For a standard 100-amp subpanel feeder under 100 feet, use 3 AWG copper or 1 AWG aluminum. For a 200-amp feeder, use 2/0 AWG copper or 4/0 AWG aluminum. These sizes assume standard 75°C terminations, an ambient temperature of 86°F (30°C), and no more than three current-carrying conductors in the raceway. If your run exceeds 100 feet, or if you are bundling multiple circuits in one conduit, you must apply derating factors and voltage drop calculations, which we cover below.
How to Read This Feeder Wire Size Chart
The most common mistake DIYers and junior electricians make is pulling wire sizes from the 90°C column of the ampacity tables. Under NEC 110.14(C), the temperature rating of the wire cannot exceed the temperature rating of the termination (the breaker or lug). Because almost all residential and light-commercial breakers and panel lugs are rated for 75°C, you must select your base wire size from the 75°C column.
The 90°C column (which applies to modern insulations like THHN, THWN-2, and XHHW-2) is strictly used as the starting baseline for derating calculations (adjusting for high ambient heat or conduit fill). Once you apply the derating math to the 90°C value, the final adjusted ampacity must still meet or exceed your load, and the physical wire size must still be large enough to satisfy the 75°C termination rule.
The Master Feeder Wire Size Chart (NEC Table 310.16)
The following data is extracted from NFPA 70 (National Electrical Code) Table 310.16. It assumes standard installation conditions: 30°C ambient temperature, single circuit in a raceway, and 75°C rated equipment terminations.
| Breaker / Load (Amps) | Copper (75°C Column) | Aluminum (75°C Column) | Copper (90°C Column - For Derating) | Aluminum (90°C Column - For Derating) |
|---|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 75A | 65A |
| 100A | 3 AWG | 1 AWG | 100A | 90A |
| 125A | 2 AWG | 1/0 AWG | 115A | 100A |
| 150A | 1 AWG | 2/0 AWG | 130A | 115A |
| 200A | 2/0 AWG | 4/0 AWG | 175A | 155A |
| 225A | 3/0 AWG | 250 kcmil | 195A | 170A |
| 250A | 4/0 AWG | 300 kcmil | 230A | 200A |
| 300A | 300 kcmil | 400 kcmil | 255A | 225A |
| 400A | 500 kcmil | 700 kcmil | 340A | 295A |
Quick-Jump: Most Common Subpanel Feeder Sizes
Bookmark this section for the most frequently pulled feeder sizes in residential and light-commercial work. These assume a maximum 3% voltage drop (runs under 100 feet) and standard 75°C lugs.
- 60A Subpanel (e.g., EV charger, small workshop): 6 AWG Copper or 4 AWG Aluminum.
- 100A Subpanel (e.g., detached garage, standard addition): 3 AWG Copper or 1 AWG Aluminum.
- 125A Subpanel (e.g., heavy machinery, large outbuilding): 2 AWG Copper or 1/0 AWG Aluminum.
- 150A Subpanel (e.g., multi-unit tenant space): 1 AWG Copper or 2/0 AWG Aluminum.
- 200A Subpanel (e.g., main service upgrade, large farm shop): 2/0 AWG Copper or 4/0 AWG Aluminum.
Derating Factors: When to Bump Up a Wire Size
Ampacity drops when heat cannot dissipate. This happens when ambient temperatures exceed 86°F (30°C) or when you pull more than three current-carrying conductors (CCCs) in a single conduit. Electrical Contractor Magazine frequently highlights that ignoring conduit fill derating is a primary cause of failed rough-in inspections.
Worked Example: 100A Feeder with 4 CCCs at 104°F (40°C)
You are pulling two 100A circuits (4 CCCs total: two hots and two neutrals for multi-wire branch circuits) in a single PVC conduit on a hot roof where the ambient temperature hits 104°F (40°C).
- Start with 90°C column: 3 AWG Copper has a 90°C ampacity of 100A.
- Apply Conduit Fill Derating: 4 to 6 CCCs requires an 80% multiplier. (100A × 0.80 = 80A). This is below your 100A load.
- Apply Ambient Temp Derating: 40°C correction factor for 90°C insulation is 0.91. (80A × 0.91 = 72.8A). Still too low.
- Bump the Wire Size: Step up to 1 AWG Copper. Its 90°C ampacity is 130A. Apply derating: 130A × 0.80 × 0.91 = 94.6A. Still slightly under 100A.
- Final Bump: Step up to 1/0 AWG Copper (90°C ampacity 150A). 150A × 0.80 × 0.91 = 109.2A. This clears the 100A load.
- Check 75°C Termination: 1/0 AWG Copper at 75°C is rated for 150A, which easily satisfies the 100A breaker termination.
Concrete Pick: You must install 1/0 AWG Copper for this specific run, not the 3 AWG listed in the base chart.
Decision Tree: Picking Your Exact Feeder Wire
Use this decision matrix to lock in your final wire purchase. Follow the steps sequentially; do not skip the voltage drop check.
| Step | Condition / Question | Action / Result |
|---|---|---|
| 1 | What is the calculated continuous and non-continuous load? | Size the breaker to 125% of continuous load. This is your target Ampacity. |
| 2 | Are terminations rated 75°C or 90°C? | If 75°C (standard), use the 75°C column from the Master Chart above to find your baseline AWG. |
| 3 | Will the conduit contain 4 or more current-carrying conductors? | If YES, apply NEC Table 310.15(C)(1) derating to the 90°C ampacity. Bump wire size if adjusted ampacity falls below target. |
| 4 | Is the one-way wire length greater than 100 feet? | If YES, calculate voltage drop. If drop exceeds 3%, bump wire size up one or two steps and recalculate. |
| 5 | Are you using Aluminum or Copper? | Aluminum is cheaper but requires larger AWG and anti-oxidant paste (Noalox) at terminations. Copper is smaller but costs 2x-3x more. |
| 6 | Final Verification | Select the largest AWG dictated by Steps 2, 3, or 4. Torque lugs to manufacturer spec (usually 25-40 in-lbs for residential). |
What This Chart Cannot Tell You
An ampacity chart is a thermal limit guide, not a complete engineering specification. Before pulling wire, you must account for three critical variables not listed in Table 310.16:
- Voltage Drop: The NEC strongly recommends (via Informational Notes in 210.19 and 310.15) keeping voltage drop under 3% for branch circuits and feeders, and 5% total. A 4/0 aluminum wire is perfectly legal for a 200A panel 20 feet away, but if that same panel is 300 feet away at the back of a property, the voltage drop will cause lights to dim and motors to overheat. For a 300-foot 200A run, you must bump up to 350 kcmil or 500 kcmil aluminum to maintain voltage.
- Conduit Fill (Chapter 9): You cannot physically stuff unlimited wires into a pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Four 4/0 AWG THHN conductors will not fit in a 1.5-inch Schedule 40 PVC conduit; you must step up to 2-inch PVC.
- Local AHJ Amendments: Your local Authority Having Jurisdiction (inspector) may have regional amendments. Some municipalities mandate copper-only for all interior residential feeders, while others require aluminum for any feeder over 100A to prevent copper theft on active construction sites. Always pull the permit and check local bulletins before ordering 500 feet of wire.
