A 100 amp service entrance cable is the heavy-gauge, multi-conductor wiring that delivers main utility power from the meter to a home's primary breaker panel, rated to safely carry a 100-ampere continuous load. This specific cable assembly dictates your whole-home load ceiling, determines the physical routing method (stapled SE cable versus pulled THHN in conduit), and mandates the exact torque specs and lug sizes required at your main breaker terminations. If you undersize this cable or misunderstand its temperature ratings, you risk overheating the main lugs, tripping the utility transformer, or failing your local electrical inspection.
The Anatomy and NEC Rules
Service entrance conductors fall under strict regulations in the National Electrical Code (NEC), primarily NFPA 70 (NEC) Article 230 and Article 338. You will typically encounter two physical configurations for a 100 amp service entrance cable:
- SE (Service Entrance) Cable: A pre-assembled, jacketed cable containing two insulated hot conductors and a bare, concentric neutral/ground wire. Type SER is common for indoor routing, while Type SEU is often used for the meter-to-panel run. It can be stapled directly to framing in some jurisdictions, provided it is not subject to physical damage.
- Individual Conductors in Conduit: Three or four individual THHN/THWN-2 wires (two hots, one neutral, one ground) pulled through rigid metal, PVC, or EMT conduit. This is the preferred method for long runs, underground laterals, or tight bends because individual wires dissipate heat better and are easier to pull than stiff SE cable.
The critical theory to grasp here is the difference between the insulation rating and the termination rating. Modern THHN and SE cable insulation is rated for 90°C. However, the lugs on your 100-amp main breaker and meter base are almost universally rated for 75°C. Under NEC 110.14(C), you must size your wire based on the 75°C column of the ampacity table, regardless of the wire's 90°C insulation capability.
Sizing the Conductors: Copper vs. Aluminum
When sizing a 100 amp service entrance cable, you must choose between copper and aluminum. Aluminum is the industry standard for service entrances due to cost and weight, but it requires a larger physical gauge to carry the same current as copper. Below are the minimum sizes based on the 75°C termination column.
| Conductor Material | Minimum AWG Size | 75°C Ampacity | Approx. Cost per Foot (2026) | Physical Handling Notes |
|---|---|---|---|---|
| Copper (THHN/THWN-2) | 3 AWG | 100 Amps | $4.50 - $6.00 | Highly flexible, easy to bend into lugs, expensive. |
| Copper (SER Cable) | 3 AWG | 100 Amps | $7.00 - $9.00 | Very stiff, requires careful bending radius management. |
| Aluminum (THHN/THWN-2) | 1/0 AWG | 100 Amps | $1.80 - $2.50 | Lightweight, requires anti-oxidant paste on terminations. |
| Aluminum (SER Cable) | 1/0 AWG | 100 Amps | $3.50 - $4.50 | Extremely stiff; difficult to route in tight panel gutters. |
Where You Meet This in Practice
You will typically specify and install a 100 amp service entrance cable in three specific scenarios:
- Upgrading an older 60A service: Homes built in the 1940s-1960s often have 60A panels. Upgrading to 100A requires pulling new service entrance conductors from the weatherhead or underground lateral to the new panel.
- Adding a detached structure (Feeder confusion): While technically a feeder and not a service entrance, many DIYers use SER cable to feed a detached garage subpanel. The physical cable is the same, but the grounding rules change drastically (neutral and ground must be separated at the subpanel).
- Replacing damaged mast cables: If a tree branch damages the service drop and the cables inside the rigid riser mast are compromised, you must replace the conductors inside the mast down to the meter base.
Real-World Scenario: The 75-Foot Aluminum Mistake
To understand how theory translates to the jobsite, let us walk through a common failure mode involving wire sizing and voltage drop.
The Setup: A homeowner is upgrading their main panel to 100 amps. The meter base is located on the exterior wall, and the interior panel is 75 feet away through the crawlspace. To save money, they purchase 2 AWG Aluminum SER cable, assuming it is 'close enough' to 100 amps and much cheaper than 1/0 AWG.
The Numbers: According to NEC Table 310.16, 2 AWG Aluminum at 75°C is rated for exactly 90 Amps. Furthermore, we must calculate the voltage drop for the 75-foot run at a realistic 80-amp continuous peak load. Using the Southwire Voltage Drop Calculator parameters for 2 AWG Al (approx 0.319 ohms/kft):
Voltage Drop = 2 x 0.319 x (75/1000) x 80 = 3.82 Volts.
Percentage Drop = (3.82 / 240) x 100 = 1.59%.
The Outcome: The voltage drop of 1.59% is perfectly acceptable (well under the 3% NEC recommendation). However, the electrical inspector red-tags the installation and refuses to sign off on the final permit. The homeowner is forced to rip out 75 feet of stapled SER cable and buy 1/0 AWG.
What Went Wrong: The homeowner confused the physical proximity of 90 amps to 100 amps with code compliance. The main breaker is rated for 100 amps, and NEC 240.4 requires the conductor ampacity to be equal to or greater than the overcurrent device rating (with specific exceptions not applicable to 100A main service lugs). Because 2 AWG Al is only rated 90A at the 75°C termination limit, it is legally and physically undersized for a 100A breaker. They needed 1/0 AWG Aluminum (rated 100A at 75°C) to pass inspection.
Common Confusions: Service Entrance vs. Feeder Cable
The most frequent mistake made by hobbyists and junior electricians is using service entrance cable rules for feeder circuits, or vice versa.
Service Entrance Cable connects the utility source to the first point of disconnect (the main breaker). At this exact point, the neutral and ground are bonded together. Type SE cable uses a bare, concentric neutral wire that serves as both the grounded conductor (neutral) and the equipment grounding conductor.
Feeder Cable connects the main breaker to a downstream subpanel. In a feeder circuit, the neutral and ground MUST remain strictly separated. You cannot use the bare concentric wire of a standard 3-wire SE cable as a ground for a subpanel if it is also carrying neutral current. For a 100-amp subpanel feeder, you must pull a 4-wire system (two hots, one insulated neutral, one separate ground) using THWN-2 in conduit or 4-wire SER cable.
FAQ: Installation and Routing
Can I bury Type SE cable directly in the ground for an underground service?
No. Type SE and SER cables are not rated for direct burial or wet locations. If your service entrance run goes underground from the utility transformer to your meter base, you must use individual THWN-2 conductors pulled through schedule 80 PVC conduit, or use a specifically rated Underground Service Entrance (USE-2) cable assembly.
What is the correct torque for the main breaker lugs?
Never guess the torque. NEC 110.14(D) requires you to use a calibrated torque tool set to the exact value printed on the breaker or panel label. For 1/0 AWG aluminum in a 100A lug, this is typically between 40 and 50 inch-pounds, but it varies by manufacturer (Square D, Eaton, Siemens). Under-torquing causes arcing; over-torquing strips the aluminum threads or deforms the lug.
Do I need to derate the cable if it passes through a hot attic?
Yes. If your service entrance cable routes through an attic where ambient temperatures exceed 86°F (30°C), you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). If the attic reaches 110°F, a 90°C rated wire must be derated to 87% of its base ampacity. This is one reason routing through conditioned space or using conduit on the exterior wall is often preferred over attic runs.
