100 amp SER (Service Entrance Rated) wire is a flat, multi-conductor cable assembly containing two insulated hot conductors, one insulated neutral, and a bare concentric ground wire, specifically designed to feed 100-ampere main service panels or subpanels without requiring a continuous conduit run. When sizing for a standard 100A breaker, you will use #3 AWG Copper or #1 AWG Aluminum based on the 75°C column of the NEC ampacity tables, though many installers step up to 1/0 AWG Aluminum to mitigate voltage drop on longer runs.

Mains Voltage Warning: Working inside a main service panel or routing service entrance conductors involves lethal voltage. Always de-energize the upstream source, use a tested non-contact voltage tester and a multimeter to verify dead circuits, and wear appropriate PPE. Local codes may require a licensed electrician for service entrance work; always defer to your local Authority Having Jurisdiction (AHJ).

What 100 Amp SER Wire Changes in Your Installation

Choosing SER cable over individual THHN/THWN conductors pulled through PVC or EMT conduit fundamentally changes the physical routing, support requirements, and cost structure of your installation. Because SER features a flame-retardant, moisture-resistant overall jacket, the National Electrical Code (NEC) permits it to be run exposed along framing members, eliminating the need to glue, strap, and pull wire through hundreds of feet of rigid conduit.

Electrically, SER changes your grounding topology. The bare concentric wire wrapped around the insulated conductors serves as your Equipment Grounding Conductor (EGC), while the insulated white wire is strictly your neutral. This is a critical distinction when feeding a subpanel: at the subpanel, the neutral and ground must remain isolated, meaning you will terminate the white wire to the neutral bar and the bare concentric wire to the ground bar.

What people commonly confuse it with: SER is frequently mistaken for USE-2 (Underground Service Entrance) or Mobile Home Feeder (MHF). USE-2 is rated for direct burial and wet locations but lacks the indoor flame-retardant jacket required for interior runs. Conversely, SER is rated for indoor use but will fail inspection if buried directly in the dirt. MHF looks similar but is specifically listed for mobile home installations and often features a different jacket color or striping.

Ampacity and Sizing Table for 100A SER Cable

Panel lugs are almost universally rated for 75°C terminations. Even if your SER cable insulation is rated for 90°C (which allows for derating adjustments in hot attics), the final ampacity at the termination point cannot exceed the 75°C column. Below is the standard sizing matrix for 100A applications based on NFPA 70 (NEC) Table 310.16.

Cable Designation Conductor Material AWG Configuration (Hot-Hot-Neutral-Ground) 75°C Ampacity (Termination Limit) 90°C Ampacity (Derating Base) Approx. O.D. (inches)
100A Cu SER Copper 3-3-3-5 100A 115A 0.95"
100A Al SER Aluminum 1-1-1-3 100A 115A 1.05"
120A Al SER (Common Upgrade) Aluminum 1/0-1/0-1/0-2 120A 135A 1.15"
125A Cu SER Copper 1-1-1-3 130A 150A 1.10"
Pro Tip: Aluminum 1/0-1/0-1/0-2 SER is the most commercially available and cost-effective choice for 100A runs. While #1 AWG Al is technically sufficient for 100A, stepping up to 1/0 AWG costs only marginally more, provides a 120A buffer, and drastically reduces voltage drop on runs over 50 feet.

Worked Numeric Example: Voltage Drop on a 150-Foot Run

Ampacity tells you if the wire will melt; voltage drop tells you if your appliances will actually run. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders. Let’s calculate the voltage drop for a 100A subpanel fed by 1/0 AWG Aluminum SER over a 150-foot one-way distance, carrying a realistic continuous load of 80 Amps at 240V.

The Formula:
Voltage Drop (VD) = (2 × K × I × D) / CM

  • K (Specific Resistance for Aluminum) = 21.2 ohms per mil-foot
  • I (Current) = 80 Amps
  • D (One-way Distance) = 150 feet
  • CM (Circular Mils for 1/0 AWG) = 105,600 (per NEC Chapter 9, Table 8)

The Calculation:
VD = (2 × 21.2 × 80 × 150) / 105,600
VD = 508,800 / 105,600
VD = 4.81 Volts

The Percentage:
(4.81V / 240V) × 100 = 2.00%

Because 2.00% is well under the 3% NEC recommendation, 1/0 AWG Aluminum SER is an excellent, code-compliant choice for this 150-foot run. If we had used #1 AWG Aluminum (CM = 83,690), the drop would be 6.08V (2.53%)—still acceptable, but leaving less margin for heavy motor starting currents.

Where You Meet SER Cable in Practice

When you are actually pulling this cable through a jobsite, the physical properties of SER dictate how you handle it. According to Southwire's SER installation guidelines and NEC Article 339, you must adhere to specific mechanical protections.

  • Stapling and Support: SER must be secured within 12 inches of every panel or junction box, and at intervals not exceeding 4.5 feet (54 inches) along the run. Use wide-crown cable staples specifically sized for the O.D. of your SER to avoid crushing the jacket and damaging the insulation beneath.
  • Bending Radius: You cannot kink SER cable. The NEC mandates a minimum bending radius of five times the overall diameter of the cable. For a 1.15-inch 1/0 SER, your sweeps and bends must have a radius of at least 5.75 inches. Use your hands to form sweeping curves rather than sharp 90-degree folds.
  • Conduit Transitions: While SER doesn't need continuous conduit, it must be protected by conduit where it is subject to physical damage. If you are dropping down a wall to a subpanel in a garage or basement, you must transition into PVC or EMT conduit for the bottom 6 to 8 feet. Use a proper SER cable connector (not a standard Romex clamp) where the jacket enters the conduit or panel to ensure a secure, code-compliant grip.
Never Bury SER Directly: SER cable is not rated for wet locations or direct burial. If your 100A run goes underground, you must either use USE-2/RHH-RHW-2 individual conductors in buried PVC conduit, or run the SER through a continuous, sealed conduit system underground. Burying bare SER will result in jacket degradation, moisture wicking into the panel, and an inevitable failed inspection.

Frequently Asked Questions

Can I use 100 amp SER wire to feed a detached garage subpanel?
Yes, but only for the interior runs. For the underground trench between the main house and the garage, you must transition to individual THWN-2 wires inside buried Schedule 40 PVC, or use direct-burial rated cable like URD or USE-2 in conduit. Once inside the garage, you can transition back to SER to route up the wall to the subpanel.

Do I need to separate the neutral and ground on SER at a subpanel?
Absolutely. The bare concentric wire is your ground, and the insulated white wire is your neutral. At any subpanel, the neutral bar must be isolated from the panel enclosure, while the ground bar must be bonded to the enclosure. Terminating both to the same bar creates a parallel neutral path, which is a severe shock hazard and a code violation.

Can I run SER through insulation in an attic?
Yes, SER is rated for interior use and can be buried in blown-in or batt insulation. However, if the attic ambient temperature regularly exceeds 86°F (30°C), you must apply the 90°C temperature correction factors from NEC Table 310.16 to ensure the derated ampacity still meets your 100A requirement. This is where the 90°C rating of the wire's insulation saves you, even though the terminations are limited to 75°C.