For a 100 amp sub panel 100 feet away, use 2 AWG copper or 1/0 AWG aluminum THHN/THWN-2 wire on a 100-amp double-pole breaker. While 3 AWG copper meets the bare minimum 100A ampacity at 75°C, upsizing to 2 AWG mitigates voltage drop over the 100-foot run and provides vital derating headroom.
- Material: Copper (primary) / Aluminum (alternative)
- Insulation: THHN/THWN-2 or XHHW-2
- Temperature Column: 75°C (per NEC 110.14(C) terminal limits)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Single raceway (PVC or EMT conduit), 3 current-carrying conductors + 1 ground
- System Voltage: 240V split-phase
The Core Sizing Calculation: Ampacity vs. Voltage Drop
The most common mistake DIYers make is sizing wire strictly to the breaker rating while ignoring the physical realities of the run. To understand why we recommend 2 AWG copper over the bare-minimum 3 AWG, we have to look at both ampacity and voltage drop.
Under NEC Table 310.16, 3 AWG copper wire is rated for exactly 100 amps in the 75°C column. However, NEC 110.14(C) dictates that we must use the 75°C column because standard panel lugs and breakers are rated for 75°C, even if the THHN wire insulation is rated for 90°C. If your subpanel will carry a continuous load (defined as running for 3 hours or more), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. If you plan to pull 80 amps continuously, you need 100A capacity. Pushing 3 AWG copper to its absolute 100A thermal limit in a conduit leaves zero margin for error.
Voltage Drop at 100 Feet
While the NEC recommends keeping feeder voltage drop under 3% (Informational Note to 210.19(A)), sensitive electronics and motor loads perform poorly when voltage sags. Let us run the math for a realistic 80-amp continuous load on a 240V circuit over 100 feet using the standard formula: VD = (2 × K × I × D) / CM.
| Wire Size (Copper) | 75°C Ampacity | Circular Mils (CM) | Voltage Drop (80A @ 100ft) | VD Percentage (240V) |
|---|---|---|---|---|
| 3 AWG | 100A | 52,620 | 3.92V | 1.63% |
| 2 AWG (Recommended) | 115A | 66,360 | 3.11V | 1.29% |
| 1 AWG | 130A | 83,690 | 2.47V | 1.03% |
As the table shows, 3 AWG technically passes the 3% voltage drop guideline. So why choose 2 AWG? Conduit fill and heat. If you pull four current-carrying conductors (e.g., feeding a subpanel with two hots, a neutral, and a separate ground, but adding a second circuit later), NEC 310.15(C)(1) requires an 80% derating factor. 100A derated by 80% is 80A—meaning 3 AWG would instantly become a code violation. 2 AWG (115A × 0.80 = 92A) keeps you safely above the 100A breaker protection threshold even when bundled. For a complete breakdown of conduit fill and voltage drop calculations, the Southwire Calculator Suite is an excellent bench reference.
Variables That Change Your Wire Size Decision
The 2 AWG copper recommendation assumes ideal conditions. Real-world jobsites rarely cooperate. Use this decision matrix to adjust your material and sizing based on your specific installation environment.
| Installation Variable | Impact on Sizing | Required Adjustment |
|---|---|---|
| Using Aluminum Wire | Aluminum has higher resistance and lower ampacity per AWG. | Must use 1/0 AWG Aluminum (XHHW-2). Never mix Al and Cu without proper anti-oxidant paste and rated lugs. |
| High Ambient Heat (e.g., Attic) | Temperatures above 30°C (86°F) require ampacity derating. | At 40°C (104°F), multiply ampacity by 0.88. Upsize to 1 AWG Copper. |
| Direct Burial (UF-B or USE-2) | Soil acts as an insulator; ampacity tables differ from conduit. | Use 1 AWG Copper or 2/0 AWG Aluminum direct burial cable to account for thermal resistivity of soil. |
| Distance > 150 Feet | Voltage drop becomes the governing factor over ampacity. | Upsize to 1 AWG Copper or 2/0 AWG Aluminum to maintain < 3% drop at full load. |
Copper vs. Aluminum Interchangeability: Never treat these materials as interchangeable. Aluminum expands and contracts more than copper under thermal cycling, which can cause lugs to loosen and arc over time. If you use 1/0 AWG aluminum, you must use lugs explicitly marked 'AL/CU' and apply a dielectric grease (like Noalox) to prevent galvanic corrosion. Torque the lugs to the exact inch-pound specification printed on the panel label—usually between 40 and 50 in-lbs for this size.
Installation Realities, Grounding, and AHJ Sign-Off
Feeding a subpanel involves more than just the hot and neutral conductors. The equipment grounding conductor (EGC) and local code enforcement dictate the final success of your installation.
The Grounding Conductor Trap (NEC 250.122)
For a standard 100-amp breaker, NEC Table 250.122 requires a minimum 8 AWG copper or 6 AWG aluminum ground wire. However, there is a critical rule that catches many off-guard: NEC 250.122(B). This rule states that if you upsize your ungrounded (hot) conductors to compensate for voltage drop, you must proportionally upsize your ground wire.
Since we are upsizing from the minimum 3 AWG to 2 AWG copper (a 26% increase in circular mil area), strict mathematical interpretation of 250.122(B) might still allow 8 AWG to pass. However, most local inspectors will automatically require you to bump the ground to 6 AWG copper when they see 2 AWG hots, simply to maintain a robust fault-current path over the 100-foot distance. Buy 6 AWG bare or green THHN for your ground to avoid a failed inspection.
When to Involve an Engineer or the AHJ
While this guide aligns with standard NFPA 70 (NEC) practices, your local Authority Having Jurisdiction (AHJ) has the final say. You must pull a permit and have an inspector verify your work. Furthermore, you should consult a licensed electrical engineer if:
- Your run exceeds 250 feet (requiring complex voltage drop and short-circuit current calculations).
- You are installing parallel feeder sets (multiple wires per phase, which requires precise balancing and specific conduit routing).
- The subpanel will serve highly inductive loads (like large HVAC compressors or industrial welders) that introduce severe power factor issues and harmonic heating.
Frequently Asked Questions
Can I use 3 AWG copper wire for a 100 amp sub panel 100 feet away?
Technically, yes, 3 AWG copper is rated for 100 amps at 75°C and will keep voltage drop under 3% on a 240V circuit over 100 feet. However, it is highly discouraged. 3 AWG leaves zero thermal headroom. If the wire is run through a hot attic, bundled with other circuits, or subjected to a high continuous load, it will exceed its safe operating temperature. Upsizing to 2 AWG copper costs slightly more but ensures code compliance under derating conditions and provides a safer, cooler run.
What size aluminum wire do I need for a 100 amp sub panel 100 feet away?
You need 1/0 AWG (one-aught) aluminum XHHW-2 or THHN wire. 1/0 AWG aluminum is rated for 120 amps in the 75°C column, which comfortably covers the 100-amp breaker requirement and provides enough circular mils (105,600 CM) to keep the voltage drop to roughly 1.33% over a 100-foot run at an 80-amp continuous load. Ensure your panel lugs are rated for aluminum and use anti-oxidant compound on the terminations.
Do I need to size the ground wire differently for a 100-foot sub panel run?
The physical length of 100 feet does not directly change the grounding table requirements, but upsizing your hot wires for voltage drop does. The baseline ground for a 100A breaker is 8 AWG copper. Because you are likely upsizing your hot wires from 3 AWG to 2 AWG to mitigate voltage drop over the 100-foot distance, NEC 250.122(B) requires proportional upsizing of the ground. To satisfy almost any local inspector and ensure a low-impedance fault path, use a 6 AWG copper equipment grounding conductor.
Will a 100 amp sub panel 100 feet away handle a 50 amp EV charger?
Yes, but you must calculate the continuous load correctly. A 50-amp EV charger is considered a continuous load by the NEC, meaning the circuit must be sized at 125% of the draw (50A × 1.25 = 62.5A). You would install a 60-amp breaker for the charger inside the subpanel. The 100-amp subpanel feeder (using 2 AWG copper) has plenty of capacity to handle the 60-amp EV charger alongside standard lighting and receptacle loads, provided the total calculated continuous load on the subpanel does not exceed 80 amps.
