For a standard 100 amp sub panel wire size, you need 3 AWG copper or 1 AWG aluminum conductors protected by a 100-amp double-pole breaker. This baseline assumes 75°C rated equipment terminals, a 30°C (86°F) ambient temperature, and standard installation conditions.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2/THHN)
- Temperature Column: 75°C (per NEC 110.14(C) terminal limitations)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit: Standard EMT, PVC Schedule 80, or direct burial
- Current-Carrying Conductors: 3 or fewer in a single raceway (2 hots + 1 neutral)
The Core Sizing Table: Copper vs. Aluminum for 100A
When provisioning a subpanel, you are not just matching the breaker; you are matching the continuous and non-continuous load capacity of the panel's main lugs. The table below extracts the critical rows from NEC Table 310.16 to show exactly why specific gauges are mandated, separating the 75°C terminal limit from the 90°C wire insulation rating.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity (Terminal Limit) | 90°C Ampacity (Derating Base) | Max Standard Breaker |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | 90A |
| 3 AWG | Copper | 100A | 115A | 100A |
| 2 AWG | Aluminum | 90A | 100A | 90A |
| 1 AWG | Aluminum | 100A | 115A | 100A |
| 1/0 AWG | Aluminum | 120A | 135A | 125A |
Note: The equipment grounding conductor (EGC) for a 100A feeder does not need to match the hot wires. Per NEC 250.122, an 8 AWG copper or 6 AWG aluminum ground wire is sufficient. Running 3 AWG for the ground is a waste of copper and conduit space.
Why 3 AWG Copper (and Not 4 AWG)?
A common point of confusion on the jobsite is why we cannot use 4 AWG copper THHN for a 100A breaker. After all, 4 AWG THHN has a 90°C ampacity of 95A, and NEC 240.4(B) allows you to round up to the next standard breaker size (which would be 100A).
The restriction comes from NEC 110.14(C) - Temperature Limitations. The wire insulation might be rated for 90°C, but the metal lugs inside standard 100A breakers and subpanel main disconnects are almost universally tested and rated for a maximum of 75°C. If you push 95A through a 75°C rated lug, the terminal will overheat, potentially melting the breaker housing or causing a high-resistance fault. Therefore, you must size the wire using the 75°C column of Table 310.16. In the 75°C column, 4 AWG copper is only good for 85A. To hit a true 100A terminal rating, you must step up to 3 AWG copper (rated exactly 100A at 75°C).
If you choose 1 AWG aluminum to save money (aluminum is roughly 40% cheaper than copper by volume), you must apply an anti-oxidant compound like Noalox to the stripped wire ends before torquing the lugs. Furthermore, verify that your breaker and subpanel lugs are explicitly marked "AL/CU" or "CO/ALR". Terminating bare aluminum into a copper-only lug will result in galvanic corrosion, increased resistance, and eventual thermal failure.
Variables That Force You to Upsize
The 3 AWG Copper / 1 AWG Aluminum rule is your baseline. However, real-world physics and NEC derating requirements frequently force you to pull larger wire. Here are the three variables that will change your answer.
1. Voltage Drop Over Distance
The NEC recommends (via Informational Note to 310.15(B)) a maximum voltage drop of 3% for branch circuits and feeders to ensure equipment operates efficiently. Let us run the math for a 100A load at 240V over a 150-foot run using 3 AWG copper.
- Formula: VD = (2 × K × I × D) / CM
- K (Copper constant): 12.9
- I (Current): 100A
- D (Distance): 150 ft
- CM (Circular Mils for 3 AWG): 52,620
VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts.
Percentage Drop = (7.35 / 240) × 100 = 3.06%.
At 150 feet, 3 AWG copper slightly exceeds the 3% recommended threshold. If your subpanel is located 150 feet or more from the main panel, you must upsize to 2 AWG copper or 1/0 AWG aluminum to keep the voltage drop under 3% and prevent motors from overheating or lights from flickering under heavy load.
2. Ambient Heat and Conduit Bundling
Table 310.16 assumes an ambient temperature of 30°C (86°F). If your conduit runs through a hot attic in the summer, or if you bundle multiple circuits in the same pipe, you must derate the wire's ampacity.
Suppose your conduit runs through an attic that reaches 110°F (43°C). According to the temperature correction factors at the bottom of Table 310.16, the multiplier for 75°C wire at 41-45°C is 0.82.
- Required Ampacity = 100A / 0.82 = 121.9A.
You must now use the 90°C column to find a wire that can handle 121.9A after derating. 3 AWG THHN (90°C rating is 115A) is no longer sufficient. You must upsize to 2 AWG THHN (90°C rating is 130A; 130A × 0.82 = 106.6A, which satisfies the 100A terminal requirement after correction).
3. Conduit Fill and Pulling Tension
While 3 AWG copper and 1 AWG aluminum are electrically equivalent for 100A, they behave very differently physically. 1 AWG aluminum is thicker but significantly lighter and more flexible than 3 AWG copper. If you are pulling wire through a 1-inch PVC conduit with multiple 90-degree sweeps, 3 AWG copper will fight you every inch of the way. In long, complex conduit runs, electricians often prefer upsizing to aluminum or using a larger conduit (1.25-inch or 1.5-inch EMT) to reduce pulling tension and prevent insulation scoring.
When to Call an Engineer or the AHJ
While the guidelines above cover 95% of residential and light-commercial subpanel installations, specific load profiles and local jurisdictions require professional verification.
| Scenario | Action Required |
| Subpanel powers standard tools, lighting, and a few 120V/240V appliances. | Use 3 AWG Cu / 1 AWG Al. Standard AHJ inspection applies. |
| Subpanel powers continuous loads (EV chargers, server racks, grow lights) expected to run at max capacity for 3+ hours. | Multiply continuous load by 1.25. If continuous load is 80A, required feeder capacity is 100A. If total panel load exceeds this, upsize to 1 AWG Cu / 2/0 Al and consult an engineer. |
| Installation is in a jurisdiction with strict local amendments (e.g., Chicago requires all wiring in EMT conduit; some areas mandate AFCI protection on subpanel feeders). | Consult the local Authority Having Jurisdiction (AHJ) or a licensed master electrician before purchasing materials. |
Always remember that the National Electrical Code (NEC) serves as the baseline standard, but your local AHJ has the final authority. If your main service panel is already operating near its maximum capacity (e.g., a 200A main panel with 180A of calculated existing load), adding a 100A subpanel will trigger a requirement for a full service upgrade or a dedicated line-side tap, which strictly requires a licensed electrician and utility coordination.
By anchoring your 100 amp sub panel wire size to the 75°C terminal column, verifying voltage drop at your specific distance, and applying ambient derating factors where necessary, you ensure a safe, code-compliant installation that will not require a costly re-pull when the inspector arrives.
