SER Cable for 100 Amp Service: Sizing, Specs, and NEC Rules

SER (Service Entrance Round) cable is a factory-assembled, multi-conductor cable with a flame-retardant PVC outer jacket, designed to carry main utility power to a residential electrical panel. Choosing the correct SER cable for 100 amp service changes your physical routing method by eliminating the need for continuous conduit in interior runs, sets your maximum continuous load limit based on termination temperature ratings, and dictates the specific lug torque values required at the panel. Most commonly, DIYers and junior installers confuse SER with USE (Underground Service Entrance, which lacks an indoor flame-retardant jacket) or NM-B (Romex, which is limited to 60°C terminations and is not rated for service entrance).

The Exact Pick: SER Cable for 100 Amp Service

If you need a direct answer for a standard 100-amp residential panel or subpanel feeder operating at 120/240V split-phase, here is the exact material you need to buy. Do not undersize based on outdated internet forums.

• Default Concrete Pick: 1-1-1-3 Aluminum SER (1 AWG Hots, 1 AWG Neutral, 3 AWG Bare Ground)
• Alternative (Copper): 3-3-3-5 Copper SER (3 AWG Hots, 3 AWG Neutral, 5 AWG Bare Ground)
• Insulation Rating: XHHN-2 or THHN/THWN-2 (90°C wire, but limited to 75°C at terminations)
• Estimated Cost (2026): ~$2.20 to $2.80 per foot for 1-1-1-3 AL SER

Decision Path: Which SER Cable Should You Pull?

Use this decision tree to confirm your exact pick based on your specific installation constraints. Follow the logic down to your terminating row.

Where You Meet SER Cable in Practice

You will encounter SER cable in three primary scenarios on a jobsite or in a heavy DIY retrofit:

1. Meter Base to Main Panel: The classic service entrance drop. The utility terminates at the meter; the SER cable routes through the exterior wall directly into the top of the 100A main breaker panel.
2. Main Panel to Subpanel: Feeding a 100A subpanel in a detached garage or a large shop addition. (Note: For detached structures, you must separate the neutral and ground bars at the subpanel, and the SER ground serves as your equipment grounding conductor).
3. Heavy Appliance Feeds: While less common for single appliances, large 100A electric tankless water heaters or heavy commercial HVAC units in residential settings sometimes require a dedicated 100A SER feed from a subpanel.

Worked Numeric Example: Voltage Drop and Ampacity

Let's run the actual math for a 100A load using our default pick (1-1-1-3 Aluminum SER) over a 100-foot one-way run. This proves why 1 AWG is required and why 2 AWG fails the math.

Step 1: Verify Ampacity (NEC Table 310.16)
According to the NEC ampacity tables, 1 AWG Aluminum at the 75°C column is rated for exactly 100 Amps. (Many DIYers mistakenly buy 2 AWG Aluminum, but 2 AWG is only rated for 90A at 75°C. Putting 2 AWG on a 100A breaker is a direct NEC violation).

Step 2: Calculate Voltage Drop
We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM

• K (Constant for Aluminum) = 21.2
• I (Current) = 100A
• D (Distance) = 100 feet
• CM (Circular Mils for 1 AWG) = 83,690

VD = (2 × 21.2 × 100 × 100) / 83,690
VD = 424,000 / 83,690 = 5.06 Volts

Step 3: Check Against NEC Recommendations
5.06V on a 240V system is a 2.1% voltage drop. The NEC recommends a maximum of 3% for branch circuits and 5% for the total feeder + branch combined. At 2.1%, your 1-1-1-3 AL SER is well within safe operating limits for a 100-foot run. If your run exceeds 140 feet, you would need to step up to 1/0 AWG Aluminum to keep the drop under 3%.

Aluminum vs. Copper: The Cost and Termination Reality

When sizing SER cable for 100 amp service, the material choice drastically impacts your budget and your termination technique. While copper is the historical gold standard, modern aluminum alloys (AA-8000 series) are the undisputed standard for service entrance feeders.

The data is clear: unless you are running a very short distance where the price difference is negligible, Aluminum SER is the correct choice. The critical failure mode with aluminum is failing to use an oxide inhibitor (like Noalox) and failing to torque the lugs to the manufacturer's exact spec. Aluminum forms a non-conductive oxide layer when exposed to air, and it expands/contracts more than copper under load. If you just strip it and shove it in the lug, it will eventually loosen, arc, and melt the panel bus bar.

Common Confusions: SER vs. USE vs. NM-B

Misidentifying cable types is the fastest way to fail an electrical inspection or start a fire. Here is how to distinguish SER from its lookalikes.

SER vs. USE (Underground Service Entrance)

USE cable looks almost identical to SER, but it lacks the flame-retardant PVC outer jacket. USE is rated for direct burial and wet locations. If you run USE cable inside your home from the meter to the panel, it is a fire hazard and a direct violation of NEC Article 338. Conversely, you cannot bury standard SER cable directly in the dirt; it must be run through PVC or rigid conduit if it transitions underground.

SER vs. NM-B (Romex)

NM-B is the standard interior wiring cable (Romex). People sometimes ask if they can use two runs of 3 AWG NM-B to feed a 100A panel. The answer is a hard no. NM-B is strictly limited to the 60°C ampacity column by NEC 334.80, meaning a 3 AWG copper NM-B is only good for 85A. Furthermore, NM-B is not rated for use as a service entrance conductor. SER's XHHN-2 insulation and specific assembly allow it to legally and safely handle the 75°C termination limits of a 100A main breaker.