To properly feed a 100 amp sub panel, install a 100A two-pole breaker at the main panel and pull 3 AWG copper THHN wire for the two hots and neutral, with an 8 AWG copper ground. This assumes a standard 75°C terminal rating, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (THHN/THWN-2 in conduit)
- Temperature Column: 75°C (NEC Table 310.16, matching standard residential breaker terminals)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: Maximum 3 current-carrying conductors (2 hots + 1 neutral; equipment ground does not count)
Baseline Wire and Breaker Sizing for a 100A Sub Panel
When planning how to install a 100 amp sub panel, the breaker at the main panel dictates the minimum wire size. A 100A two-pole breaker requires conductors with an ampacity of at least 100 amps. According to the NFPA 70 (National Electrical Code) Table 310.16, 3 AWG copper wire is rated for exactly 100A in the 75°C column.
| Conductor Role | Copper AWG | Aluminum AWG | 75°C Ampacity (Cu) | 75°C Ampacity (Al) |
|---|---|---|---|---|
| Hot (Line 1 & 2) | 3 AWG | 1 AWG | 100A | 100A |
| Neutral (Grounded) | 3 AWG | 1 AWG | 100A | 100A |
| Equipment Ground | 8 AWG | 6 AWG | N/A (NEC 250.122) | N/A (NEC 250.122) |
| Breaker Size | 100A Two-Pole | Maximum Overcurrent Protection | ||
The "Service vs. Feeder" Trap: Why Not 4 AWG?
You might be tempted to grab 4 AWG copper because you read online that it handles 100 amps. That is a classic DIY trap. NEC Article 310.12 (formerly 310.15(B)(7)) allows a special 83% derating factor for service-entrance conductors supplying the main service panel of a single-family dwelling. Under that specific rule, 4 AWG copper (rated 85A at 75°C) can be used on a 100A main service.
However, a sub panel is supplied by a feeder (NEC Article 215). Feeders do not qualify for the residential service allowance. Therefore, you must use the standard ampacity tables. In the 75°C column, 4 AWG copper is only good for 85A. To legally and safely carry 100A on a feeder, you must step up to 3 AWG copper. If you use 4 AWG on a 100A sub panel feeder, the inspector will fail the rough-in, and you will be pulling new wire.
Voltage Drop Calculations: When Distance Forces an Upsize
Ampacity tables only tell half the story; they assume a short run where resistance is negligible. Over long distances, wire resistance causes voltage drop, which can lead to dimming lights, tripping breakers, and overheating motors. The NEC recommends keeping voltage drop under 3% on a feeder.
To calculate this, we use the single-phase voltage drop formula: VD = (2 × K × I × D) / CM. For copper, K is 12.9. We will assume an actual continuous load of 80A (since a 100A panel rarely sees a true 100A continuous load, and sizing voltage drop on the breaker rating rather than actual load leads to massive, unnecessary copper costs). The circular mils (CM) for 3 AWG is 52,620.
| One-Way Distance | Wire Size | Voltage Drop | Percentage | Status |
|---|---|---|---|---|
| 100 ft | 3 AWG Cu | 3.92V | 1.63% | Pass (Under 3%) |
| 150 ft | 3 AWG Cu | 5.88V | 2.45% | Pass (Under 3%) |
| 200 ft | 3 AWG Cu | 7.84V | 3.26% | Fail (Exceeds 3%) |
| 200 ft | 2 AWG Cu | 6.22V | 2.59% | Pass (Upsized) |
If your detached garage or workshop is 200 feet away from the main panel, 3 AWG copper will result in a 3.26% drop. While not immediately dangerous, it violates NEC informational notes for optimal performance. At that distance, you must upsize to 2 AWG copper to bring the drop back down to a safe 2.59%.
Derating and Material Variables That Change Your Wire Size
The baseline assumptions only hold true for a standard, clean installation. Real-world jobsites introduce variables that force you to adjust your wire size. The two most common culprits are conduit bundling and ambient temperature.
When you bundle multiple circuits in the same conduit, the heat generated by the wires cannot dissipate. NEC Table 310.15(C)(1) requires you to apply a derating factor based on the number of current-carrying conductors (CCCs). Note that equipment grounding conductors do not count as CCCs, and neutrals only count if they carry harmonic currents or are part of a multi-wire branch circuit where they carry the same current as the hots. For a standard 240V sub panel feeder with a neutral, you have 3 CCCs. But if you pull two separate sub panel feeders through the same large PVC conduit, you now have 6 CCCs, triggering an 80% derating factor.
When derating, the NEC allows you to use the 90°C column of Table 310.16 for the math, even though your terminations are limited to 75°C. 3 AWG THHN in the 90°C column is rated for 115A. Multiply that by the 80% adjustment factor, and you get 92A. Because 92A is less than your 100A breaker, 3 AWG is no longer legal. You must upsize to 2 AWG (130A × 0.80 = 104A).
| Installation Condition | Impact on 3 AWG Copper | Required Action |
|---|---|---|
| Standard (≤3 CCC, 30°C) | 100A Ampacity | Keep 3 AWG |
| 4-6 CCC in single conduit | Derate to 80% (92A) | Upsize to 2 AWG |
| Ambient Temp 50°C (122°F) | Derate to 82% (94A) | Upsize to 2 AWG |
| Using Aluminum Conductors | 1 AWG required for 100A | Use 1 AWG Al (or 2 AWG for VD) |
| Run exceeds 150 feet | Voltage drop exceeds 2.5% | Upsize to 2 AWG for performance |
If you opt for aluminum wire to save money on long runs, remember that aluminum has higher resistance and lower ampacity per gauge. You will need 1 AWG aluminum to meet the 100A ampacity requirement at 75°C. However, because aluminum suffers from voltage drop much faster than copper, a 150-foot run of 1 AWG aluminum will push a 3.1% voltage drop at 80A. For long runs, upsizing to 1/0 AWG aluminum is often the most cost-effective solution that satisfies both ampacity and voltage drop requirements.
When to Call an Engineer or the AHJ
While sizing a feeder for a standard detached garage or home workshop is straightforward, certain scenarios require professional oversight. You must consult the Authority Having Jurisdiction (AHJ) or a licensed electrical engineer in the following situations:
- Multi-Family or Commercial Loads: If the sub panel feeds a multi-family dwelling unit or commercial equipment, the load calculations fall under NEC Article 220, which requires specific demand factors that a standard residential calculator will not cover.
- High-Ambient Environments: If your conduit runs through a space where the ambient temperature regularly exceeds 113°F (45°C)—such as an unventilated attic in a southern climate or near industrial boilers—you must apply temperature correction factors that may require upsizing to 1 AWG copper or larger.
- Detached Structure Grounding: Installing a sub panel in a detached building requires a separate grounding electrode system (grounding rods) per NEC 250.32, and the neutral must be isolated from the ground bar. Local inspectors scrutinize the bonding jumper sizing and grounding conductor heavily. Refer to resources like Electrical Contractor Magazine (ECM) Codes & Standards for the latest interpretations on grounding electrode conductors.
- Main Service Capacity: Adding a 100A sub panel feeder to an older 100A or 150A main service may overload the utility transformer or the main service entrance. An engineer or the utility company must verify that the existing service can handle the additional calculated load before you pull the permit.
Getting the wire size right on the first pull saves you from failed inspections, overheated lugs, and the misery of fishing new conductors through buried conduit. Stick to the 75°C column, respect the feeder rules over service rules, and always calculate your voltage drop before buying your copper.
