For a 100-amp residential service entrance, the correct cable is 2-2-2-4 Aluminum SER, but for a 100-amp subpanel feeder, you must upgrade to 1-1-1-3 Aluminum SER. SER (Service Entrance Rated) cable is a round, multi-conductor cable assembly containing insulated hot and neutral conductors wrapped around a bare ground wire, specifically listed for use as service-entrance conductors, feeders, and branch circuits. Selecting the correct SER gauge dictates your panel lug termination torque, the minimum bending radius at the service head, and the physical knockout size needed at the meter base. Homeowners and DIYers commonly confuse SER with SEU (Service Entrance Underground) cable, or mistakenly size aluminum conductors using the copper ampacity columns in the National Electrical Code (NEC).
SER Cable Sizing Table for 100A and Common Feeder Sizes
The table below outlines the standard SER cable configurations for residential services and feeders. These values assume copper or aluminum conductors with 75°C insulation ratings (like XHHW-2 or THHN) terminated in standard 75°C rated panel lugs, as outlined in NEC Table 310.16.
| Target Ampacity | Aluminum SER Size (AWG) | Copper SER Size (AWG) | 75°C Ampacity (Al) | 75°C Ampacity (Cu) | Typical Application |
|---|---|---|---|---|---|
| 100A (Service) | 2-2-2-4 | #4 (Rarely used) | 90A* | 85A* | Main dwelling service entrance (NEC 310.12) |
| 100A (Feeder) | 1-1-1-3 | #3 | 100A | 100A | Subpanel feeder, detached garage, workshop |
| 125A | 1/0-1/0-1/0-2 | #2 | 120A | 115A | Large subpanels, EV charger dedicated feeds |
| 150A | 2/0-2/0-2/0-1 | #1 | 135A | 130A | Upgraded residential service, small commercial |
| 200A | 4/0-4/0-4/0-2/0 | 2/0 | 180A* | 175A* | Standard modern 200A main dwelling service |
The Math Behind the 100A SER Sizing (Worked Example)
Let us run a worked numeric example to prove why 1-1-1-3 Aluminum SER is required for a 100A subpanel feeder, and verify the voltage drop over a realistic distance.
Scenario: You are running a 100A feeder from your main house panel to a detached garage subpanel located 80 feet away. The load is continuous and non-continuous mixed, protected by a 100A breaker.
- Base Ampacity Requirement: Because this is a feeder and not a main service entrance, NEC 310.12 does not apply. The conductors must have an ampacity of at least 100A. Looking at the 75°C column of NEC Table 310.16, #2 AWG Aluminum is only rated for 90A. #1 AWG Aluminum is rated for exactly 100A. Therefore, 1-1-1-3 SER is the minimum legal size.
- Voltage Drop Calculation: While the NEC recommends keeping voltage drop under 3% for feeders, it is not strictly mandatory unless specified by local AHJ. Let us calculate the drop for #1 AWG Aluminum over 80 feet at a full 100A load.
- Formula:
VD = (2 × K × I × L) / CM - K (AC resistance for Al at 75°C) ≈ 21.2
- I (Current) = 100A
- L (One-way length) = 80 ft
- CM (Circular Mils for #1 AWG) = 83,690
- VD = (2 × 21.2 × 100 × 80) / 83,690 = 4.05 Volts
- Formula:
- Percentage Drop: On a 240V system, 4.05V / 240V = 1.68%. This is well under the 3% NEC recommendation, confirming that 1-1-1-3 Aluminum SER is both code-compliant for ampacity and electrically efficient for an 80-foot run.
Where You Meet SER Cable in Practice
Theory and code tables only get you to the hardware store. On the jobsite, handling SER cable introduces physical constraints that dictate your installation success.
- Jacket Stripping: SER cable features a tough, sunlight-resistant PVC jacket. It includes a dedicated rip-cord (a piece of high-tensile string) inside the jacket. Pulling this cord with lineman pliers slices the jacket cleanly down to the panel without nicking the XHHW-2 insulation underneath—a common failure point when using a utility knife.
- Managing the Bare Ground: In standard SER, the bare ground wire is not a single solid core; it is a spiral wrap of smaller bare strands surrounding the insulated conductors. When terminating, you must twist these strands tightly into a single pigtail and secure them under the ground bar lug. Leaving strands splayed out can cause arcing or result in a failed inspection.
- Bending Radius: NEC Article 300.34 mandates minimum bending radii for shielded and metallic-sheathed cables, but for non-shielded SER, the practical rule of thumb is a bending radius of at least 5 times the overall diameter of the cable. For 1-1-1-3 SER (overall diameter roughly 1.1 inches), you need a minimum sweep of 5.5 inches. Forcing a tight 90-degree bend right at the panel knockout will stress the internal insulation and deform the conductor.
- Termination Torque: Aluminum expands and contracts more than copper under thermal cycling. If you do not torque the lugs correctly, the connection will loosen over time, creating a high-resistance hot spot that can melt the breaker lug. For #1 AWG Aluminum in standard 100A residential panels (like Square D Homeline or Siemens EQ), the manufacturer typically specifies 40 in-lbs (3.4 Nm). Always verify the exact torque value printed on the panel's wiring diagram label and use a calibrated inch-pound torque screwdriver.
SER vs. SEU and Other Common Mix-Ups
The most frequent mistake DIYers make when buying service cable is confusing SER with SEU. While both are rated for service entrance use, their internal geometry and code applications are vastly different.
| Feature | SER (Service Entrance Round) | SEU (Service Entrance Underground) |
|---|---|---|
| Physical Shape | Round | Flat |
| Neutral Conductor | Insulated (White or Gray) | Bare concentric strands wrapping the hots |
| Ground Conductor | Bare wire in the center | None (Neutral acts as ground at service) |
| Subpanel Use | Permitted (Has isolated neutral & ground) | Prohibited (No separate ground, neutral cannot be isolated) |
| Typical Use Case | Feeders, subpanels, service drops | Meter to main panel (above ground only) |
If you are running power to a detached garage, you must use SER. Subpanels require the neutral and ground to be physically isolated on separate bus bars. Because SEU lacks an insulated neutral and a dedicated ground wire, using it for a subpanel would force you to bond the neutral to the ground at the subpanel, creating a dangerous parallel neutral path that can electrify the grounding system.
Frequently Asked Questions
Can I run 100A SER cable inside PVC conduit?
Yes, but it is generally avoided because SER is designed to be run as an exposed cable assembly or fished through framing cavities. If you must transition through conduit (for example, running down an exterior wall to a buried trench), you must calculate conduit fill. A 1-1-1-3 SER cable requires a minimum of 1.5-inch Schedule 80 PVC conduit to avoid jamming, though 2-inch is highly recommended to make the physical pull manageable without damaging the jacket.
Why do big box stores sell 2-2-2-4 as '100 Amp SER'?
Retailers label 2-2-2-4 Aluminum as '100A' because it is the universal standard for a 100-amp main service entrance drop from the utility meter to the main panel, leveraging the NEC 310.12 83% rule. However, if you are feeding a subpanel, the breaker protecting that cable cannot exceed the wire's 75°C ampacity rating, which is 90A for #2 Aluminum. Always buy based on the specific application (service vs. feeder), not the retail box label.
Do I need to use anti-oxidant paste (Noalox) on aluminum SER?
While modern XHHW-2 and THHN aluminum alloys (AA-8000 series) are far more stable than the older AA-1350 aluminum used in the 1970s, applying a thin layer of anti-oxidant compound (like Noalox or Penetrox) to the stripped aluminum conductor before terminating it in a mechanical lug is still considered best practice. It prevents micro-oxidation at the lug interface, ensuring long-term thermal stability. Note that some modern breaker manufacturers explicitly state in their installation instructions that anti-oxidant paste is not required; always defer to the specific panel manufacturer's instructions.
