The correct wire size for a 100-amp service is the minimum American Wire Gauge (AWG) or kcmil conductor that can safely carry 100 amps continuously without exceeding its temperature rating or violating voltage drop limits. For a standard installation under 100 feet, the National Electrical Code (NEC) mandates #3 AWG Copper or #1 AWG Aluminum based on the 75°C ampacity column.
The Baseline: NEC Ampacity and Terminal Ratings
When sizing a feeder for a 100-amp breaker, you must look at NEC Table 310.16. However, the most common mistake DIYers make is looking at the 90°C column because modern THHN/THWN-2 wire is rated for 90°C.
Under NEC 110.14(C), you must size your wire based on the lowest temperature rating of any termination in the circuit. Almost all standard 100-amp breakers and panel lugs are rated for 75°C. Therefore, your ampacity baseline is locked to the 75°C column:
- Copper: #3 AWG is rated for exactly 100A at 75°C.
- Aluminum: #1 AWG is rated for exactly 100A at 75°C.
What Conductor Size Actually Changes in Your Installation
Choosing the right wire size for 100 amp service changes three physical realities on the jobsite: heat dissipation, conduit fill, and breaker terminal compatibility. If you undersize the wire, the insulation will degrade over time due to resistive heating, eventually leading to a melted lug or a short circuit. If you oversize the wire unnecessarily, you may find that the conductor is too thick to bend into the panel or physically cannot fit into the breaker's mechanical lug.
What people commonly confuse it with: Many builders confuse the breaker trip curve with wire ampacity. A 100-amp breaker will not instantly trip at 101 amps; it can hold 110 amps for several minutes. Because of this thermal lag, the wire must be sized to handle the continuous heat, not just the breaker's nominal trip point. Additionally, installers frequently confuse copper and aluminum sizing rules, attempting to use #3 AWG Aluminum (which is only rated for 75A at 75°C) on a 100A breaker, creating an immediate fire hazard.
Worked Example: Voltage Drop on a 150-Foot Garage Feeder
Ampacity tables only tell half the story. If your detached garage subpanel is 150 feet away from the main panel, pushing 100 amps through #1 AWG Aluminum will result in unacceptable voltage drop. The NEC recommends a maximum 3% voltage drop for branch feeders.
Let's run the math using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (DC resistance constant for Aluminum) = 21.2
- I (Current) = 100 Amps
- D (One-way Distance) = 150 Feet
- CM (Circular Mils for #1 AWG Al) = 83,690
Calculation for #1 AWG Aluminum:
VD = (2 × 21.2 × 100 × 150) / 83,690 = 7.59 Volts
Percentage = (7.59V / 240V) × 100 = 3.16%
The Fix: We bump up to the next standard size, 1/0 AWG Aluminum (CM = 105,600).
VD = (2 × 21.2 × 100 × 150) / 105,600 = 6.02 Volts (2.5%). This is well within the safe 3% limit.
Where You Meet This in Practice
You will typically encounter the need to size wire for a 100-amp service in three specific scenarios:
- Detached Garage or Workshop Subpanels: A 100A subpanel is the modern standard for a detached garage running welders, air compressors, and EV chargers. This requires a 4-wire feeder (2 hots, 1 neutral, 1 equipment grounding conductor) per NEC 250.32.
- Older Main Service Upgrades: Homes built in the 1960s and 70s often have 100A main panels. When replacing the panel, you must verify the existing service entrance conductors meet modern 75°C ampacity rules.
- Large HVAC or Heat Pump Feeds: While most residential AC units run on 30A to 60A breakers, large commercial-residential geothermal heat pumps or backup generator interlocks may require a dedicated 100A fused disconnect.
Decision Tree: Picking Your Exact Feeder Cable
Use this decision path to select your final wire size and material. This table terminates in a concrete pick based on your specific run parameters.
| If Your Scenario Is... | Material Choice | Final Wire Size Pick | Conduit Requirement (Min) |
|---|---|---|---|
| Under 100 ft, standard subpanel | Copper (THHN/THWN-2) | #3 AWG | 1-inch PVC or EMT |
| Under 100 ft, budget-conscious | Aluminum (XHHW/THWN-2) | #1 AWG | 1.25-inch PVC or EMT |
| 100 ft to 150 ft, full 100A load | Aluminum (XHHW-2) | 1/0 AWG | 1.5-inch PVC or EMT |
| Over 150 ft, or high continuous load | Aluminum (XHHW-2) | 2/0 AWG | 2-inch PVC or EMT |
| Direct Burial (SER/USE-2 cable) | Aluminum (Quadplex USE-2) | 1/0 AWG (Min) | N/A (Trench 24" deep) |
Default Recommendation: For 90% of residential detached garage subpanels under 120 feet, pull #1 AWG Aluminum XHHW-2 in 1.25-inch Schedule 40 PVC. Aluminum is roughly 60% cheaper than copper by weight, and at #1 AWG, the voltage drop is negligible for standard residential loads. Always buy XHHW-2 rather than THWN-2 for wet locations like underground conduit.
Installation Mistakes and Code Caveats
Do I need a 4-wire or 3-wire feeder for a 100A subpanel?
You must use a 4-wire feeder (two ungrounded hots, one grounded neutral, one equipment grounding conductor). The NEC eliminated the 3-wire exception for new detached structures in 2008. The neutral and ground bars in the subpanel must be kept strictly isolated.
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. Apply a UL-listed anti-oxidant compound (like Noalox or Penetrox) to the stripped aluminum conductor before inserting it into the breaker lug. This is a strict requirement for aluminum terminations.
What torque value should I use for the 100A breaker lugs?
Under NEC 110.14(D), you must tighten terminations to the manufacturer's specified torque using a calibrated torque screwdriver or wrench. For a standard 100A breaker accepting #1 AWG Aluminum, the torque spec is typically between 40 in-lbs and 50 in-lbs (check the breaker's data sheet or the label inside the panel door). Loose lugs are the number one cause of subpanel fires.
Can I use SER cable inside conduit?
Technically yes, but it is highly discouraged. SER (Service Entrance Round) cable has a flat, awkward profile that makes pulling through conduit nearly impossible and ruins conduit fill ratios. If you are running conduit, buy individual THHN/THWN-2 or XHHW-2 conductors. If you are direct-burying without conduit, use UF-B or direct-burial rated USE-2 quadplex.
Always verify your final wire size and installation method with your local Authority Having Jurisdiction (AHJ). While the Southwire voltage drop calculators and NEC tables provide the engineering baseline, local inspectors may have specific amendments regarding aluminum wire usage or conduit fill derating in high-ambient-temperature environments.
