Wire size for a 100-amp service refers to the minimum cross-sectional area of the conductor required to safely carry 100 amps of current without exceeding the insulation's temperature rating or creating a fire hazard. The direct answer depends on where the wire is going: for a standard 100-amp subpanel feeder, you need a minimum of 3 AWG copper or 1 AWG aluminum (based on the 75°C termination column). However, if this is the main service entrance for an entire single-family dwelling, NEC 310.12 allows you to use 4 AWG copper or 2 AWG aluminum.
The Core Physics: Ampacity, Heat, and the 75°C Rule
To understand why we cannot simply shove any wire that physically fits into a 100-amp breaker, you have to look at electrical resistance. When electrons are forced through a conductor, they collide with the metal's atomic lattice, generating heat. Think of it like rush-hour traffic on a two-lane highway: the higher the volume of cars (amps) relative to the road width (wire gauge), the more friction and heat you generate.
Wire size changes three critical factors in a real installation:
- Thermal Dissipation: Thicker wires have more surface area to shed heat into the surrounding air or conduit.
- Physical Bending Radius: A 1/0 AWG aluminum wire requires significantly more physical space inside a panel to bend without stressing the termination lug.
- Conduit Fill Percentage: Upsizing wire to compensate for voltage drop can push your conduit fill past the NEC 40% limit, forcing you to upsize the PVC or EMT conduit as well.
Where You Meet This In Practice (And Common Confusions)
You will encounter 100-amp wire sizing requirements in three main scenarios: feeding a detached garage subpanel, wiring a mobile home feeder, or upgrading an older 60-amp main service to a 100-amp service entrance. In the current 2026 electrical market, aluminum SER (Service Entrance Rated) cable is the dominant choice for these runs due to copper's volatile pricing, often costing 60% less per foot than equivalent copper.
What people commonly confuse it with: The most dangerous confusion is mixing up a breaker's trip curve with a wire's ampacity. A 100-amp breaker does not trip at exactly 100.0 amps. According to standard UL 489 trip curves, a 100A breaker can hold 100A indefinitely and might take several minutes to trip at 135A. If your wire is undersized and rated for only 90A, the insulation will melt and degrade long before the breaker decides to trip. The breaker protects against short circuits and massive faults; the wire sizing protects against sustained thermal overload.
Worked Numeric Example: Sizing a 100A Subpanel Feeder
Let's run the numbers for a 150-foot underground PVC conduit run to a detached garage subpanel. We will calculate voltage drop to see if the minimum code-compliant wire is actually the best choice.
| Parameter | 3 AWG Copper (Minimum) | 1 AWG Copper (Upsized) |
|---|---|---|
| Ampacity (75°C Column) | 100 Amps | 130 Amps |
| Circular Mils (CM) | 52,620 | 83,690 |
| Resistance (K factor) | 12.9 | 12.9 |
| Voltage Drop @ 80A Continuous | 6.28 Volts (2.6%) | 3.95 Volts (1.6%) |
The Math: The formula for single-phase voltage drop is VD = (2 × K × I × L) / CM. Using an 80A continuous load (worst-case scenario for a garage with EV chargers and welders), 3 AWG copper yields a 2.6% drop. While this is under the NEC's recommended 3% maximum for feeders, it leaves almost no headroom. If the garage is 200 feet away instead of 150, the 3 AWG wire will exceed 3%, and you must upsize to 1 AWG copper to maintain power quality and prevent motor burnout on heavy tools.
Real-World Scenario Walkthrough: The Melted Lug Incident
The Setup: A DIY homeowner needed to add a 100-amp subpanel for a backyard workshop, 40 feet from the main house. To save money, they purchased 2 AWG Aluminum SER cable and connected it to a 100-amp double-pole breaker.
The Numbers: According to the Southwire Ampacity Chart, 2 AWG Aluminum is rated for exactly 90 Amps in the 75°C column. The breaker was rated for 100 Amps.
The Outcome: Six months later, the homeowner ran a 15-amp table saw and a 20-amp electric space heater simultaneously while charging an EV on a 40-amp circuit. The total sustained draw hit roughly 85 amps. The 100-amp breaker didn't trip (it was well within its holding capacity). However, the 2 AWG aluminum wire was operating near its absolute thermal ceiling. Over weeks of heating and cooling, the aluminum expanded and contracted, causing the lug connection to loosen. The resulting high-resistance joint arced, melting the panel's aluminum busbar and destroying the subpanel.
What Went Wrong: The wire was undersized for the breaker's continuous capacity. Furthermore, the DIYer failed to apply Noalox (anti-oxidant paste) and failed to use a torque screwdriver. Aluminum creeps under pressure; without proper torque and oxidation prevention, undersized aluminum is a ticking time bomb. Always match the breaker to the wire's ampacity, not the other way around.
Step-by-Step Installation & Verification
When terminating 100-amp feeders, precision matters as much as the wire gauge. Follow this sequence to ensure a safe, code-compliant connection:
- De-energize and Verify: Shut off the upstream breaker. Test both the line and load sides with a non-contact voltage tester, then confirm with a multimeter set to AC voltage (reading should be 0.0V).
- Strip to Exact Length: Use a calibrated wire stripper. For standard 100A lugs, you typically need 5/8" to 3/4" of bare conductor. Do not nick the outer strands, which creates weak points for arcing.
- Apply Anti-Oxidant (Aluminum Only): If using aluminum wire, brush the bare strands with Noalox or equivalent inhibitor. This prevents galvanic corrosion and oxide buildup, which increases resistance.
- Seat and Torque: Insert the wire fully into the lug. Use an insulated torque screwdriver set to the manufacturer's exact specification (usually printed on the breaker label, typically between 45 and 60 in-lbs for 100A lugs). Never guess the torque.
- The Tug Test: Give the wire a firm, sharp pull. It should not move a millimeter. If it shifts, loosen, re-seat, and re-torque.
FAQ: 100 Amp Service Wire Sizing
Can I use 2 AWG copper for a 100-amp service?
Yes. 2 AWG copper is rated for 115A at 75°C, which exceeds the 100A requirement. It is perfectly legal and safe, though slightly more expensive and harder to bend than 3 AWG. Many electricians use 2 AWG simply because it is more commonly stocked at local supply houses than 3 AWG.
Does the NEC 83% rule apply to my 100-amp detached garage subpanel?
No. NEC 310.12(A) (the 83% derating rule that allows smaller wire for dwelling services) applies only to the main service entrance conductors supplying the entire single-family dwelling. It does not apply to subpanel feeders, detached garages, or multi-family buildings. For a 100A subpanel, you must use the standard 100% ampacity rules (3 AWG Cu / 1 AWG Al minimum).
What if I am pulling wire through a hot attic?
If your conduit runs through an attic where ambient temperatures regularly exceed 86°F (30°C), you must apply temperature correction factors from NEC Table 310.15(B)(1). For example, in a 110°F attic, you must multiply the wire's base ampacity by 0.87. In that scenario, 3 AWG copper (100A × 0.87 = 87A) is no longer sufficient, and you must upsize to 2 AWG or 1 AWG copper to maintain your 100A rating.
