The wire size for a 100 amp panel is the minimum American Wire Gauge (AWG) cross-section required to safely carry 100 amps of current without exceeding the temperature rating of the panel and breaker terminations. For a standard residential 100A subpanel or main panel, the strict NEC minimum is #3 AWG copper or #1 AWG aluminum (based on the 75°C column). What this wire size changes in a real installation is your physical conduit fill, the bending radius required in junction boxes, the specific lugs that will accept the conductor, and the voltage drop over distance. What people commonly confuse it with is using the 90°C ampacity column for breaker sizing, or assuming a 100A panel requires 100A of continuous load capacity (it doesn't; the breaker protects the wire, and the panel rating is simply a maximum busbar limit).
The Direct Answer: Minimum AWG and the 4-Wire Requirement
When sizing feeders for a 100-amp panel, you must pull four individual wires (2 hots, 1 neutral, 1 ground) or use a 4-wire cable assembly. Since the 2008 NEC cycle, 3-wire feeds to detached structures or subpanels have been strictly prohibited to prevent neutral current from traveling on equipment grounding paths.
- Copper (THHN/THWN-2 or XHHW-2): #3 AWG
- Aluminum (XHHW-2 or SER): #1 AWG
- Equipment Grounding Conductor (Copper): #8 AWG
- Equipment Grounding Conductor (Aluminum): #6 AWG
While #3 copper and #1 aluminum are the absolute minimums to prevent the breaker from tripping and the lugs from overheating, most jobsite electricians step up one size to #2 copper or #1/0 aluminum. This isn't just for future-proofing; it is a practical necessity to mitigate voltage drop and to make the physical pulling of the wire through conduit significantly easier, as #3 AWG is notoriously stiff and difficult to bend in tight panel gutters.
The 75°C Termination Rule vs. 90°C Wire Insulation
If you look at NEC Table 310.16, you will see that #3 AWG copper THHN has an ampacity of 115A in the 90°C column. This leads many DIYers to believe they can use #4 AWG copper (rated 95A at 90°C) on a 100A breaker. This is a dangerous code violation.
NEC 110.14(C) mandates that termination ampacity is limited by the lowest temperature rating of any connected component. Standard residential breakers and panelboard lugs are rated for 75°C. Therefore, you must size the wire based on the 75°C column, regardless of the fact that the wire's insulation can handle 90°C. The 90°C column is only permitted to be used for derating factors (like adjusting for high ambient temperatures or bundling more than three current-carrying conductors in a single conduit), but the final derated ampacity must still meet or exceed the 75°C termination requirement.
Worked Example: Voltage Drop on a 150-Foot Garage Feeder
Ampacity tells you if the wire will melt. Voltage drop tells you if your tools will actually run when they get to the other end of the wire. Let's calculate the voltage drop for a 100A subpanel located 150 feet from the main service.
Assuming a full 100A load (rare in reality, but required for worst-case sizing) on a 240V circuit using copper (K = 12.9):
| Wire Size | Circular Mils (CM) | Calculated Voltage Drop | Percentage Drop | Result |
|---|---|---|---|---|
| #3 AWG Cu | 52,620 | 14.7 Volts | 6.1% | FAIL (Exceeds 3%) |
| #2 AWG Cu | 66,360 | 11.6 Volts | 4.8% | MARGINAL (Acceptable for non-continuous) |
| #1 AWG Cu | 83,690 | 9.2 Volts | 3.8% | PASS (Safe for heavy motor starts) |
If you are running a 150-foot feeder to a garage where you will be running a 5HP air compressor or a table saw, starting currents can be 3 to 5 times the running current. Using the minimum #3 AWG copper will result in severe voltage sag, potentially tripping breakers or burning out motor windings. For this distance, stepping up to #1 AWG copper or #1/0 AWG aluminum is the correct engineering choice.
Where You Meet This in Practice
- Detached Garage Subpanels: This is the most common 100A application. You will pull a 4-wire feeder through underground PVC conduit (Schedule 80 where it emerges from the dirt). Because it is a detached structure, NEC 250.32 also requires you to install a local grounding electrode system (two ground rods spaced 6 feet apart) at the subpanel, bonded to the ground bar, while keeping the neutral bar isolated.
- EVSE (Electric Vehicle Supply Equipment) Upgrades: Many modern Level 2 chargers pull 40A to 48A continuous. A dedicated 100A subpanel near the driveway gives you headroom for dual-charger setups without needing to trench a new main feeder later.
- Main Service Replacements: When upgrading an older 60A or 100A main service panel to a modern 200A panel, the utility's service drop wires might still be sized for 100A. The utility will dictate the meter base and service entrance conductor sizing, but your internal feeders must match the new busbar ratings.
Decision Tree: Picking Your Exact Feeder Wire
Use this decision matrix to select the exact material and gauge for your 100A panel installation. Do not guess; follow the path based on your specific site conditions.
| Condition / Constraint | Decision Path | Concrete Pick (Part / Spec) |
|---|---|---|
| Run is under 50 feet (Indoor, dry, or short conduit) | Minimum code compliant is fine. Aluminum is cheaper and perfectly safe if torqued correctly with anti-oxidant paste. | #1 AWG Aluminum XHHW-2 (4-wire pull in 1.25" PVC) |
| Run is 50 to 120 feet (Standard detached garage) | Voltage drop becomes a factor. Step up one AWG size to compensate for distance without breaking the bank on copper. | #1/0 AWG Aluminum SER Cable (e.g., Southwire 1/0-1/0-1/0-2 Dynek) |
| Run is over 120 feet or Heavy motor/welder loads | Voltage drop is critical. Aluminum becomes too thick to bend easily in standard panel knockouts. Switch to copper for higher conductivity and smaller bending radius. | #2 AWG Copper THHN/THWN-2 (4-wire pull in 1.5" PVC conduit) |
| Direct Burial Required (No conduit trenching) | Must use wire rated for wet locations and direct earth contact. SER cable is NOT rated for direct burial. | #1/0 AWG Aluminum URD (Underground Residential Distribution) or MHF (Mobile Home Feeder) cable. |
Frequently Asked Questions
Can I use a 3-wire feed for an existing 100A subpanel?
Only if the subpanel is in the same building as the main panel and was installed before the 2008 NEC update, and even then, it is highly discouraged. For any new installation, or any feed to a detached structure, NEC 250.32 strictly requires a 4-wire feed (2 hots, 1 neutral, 1 separate equipment ground). The neutral and ground bars must be physically separated in the subpanel.
Does the ground wire need to be the same size as the hots?
No. According to NEC Table 250.122, the minimum equipment grounding conductor for a 100A overcurrent device is #8 AWG copper or #6 AWG aluminum. However, if you step up your hot wires for voltage drop (e.g., using #1 AWG copper instead of #3), NEC 250.122(B) requires you to increase the ground wire proportionally. In practice, pulling a #6 or #4 bare copper ground is cheap insurance and saves you from doing the proportional math.
Do I need to use anti-oxidant paste on aluminum wire?
Yes. Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates heat. When terminating #1 or #1/0 aluminum wire into a 100A breaker or panel lug, you must brush the stripped conductor with a wire brush, apply an anti-oxidant compound (like Noalox), and torque the lug to the exact inch-pound specification printed on the breaker label using a calibrated torque screwdriver.
