You need a minimum of 3 AWG THHN copper wire (75°C column) or 2 AWG NM-B copper cable (60°C column), protected by a 100A double-pole breaker, for a 100-amp service or subpanel feeder. Most electricians upsize to 1 AWG THHN to prevent voltage drop on longer runs and make conduit pulling easier.
Baseline Assumptions for This Guide
- Material: Copper (Aluminum requires completely different sizing; see FAQ).
- Temperature Rating: 75°C terminations (standard for modern residential/commercial panels and breakers).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Type: Standard raceway/conduit (for THHN/THWN-2) or standard NM-B (Romex) cable.
- Phase Configuration: Single-phase 240V split-phase or 120/208V 3-phase (sizing based on ungrounded hot conductors).
- Load Type: Non-continuous (operating at maximum current for less than 3 hours continuously).
NEC Ampacity and Wire Sizing Rules
Wire sizing isn't just about matching a number to a breaker; it is dictated by the insulation type and the temperature rating of the equipment terminations. According to NFPA National Electrical Code (NEC) Article 110.14(C), you must size your wire based on the lowest temperature rating of any connected device, termination, or conductor. Modern 100A breakers and panel lugs are almost universally rated for 75°C.
Here is the relevant excerpt from NEC Table 310.16 for copper conductors:
| AWG Size | 60°C Column (NM-B Cable) | 75°C Column (THHN in Conduit) | 90°C Column (Derating Only) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 115A | 115A | 130A |
| 1 AWG | 130A | 130A | 145A |
Why 3 AWG THHN and Not One Size Smaller?
You might look at 4 AWG copper (rated 85A at 75°C) and wonder if the NEC 240.4(B) "next standard size up" rule allows you to use a 100A breaker. It does not. The standard breaker sizes are 80A, 90A, and 100A. The next standard size up from 85A is 90A. Therefore, 4 AWG cannot be protected by a 100A breaker. 3 AWG copper is rated exactly 100A at 75°C, making it the absolute legal minimum for THHN in conduit.
The NM-B (Romex) Catch
If you are running NM-B cable instead of individual THHN wires in conduit, NEC Article 334.80 mandates that the ampacity must be determined using the 60°C column, even though the internal wires have 90°C insulation. In the 60°C column, 3 AWG is only rated for 85A (next size up is 90A). To hit 100A using NM-B, you must step up to 2 AWG copper, which is rated 115A in the 60°C column.
Voltage Drop: The Hidden Sizing Factor
The NEC minimums keep the wire from melting, but they don't guarantee your equipment will run correctly. NEC Article 210.19(A) Informational Note recommends a maximum voltage drop of 3% for branch circuits and feeders. For a 240V feeder, a 3% drop is 7.2 volts.
Let's run the math using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM, where K for copper is 12.9, I is 100A, D is distance, and CM is the circular mil area of the wire.
- At 100 feet using 3 AWG (52,620 CM): VD = (2 × 12.9 × 100 × 100) / 52,620 = 4.9V (2.04%). This passes the 3% rule.
- At 150 feet using 3 AWG: VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35V (3.06%). This fails. You must upsize to 2 AWG or 1 AWG.
This is why seasoned electricians rarely pull 3 AWG for a 100A subpanel. By defaulting to 1 AWG THHN (83,690 CM), the voltage drop at 150 feet drops to a highly efficient 4.6V (1.9%). Furthermore, 1 AWG THHN is physically easier to bend and pull through 1.5-inch or 2-inch PVC conduit than the stiff, short-length strands of 3 AWG or 2 AWG. You can verify these calculations using tools like the Southwire Voltage Drop Calculator.
What Changes Your Wire Size? (Decision Matrix)
The baseline answer assumes a perfect, standard installation. Real-world jobsites rarely cooperate. Use this decision tree to determine if your specific scenario requires upsizing your copper wire.
| Condition / Variable | Effect on Ampacity | Required Action |
|---|---|---|
| Run length exceeds 100 ft | Voltage drop exceeds 3% threshold. | Upsize to 2 AWG (up to 140 ft) or 1 AWG (up to 200 ft). |
| More than 3 current-carrying conductors in one raceway | NEC Table 310.15(C)(1) requires derating (e.g., 80% for 4-6 conductors). | Calculate derating using the 90°C column. Typically requires upsizing to 1 AWG or 1/0 AWG. |
| Ambient temperature exceeds 86°F (30°C) | Wire insulation degrades faster; ampacity drops. | Apply temperature correction factors from Table 310.15(B). Upsize wire accordingly. |
| Load is continuous (runs 3+ hours at max capacity) | NEC requires conductors to be sized at 125% of continuous load (125A). | Upsize to 1 AWG THHN (rated 130A at 75°C) minimum. |
| Switching to Aluminum (e.g., XHHW-2) | Aluminum has higher resistance and lower ampacity per AWG. | Do NOT use copper sizing. Use 1/0 AWG Aluminum (rated 100A at 75°C). |
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline rules, certain installations cross the line from standard DIY/subcontractor work into engineered territory. You must consult a licensed Professional Engineer (PE) or your local electrical inspector if:
- Utility Service Drops: If this 100A feed is the main service entrance coming directly from the utility meter, the utility company's "Green Book" or local service requirements will dictate exact wire types, conduit sweep sizes, and grounding electrode systems. Never guess on service entrance conductors.
- Complex Derating Scenarios: If your conduit runs through a high-temperature environment (like an uninsulated attic in a southern climate where ambient temps exceed 113°F/45°C) and contains multiple circuits, the stacked derating factors can drastically reduce ampacity. An engineer must calculate the exact thermal dissipation.
- High Fault Current Availability: If the utility transformer can deliver massive fault current (e.g., >10,000 Amps), standard 100A breakers might not have a high enough Ampere Interrupting Capacity (AIC). The AHJ or an engineer must verify the breaker's let-through current and AIC ratings to prevent a panel explosion during a dead short.
Frequently Asked Questions
Can I use aluminum wire instead of copper for a 100 amp service?
Yes, but you cannot use the same AWG size. Aluminum has lower conductivity than copper. For a 100-amp feeder using aluminum (such as XHHW-2 or THWN-2 in conduit), you must use a minimum of 1/0 AWG aluminum, which is rated exactly 100A in the 75°C column. While aluminum wire is significantly cheaper per foot than copper, it requires larger conduit due to the thicker gauge, and you must apply an anti-oxidant compound (like Noalox) to the terminations and torque them to the manufacturer's exact specifications to prevent thermal creep and arcing over time.
What size ground wire is required for a 100 amp copper feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100-amp breaker is 8 AWG copper or 6 AWG aluminum. If you had to upsize your ungrounded (hot) copper conductors to 1 AWG or larger to mitigate voltage drop over a long distance, NEC 250.122(B) requires you to proportionally increase the size of your ground wire as well. However, for standard runs up to 100-150 feet where 3 AWG or 2 AWG hot wires are used, an 8 AWG copper ground is perfectly compliant. Always keep the ground wire insulated (green) if running it in conduit to prevent confusion with neutral conductors.
Do I need a main breaker on a 100 amp subpanel?
If the 100A panel is installed as a true subpanel (fed from a main panel in the same building), it does not require a main breaker. The 100A breaker back at the main panel serves as the overcurrent protection and the disconnecting means. However, if the 100A panel is in a detached structure (like a garage or barn), NEC Article 225.31 and 225.33 require a local disconnecting means. In this case, installing a 100A main breaker panel is the most practical way to satisfy the local disconnect requirement, even though the feeder breaker and the local main breaker will be identically sized at 100A. Remember to separate the neutral and ground bars in all subpanels, regardless of whether they have a main breaker.
