature rating of your terminations, and the physical length of the run. Getting this wrong doesn't just trip breakers; it slowly degrades insulation, increases voltage drop, and creates hidden thermal hazards inside your walls.
The Core Ampacity Table for 100A Wire Size
Before you buy wire, you must know which column of the National Electrical Code (NEC) Table 310.16 applies to your installation. Most modern breakers and panel lugs are rated for 75°C, while older equipment or specific cable types (like NM-B Romex) are restricted to the 60°C column. The 90°C column is strictly for derating calculations, not for final sizing.
| Conductor Material | 60°C Column (NM-B / Older Terminals) | 75°C Column (THHN / Modern Panels) | 90°C Column (Derating Calculations Only) |
|---|---|---|---|
| Copper | 1 AWG (110A) | 3 AWG (100A) | 3 AWG (115A) |
| Aluminum | 1/0 AWG (100A) | 1 AWG (100A) | 1 AWG (115A) |
Worked Example: Sizing a 100A Subpanel Feeder
Ampacity tables assume a standard ambient temperature and a short run. In the real world, distance introduces voltage drop, and bundling wires introduces heat. Let's calculate a real-world scenario to see what 100A wire size actually changes in a physical installation.
The Scenario: You are running a 240V single-phase feeder to a detached garage subpanel protected by a 100A breaker. The one-way distance is 150 feet. You plan to use copper THHN in PVC conduit. Your calculated continuous load on the subpanel is 80A.
Step 1: Base Ampacity Sizing
Using the 75°C column, 3 AWG copper is rated for exactly 100A. This satisfies the breaker sizing requirement.
Step 2: Voltage Drop Calculation
The NEC recommends a maximum 3% voltage drop for feeders. We use the formula: VD = (2 × K × I × D) / CM.
- K (Copper constant) = 12.9
- I (Actual load current) = 80A
- D (Distance) = 150 feet
- CM (Circular mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88 Volts.
Percentage: (5.88 / 240) × 100 = 2.45%.
What if the run was 200 feet?
At 200 feet, the drop on 3 AWG jumps to 7.84V (3.26%), which exceeds the 3% recommendation. You would need to upsize to 2 AWG copper (CM = 66,360), which drops the voltage loss to 6.21V (2.58%). This demonstrates that '100A wire size' is not a static number; it scales with distance to maintain power quality.
Where You Meet This in Practice
You will typically encounter the need for 100A wire sizing in three specific residential and light-commercial scenarios:
- Detached Garage Subpanels: A 100A feeder is the modern standard for a detached garage that will house a workbench, lighting, and standard receptacles. Aluminum 1-0 AWG SER cable is the most cost-effective choice here for short runs under 100 feet.
- Hardwired EV Level 2 Chargers: Many high-speed residential EV chargers draw 80A continuously. Per NEC 210.19(A)(1), continuous loads must be multiplied by 125%. An 80A continuous load requires a 100A breaker and wire sized for 100A. Because EV chargers run for hours, thermal management in the wire is critical.
- Large Spa and Hot Tub Panels: Spas with multiple high-wattage heaters and circulation pumps frequently require a dedicated 100A GFCI-protected subpanel. Because these are often located outdoors, you must account for wet location derating and ensure your conduit fill does not exceed 40%.
Common Confusions: Breaker Sizing vs. Wire Ampacity
The most frequent mistake DIYers and junior apprentices make is confusing the breaker's trip rating with the wire's ampacity, or misunderstanding the NEC's 'next size up' rule.
Confusion 1: The 90°C Trap
Many look at Table 310.16, see that 4 AWG copper is rated for 95A in the 90°C column, and assume they can use it on a 100A breaker because '95 is close to 100'. This is a severe code violation. Your breaker lugs and panel bus bars are almost certainly rated for 75°C. The 90°C column is only used as a starting point before applying derating factors (like high ambient temperatures in an attic). The final derated ampacity cannot exceed the 75°C column value. For 100A at 75°C, you must use 3 AWG copper.
Confusion 2: The 'Next Size Up' Rule (NEC 240.4(B))
The NEC allows you to round up to the next standard breaker size if your wire ampacity doesn't match a standard breaker, provided the wire is under 800A. For example, if your calculated load requires a wire rated for 88A, you can use 2 AWG copper (rated 95A at 75°C) and protect it with the next standard breaker size up, which is 100A. However, if your actual calculated load is exactly 100A, you cannot use 2 AWG wire; the wire must be rated for the full load, meaning you must step up to 3 AWG. (Copper Development Association provides excellent visual guides on these termination limits).
Frequently Asked Questions
Can I use 2 AWG aluminum for a 100A breaker?
No. In the 75°C column, 2 AWG aluminum is only rated for 90A. While the 'next size up' rule allows a 90A wire to be protected by a 100A breaker, the wire itself will overheat if you actually pull 100A of continuous current through it. You must use 1 AWG aluminum for a true 100A load.
Does the ground wire need to be the same size as the 100A conductors?
No. Per NEC Table 250.122, the equipment grounding conductor (EGC) for a 100A breaker only needs to be 8 AWG copper or 6 AWG aluminum. However, if you upsized your current-carrying conductors for voltage drop (e.g., using 1 AWG copper instead of 3 AWG), you must proportionally increase the ground wire size as well.
Is it better to use copper or aluminum for a 100A feeder?
For runs under 50 feet, 3 AWG copper is easier to bend and terminate. For runs over 50 feet, 1 AWG aluminum (like SER cable) is significantly cheaper and lighter, making it the preferred choice for long subpanel feeders, provided you use anti-oxidant paste and torque the lugs to manufacturer specifications.
