A 100-amp service cable is the main feeder conductor sized to safely carry 100 amperes of continuous and non-continuous current from the utility meter to the main breaker panel without exceeding its thermal limits. If you are wiring a standard residential dwelling, the direct answer for what size service cable for 100 amp service is 2 AWG Aluminum or 4 AWG Copper, leveraging the NEC 310.12 residential exception. If you are wiring a commercial building or a non-dwelling feeder, you must step up to 1 AWG Aluminum or 3 AWG Copper.
Choosing the correct gauge changes everything in a real installation: it dictates your physical conduit diameter (if running individual THHN wires), the torque specifications required on the main breaker lugs, the bending radius in your panel, and your overall material cost. Getting this wrong means either failing an inspection, suffering from severe voltage drop, or melting a terminal lug because the wire was too small for the termination hardware.
The Sizing Table: Copper vs. Aluminum for 100A Services
Before pulling any wire, you need to know your baseline ampacities. The National Electrical Code (NEC) dictates wire sizing based on the temperature rating of the terminations (the lugs on your breaker and meter), not just the insulation on the wire. Most modern residential panels and meter sockets are rated for 75°C. Below is the data-dense reference chart you need to keep in your truck.
| Application Type | Conductor Material | Minimum AWG Size | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Wire Insulation Limit) | NEC 310.12 Allowance |
|---|---|---|---|---|---|
| Residential (Dwelling Service) | Aluminum (XHHW/THHN) | 2 AWG | 90A | 100A | Compliant (90A > 83A min) |
| Residential (Dwelling Service) | Copper (THHN/XHHW) | 4 AWG | 85A | 95A | Compliant (85A > 83A min) |
| Commercial / Non-Dwelling Feeder | Aluminum (XHHW/THHN) | 1 AWG | 100A | 115A | N/A (Must meet full 100A) |
| Commercial / Non-Dwelling Feeder | Copper (THHN/XHHW) | 3 AWG | 100A | 110A | N/A (Must meet full 100A) |
The Theory of Ampacity and the 83% Rule
To understand why a 90A-rated wire (2 AWG Aluminum at 75°C) is legally allowed on a 100A breaker, we have to look at NEC Article 310.12. This section contains a special exception for single-family dwellings. It states that service conductors for a dwelling unit do not need to have an ampacity equal to the main breaker's rating; they only need to carry 83% of the service rating.
For a 100-amp service, the math is straightforward: 100A × 0.83 = 83A. Therefore, any conductor with a 75°C ampacity of 83A or higher is code-compliant. 4 AWG Copper (85A) and 2 AWG Aluminum (90A) both clear this hurdle.
Think of the 83% rule like a highway designed for 100 cars per minute, but because residential traffic is naturally staggered (you don't run the oven, AC, EV charger, and dryer at 100% draw simultaneously for three continuous hours), the physical lanes only need to handle 83 cars at any exact second without gridlock. The NEC recognizes this load diversity in homes, which is why this exception does not apply to commercial buildings where continuous industrial loads are common.
For a deeper dive into the exact code language, you can reference the National Fire Protection Association (NFPA) NEC guidelines, which govern these ampacity tables and dwelling exceptions.
Worked Numeric Example: When Code Minimum Fails (Voltage Drop)
Here is where theory meets the jobsite. The NEC 310.12 exception tells you what is legal for thermal safety, but it ignores voltage drop. NEC 210.19(A) Informational Note recommends keeping voltage drop under 3% for branch circuits and feeders to ensure equipment runs efficiently.
Let's run a real-world scenario. You are installing a 100A subpanel in a detached garage, 150 feet away from the main meter. You decide to use the code-minimum 2 AWG Aluminum to save money.
- Current (I): 100A (worst-case continuous draw)
- Length (L): 150 feet (one way)
- Voltage (V): 240V
- K Factor (Aluminum at 75°C): ~21.2
- Circular Mils (CM) for 2 AWG: 66,360
Using the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM
VD = (2 × 21.2 × 100 × 150) / 66,360
VD = 636,000 / 66,360 = 9.58 Volts
Now, calculate the percentage: (9.58V / 240V) × 100 = 3.99%.
The Fix: You must upsize to 1/0 AWG Aluminum (CM = 105,600). Recalculating with 1/0 AWG yields a voltage drop of 6.02V, which is exactly 2.5%—well within the safe 3% threshold. Always use a reliable voltage drop calculator for runs over 100 feet to verify your wire size.
Where You Meet This In Practice (And Common Confusions)
When you are actually pulling wire and terminating lugs, the theoretical numbers on a chart run into physical reality. Here is where you meet this concept on the bench and in the panel, along with the most common mistakes DIYers and junior apprentices make.
1. The 90°C vs. 75°C Termination Trap
The most common confusion in service sizing is looking at the 90°C column on the ampacity chart and thinking you can use a smaller wire. You might buy 90°C rated THHN wire and assume 3 AWG Aluminum (115A at 90°C) is fine for a 100A commercial feeder. It is not.
NEC 110.14(C) states that you must size your wire based on the lowest temperature rating of any connected component. Almost all residential and commercial breaker lugs and meter sockets are rated for 75°C. Therefore, even if your wire insulation can handle 90°C, the termination cannot. You must always use the 75°C column for your final sizing, using the 90°C column only for derating calculations (like adjusting for high ambient temperatures in a hot attic).
2. SER Cable vs. THHN in Conduit
For a 100A residential service, you will typically choose between Service Entrance Round (SER) cable or individual THHN wires pulled through PVC or EMT conduit.
- SER Cable: This is a pre-assembled bundle (usually two hots, one neutral, and a bare ground wrapped in a jacket). It is rated for direct burial (if marked USE-2) or running through framing cavities. It is faster to install but much stiffer to bend. For 100A, you will buy 2-2-2-4 Aluminum SER.
- THHN in Conduit: If you are pulling individual wires through 1.5-inch or 2-inch PVC conduit underground, you are dealing with conduit fill limits and heat dissipation. THHN is easier to pull through bends, but you must ensure your conduit is large enough to prevent jamming. Three 2 AWG Al THHN wires will easily fit in 1.25-inch PVC, but stepping up to 1.5-inch makes the physical pull much easier.
3. Torque Specifications Matter
Sizing the wire correctly is only half the battle. A 100A service cable must be torqued to the manufacturer's exact specification. Since the 2017 NEC update (110.14(D)), inspectors frequently require a calibrated torque screwdriver or wrench. An under-torqued 2 AWG aluminum lug will loosen over time due to thermal expansion and contraction (aluminum creeps more than copper), creating a high-resistance joint that will eventually melt the breaker lug or start a fire. Always check the sticker inside the panel door for the exact lb-in rating.
