For a standard 100-amp residential service, you need 1/0 AWG copper or 2/0 AWG aluminum conductors, rated for 75°C, typically installed as 4-wire SER (Service Entrance Cable) or individual THHN/THWN-2 wires in conduit. A 100 amp service cable is the primary electrical feeder that carries up to 100 amps of continuous current from the utility meter to a building's main distribution panel. In a real installation, this cable dictates the physical conduit diameter, the torque requirements on the main breaker lugs, and the maximum continuous wattage (24,000W at 240V) your home can draw before the main breaker trips.
The Core Theory: Ampacity and the 75°C Termination Rule
When sizing a 100 amp service cable, the most critical theory to grasp is the difference between a wire's insulation rating and its termination rating. If you look at the Cerro Wire ampacity chart based on NEC Article 310.16, you will see that 1 AWG copper THHN (90°C column) is rated for 130 amps. So why can't you use 1 AWG for a 100-amp service?
The answer lies in the 75°C termination rule. NEC 110.14(C) requires that the ampacity of a circuit be determined by the lowest temperature rating of any connected component. While your THHN wire insulation can handle 90°C, the lugs inside your meter base and main breaker panel are typically rated for only 75°C. If you push 130 amps through a 1 AWG wire, the wire won't melt, but the 75°C breaker lugs will overheat, oxidize, and eventually fail. Therefore, you must size the wire using the 75°C column:
- Copper (75°C column): 1/0 AWG is rated for exactly 150 amps, safely covering the 100A requirement with room for standard overcurrent protection.
- Aluminum (75°C column): 2/0 AWG is rated for 150 amps, making it the minimum aluminum size for a 100A service.
Think of the cable's cross-sectional area like a multi-lane highway. A 100-amp load on a wire sized for only 60 amps is like forcing rush-hour traffic through a single lane; the friction (resistance) generates massive heat, eventually melting the asphalt (insulation). Sizing to the 75°C column ensures the 'highway' stays cool at the on-ramps (terminations).
Where You Meet 100 Amp Service Cable in Practice
You will encounter 100 amp service cable requirements in three primary real-world scenarios:
1. Main Service Entrance (Meter to Panel)
In older homes or smaller modern builds (like ADUs or manufactured homes), the utility drop connects to a meter base, which then feeds a 100A main breaker panel. Here, you will almost exclusively use SER (Service Entrance Cable, Round). SER contains two insulated hot legs, one bare neutral, and one smaller bare ground wire, all wrapped in a sunlight-resistant jacket.
2. Detached Garage Subpanel Feeds
If you are running a 100A subpanel to a detached workshop for welders and EV chargers, you are technically installing a 'feeder' rather than a 'service entrance.' However, the wire sizing math remains identical. For underground runs, you will pull individual THHN/THWN-2 conductors through Schedule 80 PVC conduit rather than burying direct-burial SER cable, as conduit allows for future upgrades and better heat dissipation.
3. Heavy Machinery Branch Circuits
While rare in residential settings, a large commercial CNC machine or a massive industrial air compressor might require a dedicated 100A branch circuit. The wire sizing is the same, but the overcurrent protection and disconnect switches will differ from a residential main panel.
Worked Numeric Example: Voltage Drop Over Distance
Ampacity tells you if the wire will catch fire; voltage drop tells you if your appliances will actually work. The NEC recommends a maximum 3% voltage drop on feeders. Let's calculate the voltage drop for a 150-foot underground run to a detached garage subpanel, drawing an 80A continuous load at 240V.
Where L = one-way length in feet, R = resistance per 1000 ft (from NEC Chapter 9, Table 8), and I = current in amps.
| Conductor Choice | Resistance (Ω/kft) | Calculation | Voltage Drop | Percentage | Result |
|---|---|---|---|---|---|
| 2/0 AWG Aluminum | 0.194 | (2 × 150 × 0.194 × 80) / 1000 | 4.65V | 1.94% | Passes (Under 3%) |
| 1/0 AWG Copper | 0.120 | (2 × 150 × 0.120 × 80) / 1000 | 2.88V | 1.20% | Passes (Under 3%) |
| 2 AWG Aluminum (Undersized) | 0.308 | (2 × 150 × 0.308 × 80) / 1000 | 7.39V | 3.08% | Fails (Over 3%) |
If your run extends beyond 180 feet, even 2/0 Aluminum will cross the 3% threshold at an 80A load. At that point, you must upsize to 4/0 AWG Aluminum or 2/0 AWG Copper strictly to mitigate voltage drop, even though the 75°C ampacity rule would technically allow the smaller wire.
Copper vs. Aluminum: The Decision Tree
The debate between copper and aluminum for service entrance cable usually comes down to budget, physical flexibility, and run length. Use this decision path to select your material:
| Condition | Choose This Material | Why? |
|---|---|---|
| Run is under 100 ft; budget is the primary constraint. | 2/0 AWG Aluminum SER | Aluminum costs roughly 40-50% less per foot than copper. At short distances, voltage drop is negligible. |
| Run is over 150 ft; pulling through multiple conduit bends. | 1/0 AWG Copper THHN | Copper has a smaller physical diameter, making it vastly easier to pull through 1.5-inch or 2-inch PVC conduit over long distances. |
| Terminating in tight, cramped meter bases with small bend radii. | 1/0 AWG Copper SER | Copper is more malleable and less prone to 'spring-back' when bending into tight lug spaces. |
The Default Pick: For 90% of standard residential meter-to-panel replacements under 100 feet, buy Southwire or Cerro Wire 2/0-2/0-2/0-1/0 AL 600V SER Cable. Expect to pay between $4.50 and $6.00 per foot in 2026. Ensure the ground wire is 1/0 AWG (or 2 AWG minimum per NEC 250.122 for a 100A overcurrent device, though 1/0 is standard in pre-packaged SER bundles for mechanical strength).
Common Confusions and Code Mistakes
When ordering and installing 100 amp service cable, DIYers and junior apprentices frequently make three critical errors:
1. Confusing the 90°C Column with the 75°C Rule
As mentioned, buying 1 AWG THHN because the '90°C column says 130A' is a code violation. Always size the wire based on the 75°C column unless your breaker, lugs, and wire are all explicitly rated for 90°C (which is virtually never true for residential main panels).
2. Using SEU Instead of SER for Indoor Routing
SEU (Service Entrance Underground/Unarmored) cable has a bare neutral that wraps concentrically around the two hot legs. While SEU is permitted for some above-ground service entrance applications, SER (Round) is required when the cable is routed indoors through framing or used as a subpanel feeder, because SER provides a dedicated, separate equipment grounding conductor.
3. Ignoring Torque Specifications
Aluminum expands and contracts more than copper under thermal cycling. If you do not torque the lugs to the manufacturer's exact specification (typically 250 to 300 inch-pounds for 2/0 Al on a 100A Square D or Eaton main breaker), the connection will loosen over time. A loose 100A aluminum lug will arc, pit, and eventually cause a panel fire. Always use a calibrated dial torque screwdriver, not a standard ratchet.
FAQ: Sizing and Code Questions
Can I use 2 AWG aluminum for a 100 amp service?
No. 2 AWG aluminum is only rated for 90 amps in the 75°C column. You must use a minimum of 2/0 AWG aluminum for a 100-amp overcurrent device.
Do I need a neutral wire for a 100 amp service?
Yes. A standard residential service requires two hot legs (120V each to neutral, 240V phase-to-phase) and a neutral to carry the unbalanced 120V load. You also need a separate equipment ground, making it a 4-wire system.
What size conduit do I need for 100 amp service?
If pulling individual 2/0 AWG THHN conductors (two hots, one neutral, one ground), NEC Chapter 9 fill tables dictate a minimum of 1.5-inch Schedule 80 PVC for underground runs, or 1.5-inch EMT for indoor surface runs. If using pre-packaged SER cable, conduit is generally not used except for physical protection sleeves where the cable passes through concrete or underground.
Sizing a 100 amp service cable correctly bridges the gap between abstract NEC tables and a safe, functional electrical system. By defaulting to 2/0 aluminum for short runs or 1/0 copper for long conduit pulls, respecting the 75°C termination limit, and torquing every lug to spec, you ensure the service will operate safely for decades without a second trip to the electrical supply house.
