The correct cable size for a 100 amp circuit is typically 3 AWG copper or 1 AWG aluminum, based on the 75°C ampacity column of the National Electrical Code (NEC). Sizing a conductor for a specific amperage isn't just about preventing the wire from melting; it changes the circuit's thermal dissipation profile, dictates the maximum continuous load you can safely draw, and determines how much voltage drops over distance. The most common mistake DIYers and even junior electricians make is confusing the base wire temperature rating (often 90°C for modern THHN) with the terminal temperature rating of the breaker or lug, which legally bottlenecks the entire assembly to a lower ampacity.
The Direct Answer: Wire Gauge and Material
When pulling wire for a 100-amp breaker, you must select a conductor that can safely carry 100 amps without exceeding the temperature limits of the termination points. Here is the baseline data for standard building wire (THHN/THWN-2 or XHHW-2) installed in a raceway or cable assembly with no more than three current-carrying conductors.
| Material | AWG Size | 75°C Ampacity | 90°C Ampacity | Approx. Cost per Foot (2026) |
|---|---|---|---|---|
| Copper (THHN/THWN-2) | 3 AWG | 100 Amps | 115 Amps | $1.80 - $2.40 |
| Aluminum (XHHW-2) | 1 AWG | 100 Amps | 115 Amps | $0.60 - $0.90 |
While the 90°C column shows higher ampacities, you generally cannot use those numbers for sizing the base wire. This brings us to the most critical rule in feeder sizing.
The 75°C Rule: Why Terminal Ratings Dictate Wire Size
Under NEC 110.14(C), the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device. Almost all standard 100-amp molded case circuit breakers and panelboard lugs are tested and rated for 75°C terminations.
If you buy 90°C rated 4 AWG copper wire (rated 95A at 90°C, but only 85A at 75°C), it will physically fit in the breaker. However, because the breaker lugs are rated for 75°C, the wire is legally limited to 85 amps. The 100-amp breaker will not protect the 85-amp termination from overheating. Always size using the 75°C column, then use the 90°C column only for applying derating factors (like ambient temperature or conduit fill) before checking back against the 75°C baseline.
Furthermore, if your 100-amp load is considered continuous (operating for 3 hours or more), NEC 210.20 requires the breaker to be sized at 125% of the continuous load. A 100-amp breaker can only handle 80 amps of continuous load. If your actual continuous load is 100 amps, you need a 125-amp breaker and 1/0 AWG copper wire.
Where You Meet 100 Amp Feeder Sizing in Practice
You will typically encounter the need for a 100-amp feeder in three specific residential or light-commercial scenarios:
- Detached Subpanels: Feeding a 100-amp main lug subpanel in a detached garage, barn, or workshop. This requires 3 AWG copper or 1 AWG aluminum for the hots and neutral, plus an equipment grounding conductor.
- High-Draw EV Chargers: While most Level 2 chargers use 40A to 60A circuits, dual-charger setups or heavy-duty commercial EVSE units can require an 80A continuous load, mandating a 100A breaker and appropriately sized feeders.
- Large Workshop Machinery: Feeding a 100-amp disconnect switch for heavy equipment like large rotary phase converters, industrial welders, or multi-axis CNC machines.
Real-World Scenario Walkthrough: The 200-Foot Subpanel Mistake
Theory meets reality when distance enters the equation. Here is a common failure mode seen in the field when installers rely solely on ampacity tables without calculating voltage drop.
- Setup: An installer runs a 100A feeder to a detached garage subpanel located 200 feet from the main service panel. They pull three strands of 3 AWG copper THHN and an 8 AWG ground through 1.5-inch PVC conduit.
- Numbers: The 3 AWG copper is perfectly rated for 100A at 75°C. The garage is equipped with a table saw, air compressor, and lighting, drawing a combined peak load of roughly 80 amps when all tools run simultaneously.
- Outcome: The breaker never trips. However, the table saw motor bogs down and overheats, LED lights flicker visibly when the compressor kicks on, and the breaker lugs in the main panel feel warm to the touch.
- What Went Wrong: The installer ignored voltage drop. Over a 200-foot run (400 feet of total wire length for the round trip), the resistance of 3 AWG copper causes a significant voltage drop under heavy load. The saw was receiving less than 220V, causing it to draw higher amperage to compensate for the lost power, which in turn generated excess heat at the terminations.
Voltage Drop and Upsizing: The Hidden Variable
The NEC recommends (via Informational Notes in Article 215 and 250) that feeder voltage drop not exceed 3%, and the total drop (feeder plus branch circuit) not exceed 5%. Let's run the exact math on the scenario above using the standard voltage drop formula to see why upsizing is mandatory for long runs.
The Formula: Voltage Drop = (2 × K × I × D) / CM
(Where K is the resistivity constant for copper [12.9], I is current [80A], D is one-way distance [200 ft], and CM is the circular mil area of the wire [3 AWG = 52,620 CM]).
VD = (2 × 12.9 × 80 × 200) / 52,620
VD = 412,800 / 52,620 = 7.84 Volts
Percentage Drop = (7.84V / 240V) × 100 = 3.26%
Result: Fails the 3% recommended maximum for feeders.
To fix this, we must upsize the wire to 1 AWG copper (CM = 83,690).
VD = (2 × 12.9 × 80 × 200) / 83,690 = 4.92 Volts.
Percentage Drop = (4.92V / 240V) × 100 = 2.05%. This is well within the 3% limit, ensuring motors run cool and lights stay steady.
Frequently Asked Questions
Do I need a separate ground wire for a 100 amp feeder, and what size?
Yes. For a subpanel, you must pull a separate Equipment Grounding Conductor (EGC); the neutral and ground must remain isolated at the subpanel. Per NEC Table 250.122, the minimum EGC size for a 100-amp breaker is 8 AWG copper or 6 AWG aluminum. If you upsize your current-carrying conductors for voltage drop, you must proportionally upsize the ground wire as well.
Can I use 2 AWG copper instead of 3 AWG if I already have it on hand?
Absolutely. In electrical work, a larger wire (lower AWG number) has less resistance and higher ampacity. Using 2 AWG or 1 AWG copper on a 100-amp breaker is perfectly safe and code-compliant, provided the wire physically fits into the breaker's terminal lugs without excessive force or trimming of strands.
What torque value should I use for the breaker lugs?
Never guess the tightness. Loose connections cause arcing and fires; overtightened connections strip threads and crack lugs. Check the breaker's datasheet or the label inside the panel door. Most 100-amp Square D or Eaton breakers require between 25 and 45 inch-pounds of torque. Use a calibrated torque screwdriver or torque wrench to verify.
Is aluminum wire safe for a 100 amp subpanel feeder?
Yes, modern AA-8000 series aluminum alloy wire (like XHHW-2) is perfectly safe and highly cost-effective for feeders. The historical issues with aluminum wiring in the 1970s were related to solid aluminum branch circuit wire and incompatible terminations, not modern stranded feeder cable. Just ensure you use an antioxidant compound (like Noalox) on the stripped aluminum ends before torquing them into the lugs to prevent oxidation.
Getting the cable size for a 100 amp circuit right means looking past the basic ampacity chart. By respecting the 75°C termination rule, calculating voltage drop for runs over 50 feet, and applying proper torque to your lugs, you ensure a feeder that is not just code-compliant, but engineered for decades of reliable service.
