The Baseline Spec Sheet for a 100A Feeder
Before pulling any wire, we must establish the baseline assumptions for this installation. Wire ampacity is not a single fixed number; it changes based on insulation type, termination temperature ratings, and ambient heat. The specifications below are derived from the NFPA 70 National Electrical Code (NEC), specifically Table 310.16.
- Conductor Material: Copper (unless otherwise specified in the decision tree)
- Insulation Type: THHN/THWN-2 (rated 90°C, but we must use the 75°C column for ampacity)
- Termination Rating: 75°C (standard for modern breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors (no derating required)
| Component | Specification | NEC Reference |
|---|---|---|
| Main Panel Breaker | 100A, 2-Pole | NEC 240.4 |
| Hot Conductors (x2) | 3 AWG Copper THHN | NEC Table 310.16 (75°C Col = 100A) |
| Neutral Conductor | 3 AWG Copper THHN (White) | NEC 220.61 |
| Equipment Ground | 8 AWG Copper (Green/Bare) | NEC Table 250.122 |
| Conduit Size (Min) | 1-inch PVC Sch 80 or EMT | NEC Chapter 9, Table 1 |
The Decision Tree: When to Upsize Your Wire
The baseline spec works for 90% of residential garage, workshop, or detached building subpanels. However, environmental factors and physical routing can force you to upsize. Use this decision path to determine your final wire gauge.
| Condition / Variable | Impact on Ampacity | Required Action & Final Wire Pick |
|---|---|---|
| Run is under 100 ft, standard temp | None. Baseline applies. | Pick: 3 AWG Copper |
| Run is 100 ft to 150 ft | Voltage drop approaches 3% at full 80A continuous load. | Pick: Upsize to 2 AWG Copper to maintain <3% drop. |
| Run is over 150 ft | Voltage drop exceeds 3% on 2 AWG. | Pick: Upsize to 1 AWG Copper (or 1/0 Al). |
| 4 to 6 current-carrying conductors in conduit | NEC 310.15(C)(1) requires 80% derating. | Pick: 2 AWG Copper (115A x 0.8 = 92A - wait, use 1 AWG to be safe: 130A x 0.8 = 104A). |
| Ambient temp is 40°C (104°F) in attic | THHN 90°C column correction factor is 0.91. | Pick: 2 AWG Copper to compensate for thermal derating. |
| Budget constraint / Long distance | Copper becomes prohibitively expensive. | Pick: 1 AWG Aluminum (XHHW-2) for runs over 50 ft. |
Voltage Drop Verification at 100 Feet
Sizing wire solely by the ampacity table is a rookie mistake. The NEC recommends a maximum 3% voltage drop for branch circuits and feeders to ensure equipment operates efficiently and motors don't overheat. Let us run the math for our baseline 3 AWG copper wire at a 100-foot distance.
We use the single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9 ohms per mil-foot
- I (Current) = 80A (We calculate using the 80% continuous load rule, as a 100A panel is rarely loaded to 100A continuously without tripping thermal limits).
- D (Distance) = 100 feet
- CM (Circular Mils for 3 AWG) = 52,620
Calculation:
VD = (2 × 12.9 × 80 × 100) / 52,620
VD = 206,400 / 52,620 = 3.92 Volts
Percentage Drop:
(3.92V / 240V) × 100 = 1.63%
At 1.63%, 3 AWG copper easily passes the 3% NEC recommendation for a 100-foot run. If your run extends to 180 feet, the drop hits 2.93%, right on the edge. At 185 feet, you must upsize to 2 AWG copper.
Why 4 AWG Copper Fails the 100A Test
A common question on forums is why we cannot use 4 AWG copper, since THHN wire is rated for 90°C, and 4 AWG at 90°C is rated for 95 Amps. The answer lies in NEC 110.14(C), which governs termination temperature provisions.
Even if your wire insulation is rated for 90°C, the lugs inside standard residential breakers and subpanel neutral bars are typically only tested and rated for 75°C. The NEC mandates that you must size your wire based on the lowest temperature rating in the circuit. Looking at Mike Holt Enterprises' breakdown of NEC Table 310.16, 4 AWG copper in the 75°C column is only rated for 85 Amps. You cannot protect an 85A wire with a 100A breaker. Therefore, 3 AWG (rated 100A at 75°C) is the absolute minimum legal size for copper.
NEC 110.14(D) requires that all terminations be tightened to the manufacturer's specified torque. For a 100A breaker accepting 3 AWG wire, this is typically between 40 and 50 in-lbs. Do not guess. Use a calibrated digital torque screwdriver. Under-torqued lugs cause high resistance, arcing, and panel fires; over-torqued lugs strip threads or snap the wire strands.
Aluminum vs. Copper: The Cost-to-Size Tradeoff
For short runs inside a garage, copper is the default. But if you are trenching 150 feet to a detached workshop, the cost of 3 AWG copper becomes staggering. This is where aluminum becomes the logical choice, provided you respect the sizing and preparation rules.
To carry 100 Amps at 75°C, you must use 1 AWG Aluminum (XHHW-2 or THHN). Aluminum has higher resistance and expands/contracts more than copper under thermal cycling. If you choose aluminum, you must execute the following:
- Brush and Paste: Use a wire brush to clean the aluminum conductor, then immediately apply an anti-oxidant compound like Noalox or Ideal Noalox. This prevents aluminum oxide (an insulator) from forming at the lug.
- Check Lug Ratings: Ensure the subpanel lugs are explicitly marked "AL" or "AL/CU". Most modern panels are, but older stock may be copper-only.
- Retorque: Because aluminum creeps under pressure, some inspectors require you to retorque the lugs after 24 hours of thermal cycling, though modern AA-8000 series aluminum alloys have largely mitigated this.
When the AHJ or an Engineer Must Step In
While this guide provides the definitive baseline for standard residential and light-commercial subpanels, certain scenarios cross the line from DIY territory into engineered requirements. You must consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if:
- Parallel Runs: You want to run two sets of smaller wires in parallel to achieve 100A. (NEC 310.10(G) generally prohibits paralleling conductors smaller than 1/0 AWG).
- High Fault Current: Your utility transformer provides available fault current exceeding the 10,000 AIC (Ampere Interrupting Capacity) rating of standard residential breakers. You will need breakers with a higher AIC rating (e.g., 22k or 42k AIC).
- Complex Grounding: The subpanel is in a separate building with its own grounding electrode system (ground rods). You must isolate the neutral from the ground bar in the subpanel and bond only the ground bar to the local electrode per NEC 250.32.
The Final Verdict: For a standard install 100 amp sub panel setup under 100 feet, buy a 100A 2-pole breaker, a spool of 3 AWG copper THHN (black, red, white), and 8 AWG green/bare copper for the ground. Keep your terminations at 75°C ratings, torque them to spec, and isolate your neutral bar in the destination subpanel.
