The Direct Answer: 100 amp SER (Service Entrance Round) cable is a multi-conductor, unarmored feeder cable rated for 100 amps, typically containing two insulated hot wires, one insulated neutral, and a bare ground, used to supply main panels or large subpanels. For a standard 100A subpanel feeder under 100 feet, your default pick is 1/0-1/0-1/0-2 AWG Aluminum SER.
What 100 Amp SER Cable Actually Is (and Isn't)
When you pull 100 amp SER cable off the spool, you are handling a flat or round assembly of THHN/THWN-2 conductors bundled inside a sunlight-resistant, flame-retardant PVC or XLPE jacket. What this cable changes in a real installation is your physical routing freedom: unlike individual THHN wires that require continuous conduit, or UF-B cable that is limited to direct burial, SER cable can be run through wall cavities, across joists, and surface-mounted in exposed basement areas without the need for metal or PVC raceways, provided it is properly secured.
The most common and costly mistake DIYers make is confusing SER with SEU (Service Entrance Unarmored). SEU cable contains two insulated hot conductors and a bare, stranded neutral that wraps concentrically around the hots. SEU has no separate equipment grounding conductor and is strictly limited to service entrance use (from the weatherhead to the main disconnect). SER cable, by contrast, includes an insulated white neutral and a dedicated bare copper or aluminum ground wire, making it legal and safe for use as a feeder to a subpanel where neutral and ground must remain isolated.
Another frequent mix-up is assuming the NEC 310.12 service conductor sizing rules apply to subpanel feeders. NEC 310.12 allows service entrance conductors to be sized at 83% of the service rating (meaning an 83A ampacity wire can feed a 100A main service). However, if you are running a 100A feeder from your main panel to a detached garage subpanel, the 83% rule does not apply. You must size the feeder for the full 100A load (or 125A if the load is continuous, which is rare in residential settings).
The Math: Ampacity, Temperature Columns, and Voltage Drop
To safely carry 100 amps, we must look at the 75°C column of NEC Table 310.16. Even though the THWN-2 insulation inside modern SER cable is rated for 90°C, standard residential breakers and panel lugs are only rated for 75°C. Therefore, the 75°C column dictates our baseline ampacity.
- 2 AWG Copper: 115A at 75°C. (Sufficient for 100A).
- 1/0 AWG Aluminum: 120A at 75°C. (Sufficient for 100A).
- 1 AWG Aluminum: 100A at 75°C. (Technically meets the bare minimum, but leaves zero margin for voltage drop or ambient temperature derating).
Worked Numeric Example: Voltage Drop at 100 Feet
Let us calculate the voltage drop for a 240V, 100A load located 100 feet from the main panel. The NEC recommends keeping voltage drop under 3% for feeders.
Scenario A: 2 AWG Copper SER
Using NEC Chapter 9, Table 8, the resistance for uncoated 2 AWG copper is 0.194 ohms per 1,000 feet.
Voltage Drop = (2 × Current × Length × Resistance) / 1,000
Voltage Drop = (2 × 100A × 100ft × 0.194) / 1,000 = 3.88 Volts.
Percentage = (3.88 / 240) × 100 = 1.61%. (Well within the 3% limit).
Scenario B: 1/0 AWG Aluminum SER
The resistance for 1/0 AWG aluminum is 0.200 ohms per 1,000 feet.
Voltage Drop = (2 × 100A × 100ft × 0.200) / 1,000 = 4.00 Volts.
Percentage = (4.00 / 240) × 100 = 1.66%. (Also well within the 3% limit).
Bench Tip: If your run exceeds 120 feet, 1/0 Aluminum will push past the 2% optimal threshold for sensitive electronics. At 150 feet, step up to 2/0 AWG Aluminum SER to maintain voltage stability under heavy inductive loads like well pumps or HVAC compressors.
Where You Meet This in Practice
Handling 100 amp SER cable on a jobsite requires respecting its physical stiffness and the specific manufacturer routing specifications governed by NEC Article 339.
Stapling and Support: SER cable must be secured within 12 inches of every panel box or junction box, and at intervals not exceeding 4.5 feet along the run. Because 1/0 Aluminum SER is roughly 0.75 inches thick and highly rigid, standard 1-inch plastic cable staples will crush the jacket or fail to seat. You must use heavy-duty, UV-rated screw-down cable ties or wide-crown metal staples specifically rated for SE cable.
Bending Radius: You cannot fold SER cable at sharp 90-degree angles into a panel knockout. NEC 339.24 mandates a minimum bending radius of five times the diameter of the cable. For a 1/0 SER cable with an overall diameter of roughly 1.1 inches, you need a minimum sweep radius of 5.5 inches. If you force a tighter bend, you risk fracturing the internal aluminum conductors or compromising the dielectric insulation, leading to a high-resistance fault inside the wall.
Framing Penetrations: When drilling through wooden studs or joists, the edge of the bored hole must be at least 1.25 inches from the nearest edge of the framing member. If your framing does not allow this clearance, you are required by code to install a steel nail plate over the stud face to prevent drywall screws or trim nails from piercing the SER jacket and shorting the 100A hots to the ground.
Decision Tree: Picking Your Exact 100A SER Part Number
Use this decision matrix to finalize your material order. Do not default to copper unless your run is exceptionally long or your panel lugs specifically forbid aluminum (which is rare in modern load centers rated for 75°C).
| Installation Condition | Material Choice | AWG Size | Concrete Part Pick / Spec |
|---|---|---|---|
| Run is UNDER 100 feet, standard indoor residential subpanel. | Aluminum | 1/0 AWG Hots/Neutral, 2 AWG Ground | 1/0-1/0-1/0-2 AL SER (e.g., Southwire #203805 or Cerro equivalent) |
| Run is 100 to 140 feet, standard indoor residential subpanel. | Aluminum | 2/0 AWG Hots/Neutral, 1 AWG Ground | 2/0-2/0-2/0-1 AL SER (Steps up to mitigate voltage drop) |
| Run is OVER 140 feet, or pulling through multiple tight conduit sweeps. | Copper | 1 AWG Hots/Neutral, 3 AWG Ground | 1-1-1-3 CU SER (Copper is more flexible and has lower resistance per foot) |
| Feeding a 100A Main Service Drop (Utility to Main Disconnect). | Aluminum | 2 AWG Hots/Neutral, 4 AWG Ground | 2-2-2-4 AL SER (Permitted by NEC 310.12 83% rule for services only) |
The Default Recommendation: For 90% of DIYers and contractors adding a 100A subpanel to a detached garage, workshop, or basement addition within 100 feet of the main panel, buy 1/0-1/0-1/0-2 AWG Aluminum SER cable. It costs roughly $3.00 to $4.00 per foot (compared to $9.00+ for copper equivalents), terminates cleanly in 75°C rated lugs with proper anti-oxidant paste, and easily passes inspection without requiring continuous conduit.
Frequently Asked Questions
Can I run 100 amp SER cable outdoors or underground?
No. While the outer jacket of SER cable is typically sunlight-resistant (marked SUN RES on the printing), it is not rated for direct burial or continuous wet locations. If you need to run a 100A feeder underground to a detached structure, you must either switch to URD (Underground Residential Distribution) cable, use individual THWN-2 wires inside buried PVC conduit, or transition from indoor SER to conduit before exiting the building.
Do I need to use anti-oxidant paste on aluminum SER terminations?
Yes. Whenever you terminate aluminum conductors into panel lugs, you must coat the stripped wire ends with a UL-listed anti-oxidant compound (such as Noalox or Penetrox). Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates heat under load. Furthermore, you must torque the breaker and neutral bar lugs to the exact inch-pound specification printed on the panel label using a calibrated torque screwdriver; aluminum is highly susceptible to cold flow and will loosen over time if under-torqued.
Why does my 100A SER cable only have a 2 AWG ground instead of 1/0?
This is standard and code-compliant. The equipment grounding conductor (EGC) does not carry current during normal operation; it only needs to handle fault current long enough to trip the breaker. NEC Table 250.122 dictates that for a 100A overcurrent device, the minimum copper ground is 8 AWG, and the minimum aluminum ground is 6 AWG. Manufacturers include a 2 AWG aluminum ground in 1/0 SER cable to provide a robust mechanical ground that exceeds the bare minimum, while saving weight and cost compared to matching the hot conductor size.
