For a standard 100-amp residential subpanel or service, you need #2 AWG copper wire or #1/0 AWG aluminum wire, protected by a 100-amp double-pole breaker. This assumes THHN/THWN-2 insulation in conduit, a 75°C temperature rating, and an ambient temperature of 30°C (86°F).
- Material: Copper (primary) and Aluminum (alternative)
- Insulation: THHN/THWN-2 (rated 90°C, but terminated at 75°C)
- Temperature Column: 75°C (per NEC 110.14(C) for 100A+ equipment)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Up to 3 current-carrying conductors in EMT or PVC conduit
The 100 Amp Wire Sizing Table (NEC 310.16)
Sizing a 100-amp feeder requires looking at the NEC Table 310.16 ampacity charts. The most common mistake DIYers make is reading the 90°C column because THHN wire is rated for 90°C. However, the lugs on your breaker and panel are almost universally rated for 75°C. Under NEC 110.14(C) temperature limitations, you must use the 75°C column to determine your final wire size.
| Conductor Material | AWG Size | 60°C Ampacity | 75°C Ampacity (Use This) | 90°C Ampacity (Derating Only) | 100A Feeder Verdict |
|---|---|---|---|---|---|
| Copper | #4 AWG | 70A | 85A | 95A | Too Small |
| Copper | #3 AWG | 85A | 100A | 115A | Minimum / Short Runs |
| Copper | #2 AWG | 95A | 115A | 130A | Standard / Best Practice |
| Aluminum | #2 AWG | 75A | 90A | 100A | Too Small |
| Aluminum | #1 AWG | 85A | 100A | 115A | Minimum / Short Runs |
| Aluminum | #1/0 AWG | 100A | 120A | 135A | Standard / Best Practice |
Why #2 AWG Copper and Not #3 or #4?
If you look closely at the table above, you will see that #3 AWG Copper is rated for exactly 100 amps at 75°C. So why do professional electricians almost universally pull #2 AWG for a 100-amp feeder instead of #3? There are three practical and code-driven reasons:
- The 'Next Size Up' Rule Trap: Many DIYers look at #4 AWG Copper (95A in the 90°C column) and assume they can use NEC 240.4(B) to protect it with a 100A breaker. This is illegal here. The next-size-up rule only applies if the calculated load is less than the wire's ampacity, but the breaker is the next standard size up. Because your termination is limited to the 75°C column (85A for #4 AWG), you cannot use a 100A breaker on #4 wire under any circumstance.
- Voltage Drop Buffer: The National Electrical Code (NFPA 70) recommends keeping voltage drop under 3% for feeders. #3 AWG copper has a resistance of roughly 0.245 ohms per 1,000 feet. If your subpanel is 100 feet away (200 feet of total wire round-trip), a 100A load will cause a 4.9V drop (about 2% on a 240V system). While technically acceptable, it leaves zero room for startup surges from well pumps or HVAC compressors. Upsizing to #2 AWG drops the resistance to 0.194 ohms/kft, giving you a much healthier buffer.
- Derating and Availability: If you pull a neutral and a ground in the same conduit as your two hot legs, you have 3 current-carrying conductors. If you ever add a second circuit to that conduit (4 current-carrying conductors), NEC 310.15(C)(1) requires you to derate the wire's capacity to 80%. 80% of #3 AWG's 90°C rating (115A) is 92A—which is below your 100A breaker. #2 AWG provides the necessary headroom to survive bundling deration. Furthermore, big-box stores rarely stock #3 AWG; they jump from #4 to #2.
Variables That Change Your Wire Size
The #2 AWG Copper / #1/0 AWG Aluminum baseline assumes a perfect-world installation. In the real world, jobsite conditions force you to upsize. Use this decision matrix to adjust your sizing:
| Installation Variable | Condition | Required Action |
|---|---|---|
| Distance (Voltage Drop) | Run exceeds 150 feet | Upsize to #1 AWG Copper or #2/0 AWG Aluminum to maintain <3% drop at 240V. |
| Conduit Fill (Bundling) | 4 to 6 current-carrying conductors in one raceway | Apply 80% derating factor. #2 AWG Cu (130A at 90°C * 0.8 = 104A) barely passes. Upsize to #1 AWG Cu for safety. |
| Ambient Temperature | Attic or rooftop conduit exceeds 86°F (30°C) | Apply NEC Table 310.15(B)(1) correction factors. At 110°F (43°C), multiply ampacity by 0.87. |
| Aluminum Terminations | Using Aluminum wire on standard lugs | Must use Noalox anti-oxidant paste and a calibrated torque screwdriver set to the lug's exact in-lb spec. |
A Note on Aluminum Torque: If you choose #1/0 AWG Aluminum to save money (aluminum is roughly 40% cheaper than copper per foot), you must treat the terminations differently. Aluminum creeps and oxidizes. You must coat the stripped wire ends with Noalox (or an equivalent oxide inhibitor) before inserting them into the lugs. More critically, you must use a torque screwdriver to tighten the lugs to the exact inch-pound specification printed on the breaker or panel label. Loose aluminum connections arc and melt down over time due to thermal expansion and contraction cycles.
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline for safe conductor sizing, your local Authority Having Jurisdiction (AHJ) or the local utility company holds the final veto power. You must pull a permit and consult a professional in the following scenarios:
- Service Entrance Drops vs. Subpanels: The wire sizing above applies to feeders (e.g., running power from your main 200A panel to a 100A detached garage subpanel). If you are installing a completely new 100A utility service drop from the transformer to your meter mast, the utility company dictates the exact wire type, size, and mast height. Many utilities mandate #1/0 Aluminum minimum for overhead drops regardless of NEC minimums.
- Continuous Loads: If your 100A panel is dedicated to a continuous load (defined by the NEC as running at maximum current for 3 hours or more, like an EV charger array or commercial lighting), the wire and breaker must be sized at 125% of the load. A 100A continuous load requires wire rated for 125A, pushing you to #1 AWG Copper.
- Long Underground Trenches: If you are burying direct burial wire (like USE-2 or UF) in a trench exceeding 200 feet, the cost of copper becomes prohibitive, and the voltage drop calculations become complex. An electrical engineer should calculate the exact voltage drop based on the specific power factor of your loads to determine if #2/0 or #4/0 Aluminum is required.
By sticking to #2 AWG Copper or #1/0 AWG Aluminum for standard 100-amp subpanel feeders under 150 feet, you satisfy the NEC, eliminate voltage drop headaches, and ensure your lugs run cool under heavy summer loads.
