For a 100-amp subpanel feeder, use 3 AWG copper or 1 AWG aluminum wire with a 100A breaker. For a main dwelling service entrance, NEC 310.12 permits 4 AWG copper or 2 AWG aluminum. This assumes 75°C rated terminals, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.
In NEC terms, a "service" is the main utility drop to your house. A "feeder" supplies a subpanel (like in a detached garage). DIYers often use the word "service" for both. Sizing rules differ drastically between the two. Never use main service rules for a subpanel feeder.
- Material: Copper (THHN/THWN-2) and Aluminum (XHHW-2) evaluated separately.
- Temperature Column: 75°C column (per NEC 110.14(C) termination limits).
- Ambient Temperature: 30°C (86°F) baseline.
- Conduit: PVC or EMT, with a maximum of 3 current-carrying conductors.
The Ampacity Baseline: Why 3 AWG Copper and 1 AWG Aluminum?
When determining what size wire for a 100 amp service feeder, you must look at NEC Table 310.16. The most common mistake beginners make is looking at the 90°C column because THHN wire is rated for 90°C. However, the breakers and panel lugs you are terminating into are almost universally rated for a maximum of 75°C. Therefore, the 75°C column dictates your maximum allowable ampacity.
Why not use 4 AWG copper? In the 75°C column, 4 AWG copper is only rated for 85 amps. Under NEC 240.4, you cannot protect an 85A conductor with a 100A breaker. You must step up to the next standard size that meets or exceeds 100A, which is 3 AWG (rated exactly 100A at 75°C). If you choose aluminum to save money on long runs, 1 AWG aluminum hits exactly 100A in the 75°C column.
| Wire Size (AWG/kcmil) | Material | 60°C Ampacity | 75°C Ampacity (Termination Limit) | 90°C Ampacity (THHN Insulation) |
|---|---|---|---|---|
| 4 AWG | Copper | 70A | 85A | 95A |
| 3 AWG | Copper | 85A | 100A | 115A |
| 2 AWG | Copper | 95A | 115A | 130A |
| 2 AWG | Aluminum | 70A | 90A | 100A |
| 1 AWG | Aluminum | 80A | 100A | 115A |
| 1/0 AWG | Aluminum | 100A | 120A | 135A |
Note: Always apply anti-oxidant compound (like Noalox) to aluminum wire terminations to prevent galvanic corrosion and high-resistance heating at the lugs.
Voltage Drop: When Length Forces a Larger Wire
Ampacity tables only tell you what size wire will prevent the insulation from melting. They do not account for voltage drop over distance. The NEC recommends (via Informational Notes in articles like 210.19 and 215.2) a maximum 3% voltage drop for branch circuits and feeders to ensure equipment operates efficiently.
Let us run the exact math for a 100A load on a 240V circuit using 3 AWG copper. To be highly precise, we use the AC resistance value from NEC Chapter 9, Table 9 (for uncoated copper in PVC conduit), which is 0.25 ohms per 1,000 feet, rather than the DC resistance from Table 8.
- Maximum Allowable Drop: 3% of 240V = 7.2 Volts
- Formula: Voltage Drop = (2 × Length × Current × AC Resistance) / 1000
- Calculation: 7.2 = (2 × L × 100 × 0.25) / 1000
- Solving for L: 7.2 = 50L / 1000 → 7200 = 50L → L = 144 feet
If your subpanel is 144 feet or less from the main panel, 3 AWG copper is perfectly adequate. If the run is 150 feet, your voltage drop exceeds 3%, and you must upsize to 2 AWG copper to maintain power quality. If you are running a 120V feeder instead of 240V, that maximum distance halves to just 72 feet before you must upsize.
Derating Factors and Continuous Loads
The baseline answer assumes standard residential use. Two major factors will force you to buy thicker wire: continuous loads and ambient temperature derating.
Under NEC Article 100, a continuous load is one where the maximum current is expected to continue for 3 hours or more. Examples include EV chargers, server racks, or heavy commercial lighting. If your 100A load is continuous, NEC 215.2 requires the conductors to be sized at 125% of the load. 100A × 1.25 = 125A. You can no longer use 3 AWG copper (100A); you must jump to 1 AWG copper (130A at 75°C).
| Scenario / Condition | Required Copper Size | Required Aluminum Size | NEC Reference |
|---|---|---|---|
| Standard Subpanel Feeder (<144 ft) | 3 AWG | 1 AWG | 310.16 (75°C Col) |
| Main Dwelling Service Entrance | 4 AWG | 2 AWG | 310.12(A) |
| Continuous Load (EV Charger, 3+ hrs) | 1 AWG | 1/0 AWG | 215.2(A)(1) |
| Run exceeds 144 ft (at 240V, 100A) | 2 AWG | 1/0 AWG | 210.19 Info Note |
| 4 current-carrying conductors in conduit | 1 AWG | 1/0 AWG | 310.15(C)(1) |
Ambient temperature also plays a role. If you are routing your feeder through an unconditioned attic in a southern climate where temperatures routinely exceed 86°F (30°C), you must apply the temperature correction factors from the bottom of Table 310.16. For example, at 113°F (45°C), the 90°C column derating factor is 0.87. While you still terminate based on the 75°C column, the wire's actual thermal capacity in that hot space drops, which may force an upsize depending on your exact local climate data.
When an Engineer or the AHJ Must Confirm
While sizing a feeder to a detached garage is a standard DIY-permitted task in many jurisdictions (provided you pull a permit), working on the actual main service entrance crosses a strict boundary. If you are upgrading your home's main service mast from 60A to 100A, you are dealing with utility-side power.
NEC 310.12 provides specific, often smaller, wire sizes for dwelling service entrances (4 AWG Cu / 2 AWG Al for 100A) because of the diversity factors applied to whole-home loads. However, the utility company has the final say on the service drop wires they run from the pole to your weatherhead. Furthermore, your local Authority Having Jurisdiction (AHJ) or building inspector must verify the grounding electrode system, the bonding of the neutral bar, and the physical clearance of the mast.
Never attempt to swap out a main service meter or work on the line-side of the main breaker without the utility company disconnecting the drop. The fault current available on the line side of the meter can exceed 10,000 amps, which will vaporize standard hand tools and cause fatal arc flashes. For any main service upgrade, hire a licensed electrical contractor who carries the specific insurance and utility approvals required for service entrance work in your municipality.
