For a 100 amp subpanel, use 3 AWG copper wire or 1 AWG aluminum wire for the hot and neutral conductors, paired with an 8 AWG copper or 6 AWG aluminum equipment grounding conductor. The overcurrent protection must be a 100A breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.
- Material: Copper (unless Aluminum is explicitly specified)
- Temperature Column: 75°C (Standard for residential breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: Single raceway (PVC or EMT) with a maximum of 3 current-carrying conductors
- System: Single-phase 120/240V, non-continuous load
Safety Note: Working inside a main service panel or subpanel involves lethal voltage. Always de-energize the upstream breaker, verify dead with a calibrated CAT III/IV multimeter, and torque all lugs to manufacturer specifications. Local AHJ (Authority Having Jurisdiction) approval is required for all permanent feeder installations.
| Material | Insulation Type | Hot/Neutral AWG | Ground AWG (Table 250.122) | 75°C Ampacity |
|---|---|---|---|---|
| Copper | THHN / THWN-2 | 3 AWG | 8 AWG | 100A |
| Copper | XHHW-2 | 3 AWG | 8 AWG | 100A |
| Aluminum | THHN / THWN-2 | 1 AWG | 6 AWG | 100A |
| Aluminum | XHHW-2 | 1 AWG | 6 AWG | 100A |
The Baseline: Why 3 AWG Copper and Not 4 AWG?
The most common mistake DIYers make when sizing wire for a 100 amp panel is looking at the 90°C column in NEC Table 310.16. In the 90°C column, 4 AWG copper THHN is rated for 95A, which seems close enough, and 3 AWG is rated for 115A. However, you cannot use the 90°C column for your final ampacity.
Under NEC 110.14(C), the ampacity of a conductor is limited by the lowest temperature rating of any connected component in the circuit. Standard residential circuit breakers, panelboard lugs, and terminal strips are almost universally rated for 75°C. Therefore, even if you pull 90°C-rated THHN wire, you must size the wire using the 75°C column.
In the 75°C column, 4 AWG copper is only rated for 85A. Because 85A is less than your 100A breaker, the wire will overheat before the breaker trips. You must step up to 3 AWG copper, which lands exactly at 100A in the 75°C column. For aluminum, 2 AWG is rated for 90A at 75°C, forcing you to use 1 AWG aluminum to hit the 100A threshold.
What Changes the Answer: Distance, Bundling, and Temperature
The baseline sizes above assume a perfect, short run in a cool basement. Real-world jobsites introduce voltage drop, conduit fill derating, and high ambient temperatures. Here is how those variables force you to upsize your wire.
Voltage Drop and Distance Limits
While the NEC treats voltage drop as an informational note (NEC 215.2) rather than a strict enforceable rule for most residential feeders, best practice and many local inspectors mandate a maximum 3% voltage drop for feeder circuits. A 3% drop on a 240V system equals 7.2V.
Using the standard AC voltage drop formula VD = (2 × L × I × R) / 1000, where 3 AWG uncoated copper has an approximate resistance (R) of 0.245 Ω/kft at 75°C:
- At 100 feet (100A load): VD = (2 × 100 × 100 × 0.245) / 1000 = 4.9V. This is a 2.04% drop. (3 AWG passes).
- At 150 feet (100A load): VD = (2 × 150 × 100 × 0.245) / 1000 = 7.35V. This is a 3.06% drop. (3 AWG fails; you must upsize to 2 AWG copper, which drops the resistance to 0.194 Ω/kft, yielding a 2.4% drop).
| One-Way Distance | Calculated Drop (3 AWG) | Required Wire Size | Conduit Size (PVC Sch 40, 40% Fill) |
|---|---|---|---|
| Up to 115 ft | ≤ 2.35% | 3 AWG | 1 inch |
| 116 ft to 145 ft | 2.36% - 2.95% | 2 AWG | 1 inch |
| 146 ft to 190 ft | 2.96% - 3.80% | 1 AWG | 1.25 inch |
Bundling and Conduit Fill Derating
If you pull multiple circuits through the same conduit, the wires heat each other up. NEC Chapter 9, Table 1 requires derating when you have more than three current-carrying conductors in a raceway. If you run two 240V circuits (4 hot wires) plus a neutral in one pipe, you have 5 current-carrying conductors. This triggers an 80% derating factor.
Here is where the 90°C column finally becomes useful. You apply the derating factor to the 90°C ampacity, but the final result cannot exceed the 75°C column limit. For 3 AWG THHN, the 90°C ampacity is 115A. Multiply by 0.80 (80%) = 92A. Because 92A is less than the 100A breaker, you must upsize to 2 AWG copper (90°C ampacity 130A × 0.80 = 104A, which safely covers the 100A breaker).
Equipment Grounding and Neutral Conductor Rules
Sizing the hot wires is only half the job. The neutral and ground conductors have distinct NEC rules that dictate their minimum sizes.
The Equipment Grounding Conductor (EGC)
The ground wire does not carry current under normal operation; it only carries fault current long enough to trip the breaker. Therefore, it is sized based on the breaker rating, not the load. According to NEC Table 250.122, a 100A overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. If you upsized your hot wires to 1 AWG for voltage drop, NEC 250.122(B) requires you to proportionally upsize the ground wire as well to maintain fault-current capacity.
The Neutral Conductor
For a 120/240V single-phase subpanel feeder, the neutral carries the unbalanced load. While NEC 220.61 allows you to calculate the maximum unbalanced load and potentially downsize the neutral, doing so on a 100A subpanel feeder is a false economy. The neutral must never be smaller than the required equipment grounding conductor (8 AWG Cu). In practice, pulling a full-size 3 AWG neutral prevents future bottlenecking if the panel's load profile changes, ensures adequate fault-current return paths, and satisfies the strictest local inspectors who dislike calculated neutral reductions on subpanel feeders.
When an Engineer or the AHJ Must Confirm
The 3 AWG copper / 1 AWG aluminum baseline holds true for standard residential subpanels feeding general lighting, receptacles, and mixed appliances. However, specific load profiles and environmental conditions invalidate this baseline and require professional sign-off.
Continuous Loads (The 125% Rule)
NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include EV chargers, baseboard heaters, and server racks. If your 100A panel is feeding a calculated continuous load of 85A, NEC 215.2(A)(1) requires the feeder conductors to be sized at 125% of the continuous load before any correction factors are applied. 85A × 1.25 = 106.25A. In this scenario, 3 AWG copper (100A) is illegal. You must pull 1 AWG copper (130A at 75°C) to safely handle the continuous thermal stress without nuisance tripping or lug degradation.
High Ambient Temperature Environments
If your conduit runs through an unconditioned attic in a hot climate, the ambient temperature easily exceeds 30°C. In a 40°C (104°F) attic, you must apply a temperature correction factor. For THHN in the 90°C column, the correction factor at 40°C is 0.91. While this often doesn't force an upsize for a single 100A feeder (115A × 0.91 = 104.6A, which still exceeds the 100A 75°C limit), combining high ambient heat with conduit bundling derating creates a compounding penalty that frequently forces a jump to 1 AWG or 1/0 AWG. Always consult manufacturer technical data for exact derating matrices when routing through hot spaces.
Utility Service Entrances vs. Subpanels
If this 100A panel is a main service disconnect fed directly from the utility transformer (not a subpanel fed from an existing main breaker), different rules apply. Utility companies often have specific minimum service entrance conductor sizes (frequently 2 AWG copper or 1/0 aluminum minimum regardless of calculated load) to handle available fault current and mechanical strength. Never assume subpanel feeder rules apply to a service drop; always verify with the local utility and the AHJ.
