100 amp SEU (Service Entrance Unarmored) cable is a flat, multi-conductor assembly featuring two insulated phase wires and a bare concentric neutral, specifically rated to carry a continuous 100-ampere load from the utility meter to the main distribution panel. In a real installation, choosing SEU dictates your physical routing constraints, panel knock-out fitting requirements, and strictly limits the cable to main service applications where the neutral and ground are bonded at the main disconnect. It is a specialized feeder that solves the utility-to-panel connection but introduces specific bending radius and termination temperature challenges that generic NM-B or THHN-in-conduit setups do not.

The Anatomy and Ampacity of 100 Amp SEU Cable

To size a 100A service entrance correctly, you must look at the conductor material, the insulation temperature rating, and the specific NEC tables governing dwelling services. SEU cable consists of two insulated conductors (typically XHHW-2 or THHN/THWN-2, rated for 75°C or 90°C) and a bare concentric neutral wire wrapped around the core. Because it lacks an armored jacket, it relies on its flat PVC or XLPE outer jacket for physical protection, meaning it must be run through bored holes or along framing, never directly buried.

For residential single-phase dwelling services, NEC Table 310.12(A) provides a specific shortcut for sizing service entrance conductors, bypassing the standard derating calculations required for general branch circuits. Below is the critical spec sheet for a 100A SEU installation.

Parameter Aluminum SEU (Standard) Copper SEU (Premium)
Minimum AWG Size (NEC 310.12) 1/0 AWG #2 AWG
Ampacity @ 60°C Column 100A 95A (Requires 75°C equipment rating)
Ampacity @ 75°C Column 120A 115A
Approx. Overall Diameter 0.95 inches (Flat) 0.68 inches (Flat)
Conductor Configuration 2 Insulated Hots + 1 Bare Neutral 2 Insulated Hots + 1 Bare Neutral
Typical Cost per Foot (2026) $3.50 - $4.50 $9.00 - $12.00
Termination Temperature Rule (NEC 110.14(C)): Even though the XHHW-2 insulation inside SEU cable is rated for 90°C, you must size the wire based on the temperature rating of the terminations. Most modern 100A main breaker lugs are rated for 75°C. However, if you are connecting to an older meter base or a panel explicitly marked for 60°C, you must use the 60°C ampacity column. As shown in the table, #2 Copper only yields 95A at 60°C, meaning you would be forced to upgrade to #1 Copper if your terminations are strictly 60°C rated. Always verify the lug stamping.

Worked Example: Voltage Drop on a 150-Foot 100A Run

Ampacity tables tell you what the wire can handle thermally, but they do not account for voltage drop over distance. If your meter base is on the street-side pole and your main panel is on the rear of a deep property, a 150-foot run of 1/0 AWG Aluminum SEU requires a voltage drop calculation to ensure your 240V nominal service doesn't sag below acceptable limits under heavy load.

The Scenario:
Load: 100 Amps (continuous max)
Voltage: 240V Single-Phase
Length: 150 feet (one-way)
Wire: 1/0 AWG Aluminum SEU
Resistance (R): ~0.20 ohms per 1,000 feet for 1/0 AL at 75°C.

The Math:
The formula for single-phase voltage drop is: VD = (2 × L × I × R) / 1000
VD = (2 × 150 × 100 × 0.20) / 1000
VD = 6,000 / 1000 = 6.0 Volts

The Result:
Percentage Drop = (6.0V / 240V) × 100 = 2.5%.
NEC 210.19(A) Informational Note recommends a maximum of 3% voltage drop for branch circuits and feeders, and a combined 5% for the entire system. At 2.5%, the 1/0 AWG Aluminum SEU is perfectly adequate for this 150-foot run. However, if the run extended to 200 feet, the drop would hit 3.33%, and you would need to upsize to 2/0 AWG Aluminum to maintain power quality for sensitive electronics and HVAC compressors.

Where You Meet SEU in Practice (And Why Flat Matters)

You will almost exclusively encounter SEU cable in the "service drop" or "service lateral" transition—specifically the run from the utility meter socket to the main service disconnect panel. Because SEU is a 3-wire assembly (Line 1, Line 2, Neutral), it relies on the main panel's bonding screw or strap to tie the neutral bus to the ground bus.

The Subpanel Gotcha: A common, dangerous mistake DIYers make is trying to use leftover 100A SEU cable to feed a detached garage subpanel. Under modern NEC rules (Article 250.32), a subpanel requires a 4-wire feed (two hots, a dedicated neutral, and a dedicated equipment grounding conductor) with the neutral and ground isolated. Because SEU only provides three conductors and uses the bare wrap as a combined neutral/ground, you cannot legally use SEU to feed a subpanel in new construction. It is strictly for the main service entrance where the bonding occurs.

Physical Routing Constraints: The flat profile of SEU is great for slipping through narrow stud bays, but it creates headaches at the panel knockouts. You cannot use standard circular conduit connectors. You must use specific SE cable clamp fittings designed to grip the flat jacket without crushing the internal insulation. Furthermore, NEC 334.24 dictates the bending radius for SE cable. You cannot kink it into a sharp 90-degree turn right at the panel hub; the bend radius must not be less than five times the overall diameter of the cable (approx. 4.75 inches for 1/0 AL SEU), requiring a wide, sweeping loop before it enters the knockout.

Common Confusions: SEU vs SER, and SE vs USE

The alphabet soup of service entrance cables causes frequent misorders at the electrical supply house. Confusing these types can result in failed inspections or unsafe underground installations.

What is the difference between SEU and SER cable?

SEU (Service Entrance Unarmored) is flat and contains only three wires: two insulated hots and one bare concentric neutral. SER (Service Entrance Round) is round and contains four wires: two insulated hots, one insulated neutral (usually white), and one bare equipment ground. SER is the correct choice for indoor subpanel feeders because it provides the isolated 4-wire path required by modern code. SEU is restricted to the main service entrance.

Can I bury SEU cable underground?

No. SEU is rated for above-ground, dry locations. If you need to run a service entrance underground (from a pole-mounted transformer to a house foundation), you must use USE-2 (Underground Service Entrance) cable. USE-2 features moisture-resistant, sunlight-resistant XLPE insulation designed for direct burial, but it lacks the flame-retardant jacket required for indoor routing. For runs that transition from underground to indoors, electricians often pull individual THWN-2 conductors inside PVC conduit to satisfy both burial and interior fire-code requirements.

Does the bare neutral in SEU count as a ground?

At the main service disconnect, yes. The utility grounds the neutral at the transformer, and the NEC requires the neutral and ground to be bonded at the first point of disconnect (your main panel). Therefore, the bare concentric wire in SEU serves as both the grounded (neutral) conductor and the grounding path for the main panel enclosure. Downstream of that main breaker, neutral and ground must remain strictly separated.

⚠️ Mains Safety Warning: Working on the line-side of a main breaker involves exposed utility voltage that cannot be shut off by your home's breakers. Before terminating 100A SEU cable, the utility meter must be pulled or the upstream disconnect must be locked out. Always verify dead with a Category III or IV rated multimeter before touching conductors. Local codes frequently require a licensed electrician and utility coordination for service entrance work; this guide serves as NEC-style educational guidance, not a substitute for local AHJ authority.

Understanding the exact physical and electrical boundaries of 100 amp SEU cable prevents costly rework. By respecting the 3-wire limitation, adhering to the 75°C termination rules, and calculating voltage drop for longer runs, you ensure a safe, code-compliant service entrance that will reliably power a modern home's electrical demands.