The 100 amp service wire size is the minimum American Wire Gauge (AWG) cross-sectional area required to safely carry 100 amps of electrical current from the utility meter to the main breaker panel without exceeding the insulation's thermal limits or causing excessive voltage drop.
• Dwelling Service Entrance (NEC 310.12): 4 AWG Copper or 2 AWG Aluminum
• Standard Feeder / Subpanel (NEC 310.16 @ 75°C): 3 AWG Copper or 1 AWG Aluminum
• Equipment Ground: 8 AWG Copper or 6 AWG Aluminum
Choosing the correct gauge changes three physical realities in your installation: the minimum PVC or EMT conduit diameter required to avoid jamming, the physical bending radius needed to make clean sweeps into the panel, and whether the wire will actually fit into the main breaker's terminal lugs. Undersizing causes thermal degradation of insulation; oversizing can result in wires that physically cannot be bent into tight panel gutters or stripped to fit standard 100A lugs.
The most common confusion among DIYers is looking at the 90°C ampacity column on wire charts and assuming they can use a smaller wire. While THHN wire is insulated for 90°C, the terminal lugs on almost all residential breakers and busbars are only rated for 75°C. Per NEC 110.14(C), your final wire size must be based on the 75°C column, regardless of the wire's higher insulation rating.
NEC Rules: Dwelling Services vs. Standard Feeders
The National Electrical Code (NEC) treats the main service entrance of a home differently than a feeder running to a detached garage or subpanel. This distinction is where most online forums give out incorrect advice.
The Dwelling Service Exception (NEC 310.12)
For the main service entrance conductors supplying a single-family dwelling, the NEC allows a slight ampacity bump based on the assumption that a residential load profile rarely hits 100% continuous simultaneous draw across all circuits. Under NEC 310.12(A), a 100-amp residential service requires a minimum of 4 AWG Copper or 2 AWG Aluminum.
Standard Feeders and Subpanels (NEC 310.16)
If you are running a 100-amp feeder to a detached workshop, an EV charger subpanel, or a commercial space, the dwelling exception does not apply. You must use the standard 75°C column of Table 310.16. Here, 4 AWG copper (rated 85A at 75°C) is insufficient. You must step up to 3 AWG Copper (100A) or 1 AWG Aluminum (120A).
| Application | Copper (THHN/XHHW) | Aluminum (XHHW-2/SER) | NEC Article |
|---|---|---|---|
| Main Dwelling Service | 4 AWG | 2 AWG | 310.12(A) |
| Subpanel Feeder | 3 AWG | 1 AWG | 310.16 |
| 60°C Rated Equipment (Older) | 1 AWG | 1/0 AWG | 110.14(C)(1) |
| Equipment Grounding | 8 AWG | 6 AWG | 250.122 |
Source: Cerrowire Ampacity Charts and NFPA 70 (National Electrical Code).
Worked Example: Voltage Drop on a 150-Foot Subpanel Run
Ampacity tables only tell you what size wire will prevent a fire. They do not account for voltage drop over distance. If your 100-amp subpanel is located 150 feet from the main panel, pushing 80 amps of continuous load (like an EV charger and a welder running simultaneously) through 3 AWG copper might result in unacceptable voltage sag.
The Parameters:
- Load: 80 Amps (Continuous)
- Voltage: 240V (Split-phase)
- Distance: 150 feet (one-way)
- Wire: 3 AWG Copper (Circular Mils = 52,620)
- K-Factor (Copper): 12.9
The Voltage Drop Formula:
VD = (2 × K × I × D) / CM
The Math:
VD = (2 × 12.9 × 80 × 150) / 52,620
VD = 309,600 / 52,620 = 5.88 Volts
The Result:
5.88V drop on a 240V system is a 2.45% drop. The NEC recommends a maximum of 3% voltage drop for feeders (and 5% total combined feeder + branch). Therefore, 3 AWG Copper is perfectly acceptable for this 150-foot run. However, if your continuous load was the full 100 amps, the drop would hit 3.06%, requiring you to upsize to 2 AWG Copper to stay under the 3% threshold.
Where You Meet This in Practice
You will typically encounter 100-amp wire sizing decisions in three specific jobsite scenarios:
- Adding an EV / Workshop Subpanel: Modern Level 2 EV chargers draw 40A to 48A continuous. Pair that with a 240V welder or air compressor in a detached garage, and a 100A subpanel is the standard choice. You will likely pull 1-1-1-3 Aluminum SER (Service Entrance Rated) cable through 2-inch PVC conduit to accommodate the physical stiffness of the aluminum.
- Upgrading an Older 60A Service: Homes built in the 1950s often have 60A services with 6 AWG copper. Upgrading to 100A requires pulling new 4 AWG copper or 2 AWG aluminum from the weatherhead to the new main breaker panel, and coordinating a meter jaw inspection with the utility.
- Torquing Terminal Lugs: The physical reality of 1 AWG aluminum or 3 AWG copper is that they are incredibly stiff. When terminating these into a 100A main breaker, you must use a calibrated torque screwdriver or wrench. NEC 110.14(D) mandates that terminations be torqued to the manufacturer's specifications (usually printed on the breaker label, often around 45-50 inch-pounds for these sizes). Hand-tightening leads to high-resistance connections, arcing, and melted busbars.
Frequently Asked Questions
Can I use 2 AWG copper wire for a 100 amp service?
Yes, you can always use a larger wire than the minimum required, provided it physically fits into the breaker lugs. 2 AWG copper is rated for 115A at 75°C, making it an excellent choice for a 100A feeder where you want to minimize voltage drop on a long run. However, check the breaker datasheet; some compact 100A breaker lugs are only rated to accept a maximum of 3 AWG or 2 AWG, and forcing a larger wire can damage the terminal cage.
Is aluminum wire safe for a 100 amp main panel feed?
Absolutely. Aluminum is the industry standard for service entrance conductors due to its cost-effectiveness and weight. The key to safety is using the correct alloy (AA-8000 series, as required by NEC 310.14) and proper installation techniques. You must clean the wire strands with a wire brush, apply an anti-oxidant compound like Noalox to prevent galvanic corrosion, and torque the lugs to the exact inch-pound specification. Never use old, uncoated aluminum branch circuit wire from the 1960s, but modern XHHW-2 and SER aluminum is highly reliable.
What size ground wire do I need for a 100 amp service?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100-amp overcurrent device is 8 AWG Copper or 6 AWG Aluminum. If you are running a separate ground wire alongside your feeder conductors in conduit, this is the size you must use. Note that if you are using a metal conduit (like Rigid Metal Conduit or EMT) as your grounding path per NEC 250.118, a separate ground wire may not be required, though many inspectors prefer a pulled copper ground for subpanels regardless.
Do I need to upsize the wire if running it through hot attic insulation?
Yes, this is a frequently missed code violation. NEC Table 310.15(B)(1) requires ambient temperature derating. If your conduit runs through an attic where the ambient temperature regularly exceeds 86°F (30°C) — which is almost guaranteed in the summer — the wire's ability to shed heat drops. If the attic hits 114°F to 122°F, you must apply a derating factor of 0.82 to the 90°C column ampacity. In high-heat environments, upsizing from 3 AWG to 2 AWG copper ensures you maintain your 100A capacity without violating thermal limits.
