For a 100-amp circuit, use 3 AWG copper wire or 1 AWG aluminum wire with a 100-amp breaker. This assumes THHN/THWN-2 insulation in a raceway, evaluated at the 75°C temperature column per NEC 310.16, with an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors.
- Material: Copper (unless Aluminum is explicitly stated)
- Temperature Column: 75°C (Standard for 100A breaker lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Single raceway/conduit, maximum 3 current-carrying conductors
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Baseline: NEC Ampacity and Wire Sizing
The most common mistake DIYers and junior apprentices make is sizing wire based on the 90°C column because they bought THHN wire. While THHN is rated for 90°C, the lugs on almost all standard 100-amp breakers and panelboards are only rated for 75°C. Per NFPA National Electrical Code (NEC) 110.14(C), you must use the 75°C column to determine your final ampacity.
| Material | Insulation Type | Temp Column Used | Required AWG | Base Ampacity |
|---|---|---|---|---|
| Copper | THHN/THWN-2 | 75°C | 3 AWG | 100A |
| Copper | NM-B (Romex) | 60°C | 2 AWG | 115A |
| Aluminum | XHHW-2 / THWN-2 | 75°C | 1 AWG | 100A |
Note on NM-B: If you are running Romex (NM-B), the NEC restricts it to the 60°C column. Since 3 AWG NM-B is not a standard manufactured size, you must jump to 2 AWG NM-B (rated 115A) to safely feed a 100A panel.
Why Not One Size Smaller? (And What Changes the Math)
A frequent question on the jobsite is why we cannot use 4 AWG copper wire. At 75°C, 4 AWG copper is rated for 85 amps. The NEC 'next size up' rule (240.4(B)) allows you to round up to the next standard breaker size if your wire ampacity doesn't match a standard breaker rating. However, 100A is a standard breaker size. You cannot protect an 85A wire with a 100A breaker if the calculated load could exceed 85A. For a true 100-amp rated feeder, the wire's baseline ampacity must be at least 100A.
Several real-world conditions will force you to upsize from the baseline 3 AWG copper or 1 AWG aluminum:
| Condition | Trigger Threshold | Required Action |
|---|---|---|
| Long Distance (Voltage Drop) | Run exceeds 100-120 feet | Upsize to 2 AWG or 1 AWG Cu to maintain <3% drop |
| Conductor Bundling | 4 to 6 current-carrying conductors in one conduit | Apply 80% derating factor; upsize to 2 AWG Cu (90°C column math) |
| High Ambient Heat | Attic or rooftop conduit > 30°C (86°F) | Apply NEC 310.15(B)(1) temperature correction factors |
| Continuous Loads | Load runs at max capacity for 3+ hours | Multiply load by 1.25; size wire for 125A (1 AWG Cu) |
Voltage Drop Check Example: Let's say you are feeding a detached garage subpanel 150 feet away. You have a 240V split-phase feeder carrying an 80A continuous load. Using the Southwire Voltage Drop Calculator or standard VD formulas, 3 AWG copper yields a voltage drop of roughly 5.88V. On a 240V line-to-line feeder, that is a 2.45% drop, which is acceptable (under the 3% NEC recommendation). However, if your load pushes closer to the full 100A, or if the run extends to 200 feet, you must upsize to 2 AWG copper to prevent dimming lights and motor burnout at the subpanel.
Copper vs. Aluminum for 100-Amp Feeders
When pulling a 100-amp feeder, especially for long runs to a detached workshop or barn, the cost difference between copper and aluminum becomes massive. Here is how they compare on the bench and in the trench:
- Cost: 1 AWG Aluminum XHHW-2 is typically 40% to 60% cheaper per foot than 3 AWG Copper THHN. For a 200-foot run (requiring 800 feet of total conductor for 2 hots, 1 neutral, 1 ground), aluminum saves hundreds of dollars.
- Conduit Fill: Aluminum wire is physically thicker to carry the same current. 1 AWG aluminum requires more cross-sectional area. You may need to bump your PVC conduit from 1-inch to 1.25-inch to meet NEC Chapter 9 conduit fill tables.
- Termination & Oxidation: Aluminum oxidizes rapidly when exposed to air, creating a resistive layer that causes heat. If your panel lugs are not explicitly rated for Aluminum (marked AL or AL/CU), you cannot use it. Even with rated lugs, applying an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before torquing is a critical best practice to prevent future thermal failure.
- Torque Requirements: Per NEC 110.14(D), you must use a calibrated torque screwdriver or wrench to tighten breaker and lug terminals to the manufacturer's exact inch-pound specification. This is non-negotiable for aluminum, as it creeps and loosens under thermal cycling faster than copper.
When an Engineer or the AHJ Must Confirm
While the guidelines above cover 95% of residential subpanel and feeder installations, you must step back and consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- Service Entrance Work: If this 100-amp wire is the main service entrance from the utility meter to the main panel, utility companies often have specific, rigid wire sizing and routing requirements that supersede standard NEC feeder rules.
- Continuous Industrial Loads: If the 100A load is continuous (e.g., commercial HVAC, server rooms, or heavy shop equipment running 3+ hours), NEC 210.19(A)(1) requires the conductors to be sized at 125% of the continuous load. This means sizing for 125A, requiring 1 AWG Copper or 1/0 AWG Aluminum.
- Parallel Runs: If you are attempting to parallel smaller wires to achieve 100A (which is generally not permitted for sizes under 1/0 AWG per NEC 310.10(H)), an engineer must design and stamp the installation.
Frequently Asked Questions
What size wire for 100 amp sub panel 50 feet away?
For a standard 50-foot run to a 100-amp subpanel, voltage drop is negligible. You can safely use the baseline sizes: 3 AWG copper (THHN/THWN-2 in conduit) or 1 AWG aluminum (XHHW-2 in conduit). Ensure you pull two hots, one neutral, and a separate equipment grounding conductor. Do not bond the neutral and ground at the subpanel.
Can I use 4 AWG wire for a 100 amp breaker?
No, not for a standard 100-amp feeder. 4 AWG copper wire is rated for 85 amps at the 75°C termination column. While the NEC 'next size up' rule allows rounding up to the next standard breaker when wire ampacity falls between standard sizes, 100A is already a standard size. Putting an 85A wire on a 100A breaker leaves the wire unprotected if the load pulls between 86A and 99A, creating a severe fire hazard.
What size ground wire do I need for a 100 amp circuit?
Per NEC Table 250.122, the minimum size equipment grounding conductor for a 100-amp breaker is 8 AWG copper or 6 AWG aluminum. However, if you had to upsize your current-carrying conductors for voltage drop (e.g., you bumped your hots from 3 AWG to 1 AWG copper), NEC 250.122(B) requires you to increase the ground wire proportionately. In that case, you would need to upsize the ground to 6 AWG copper.
Is it safe to mix copper and aluminum on a 100 amp feeder?
You should never directly splice copper and aluminum wire together using standard wire nuts or basic crimps; the galvanic reaction between the two dissimilar metals will cause corrosion, high resistance, and eventually a fire. If you must transition from an aluminum feeder to copper panel pigtails, you must use specialized, UL-listed bi-metallic lugs (like the Ilsco MAC series) or dual-rated AL/CU mechanical split bolt connectors, properly torqued and sealed.
