For a standard 100A subpanel load calculation, use a 100A 2-pole breaker and 3 AWG copper THHN/THWN-2 wire in conduit, or 2 AWG copper if using NM-B cable. This assumes a 120/240V split-phase system, 75°C terminations, 30°C ambient, and a run under 120 feet.
The Baseline: 100A Subpanel Sizing and Assumptions
When you perform a load calculation for sub panel feeders, the National Electrical Code (NEC) requires you to size the conductors and overcurrent protection based on the calculated load, not just the physical panel rating. A 100A subpanel is the modern standard for detached garages, workshops, and backyard studios running welders, EV chargers, and heavy compressors. However, slapping a 100A breaker on undersized wire is a fast track to a melted lug or a structure fire.
Before pulling any wire, you must lock in your baseline assumptions. If your installation deviates from these, the wire size must change.
- Conductor Material: Copper
- Temperature Column: 75°C (THHN in conduit), 60°C (NM-B cable)
- Ambient Temperature: 30°C (86°F)
- Conduit Type: PVC Schedule 80 or NM-B clipped to joists
- Phases: 120/240V Single-phase, 3-wire (2 hots, 1 neutral, 1 ground)
- Load Type: Non-continuous (runs less than 3 hours at a time)
The NEC Ampacity Table: Why 3 AWG and Not 4 AWG?
A common and dangerous mistake DIYers make is reading the 90°C column of NEC Table 310.16 and assuming 4 AWG THHN (rated 95A at 90°C) is close enough for a 100A breaker. It is not.
NEC 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected termination, device, or conductor. Most standard 100A breakers and subpanel lugs are rated for 75°C. Therefore, you must use the 75°C column to determine your baseline ampacity.
| AWG Size | 75°C Ampacity | Sufficient for 100A Breaker? |
|---|---|---|
| 6 AWG | 65A | No |
| 4 AWG | 85A | No (Will trip/overheat) |
| 3 AWG | 100A | Yes (Exact match) |
| 2 AWG | 115A | Yes (Oversized) |
At the 75°C column, 4 AWG copper is only good for 85A. If you pull 4 AWG and terminate it on a 100A breaker, a sustained 90A load will overheat the breaker lug long before the breaker trips. 3 AWG copper provides exactly 100A at 75°C, making it the minimum legal size for a 100A feeder in conduit.
The NM-B (Romex) Exception: If you are running NM-B cable instead of individual THHN wires in conduit, NEC 334.80 restricts you to the 60°C column, regardless of the cable's internal wire rating. Because 3 AWG is not a standard NM-B size, you must step up to 2 AWG NM-B, which is rated 115A at 60°C.
Voltage Drop Check: The 120-Foot Threshold
Ampacity keeps the wire from catching fire; voltage drop keeps your equipment from burning out. NEC 210.19(A) Informational Note recommends keeping feeder voltage drop under 3%. For a 240V system, that means a maximum drop of 7.2V.
Let us run the math for our baseline 3 AWG copper THHN feeder carrying a full 100A load over a 120-foot run. Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper constant): 12.9 ohms
- I (Current): 100A
- D (Distance): 120 feet
- CM (Circular Mils for 3 AWG): 52,620
VD = (2 × 12.9 × 100 × 120) / 52,620 = 5.88V
A 5.88V drop on a 240V system is a 2.45% drop. This is well under the 3% NEC recommendation. However, if your detached garage is 160 feet away from the main panel, that same 3 AWG wire yields a 3.27% drop. At that distance, you must step up to 2 AWG THHN to maintain power quality for sensitive electronics and motor starting torque. You can verify your specific run using the Southwire Voltage Drop Calculator.
Decision Tree: Adjusting for Real-World Variables
Real jobsites rarely match textbook baselines. Use this decision tree to adjust your wire size when variables change. This path terminates in a concrete part pick for your specific scenario.
| Variable Change | Code/Physics Impact | Required Action & Final Pick |
|---|---|---|
| Run exceeds 120 ft. | Voltage drop exceeds 3% at full load. | Step up one size. Pick: 2 AWG Copper THHN. |
| Using NM-B Cable instead of conduit. | NEC 334.80 limits ampacity to 60°C column. | Jump to 60°C rated size. Pick: 2 AWG Copper NM-B. |
| Switching to Aluminum (SER/USE-2). | Aluminum has higher resistance and lower ampacity per AWG. | Use 75°C AL column (1 AWG = 100A). Pick: 1 AWG Aluminum SER. |
| 4 Current-Carrying Conductors in conduit. | NEC 310.15(C)(1) requires 80% derating for bundling. | 3 AWG THHN (115A at 90°C × 0.8 = 92A) fails. Pick: 2 AWG Copper THHN. |
| Ambient temp is 40°C (104°F) attic. | NEC Table 310.15(B)(1) requires 0.88 correction factor. | 3 AWG THHN (115A × 0.88 = 101.2A) barely passes, but 2 AWG is safer. Pick: 2 AWG Copper THHN. |
When to Call the AHJ or an Engineer
While the math above covers 95% of residential 100A feeder installations, certain conditions require a licensed professional and formal approval from your local Authority Having Jurisdiction (AHJ). Do not proceed with DIY sizing if your project involves any of the following:
- Continuous Loads: If your subpanel will supply loads that run for 3 hours or more continuously (like a commercial server rack, a kiln, or continuous-duty HVAC), NEC 210.20 requires you to multiply the continuous load by 125% before sizing the breaker and wire. A 100A continuous load requires a 125A breaker and 1 AWG copper wire.
- Complex Derating Scenarios: If you are pulling multiple subpanel feeders through the same conduit trench or raceway, the bundling derating factors compound rapidly. An engineer must calculate the thermal dissipation limits.
- Service Entrance vs. Feeder: If this 100A panel is a main service disconnect (fed directly from the utility meter) rather than a subpanel fed from an existing main breaker, different NEC Article 230 rules apply regarding grounding electrodes and service drop clearances.
For a standard non-continuous workshop load under 120 feet, default to 3 AWG copper THHN in PVC conduit. If your run is longer, or you are pulling NM-B, step up to 2 AWG copper. If your scenario falls outside these explicit boundaries, stop and consult your local electrical inspector.
