A 100 amp SE (Service Entrance) cable is a multi-conductor electrical assembly specifically rated to carry 100 amperes of current from a utility drop to a main service panel or from a main panel to a subpanel. If you are wiring a standard 100-amp residential main service, NEC 310.12(A) permits 1/0 AWG aluminum or 2 AWG copper; however, if you are running a 100-amp feeder to a subpanel, NEC 310.16 requires 1 AWG aluminum or 3 AWG copper based on the 75°C termination column.
The Core Confusion: Main Service vs. Subpanel Feeder
In a real installation, whether you classify the 100 amp SE cable as a "main dwelling service" versus a "subpanel feeder" changes the minimum required wire gauge by a full AWG size, altering both the physical pulling tension and the material cost. This is the most common point of failure in DIY electrical planning.
Under NEC Article 310.12(A), single-phase dwelling services and feeders that supply the entire dwelling load are granted an ampacity bonus. This allows 1/0 AWG aluminum (normally rated for 120A) and 2 AWG copper (normally rated for 115A) to be used for a 100A main service. But if that same 100-amp cable is only feeding a detached garage subpanel that doesn't serve the whole house, that bonus disappears, and you must size strictly to the 75°C column in Table 310.16.
| Application | NEC Article | Min. Aluminum AWG | Min. Copper AWG | Equipment Ground Required? |
|---|---|---|---|---|
| Main Dwelling Service (Entire House) | 310.12(A) | 1/0 AWG | 2 AWG | No (Neutral acts as EGC at main disconnect) |
| Subpanel Feeder (Garage, Workshop) | 310.16 | 1 AWG | 3 AWG | Yes (Separate EGC required, neutral isolated) |
SE-R vs. SE-U and the Underground Trap
Builders and DIYers most commonly confuse SE cable with USE (Underground Service Entrance) cable, assuming the "S" stands for underground, or they confuse the two physical styles of SE cable. Understanding what people commonly confuse it with prevents catastrophic installation failures.
When buying above-ground SE cable, you will encounter two variants:
- SE-R (Round): Contains individually insulated hot conductors, an insulated neutral, and a bare equipment grounding conductor (EGC). This is mandatory for subpanel feeders because the neutral and ground must remain isolated at the subpanel.
- SE-U (Unarmored/Style U): Contains insulated hot conductors wrapped by a bare, concentric neutral wire. There is no separate ground wire. This is only permitted for main service drops where the neutral and ground are bonded at the main disconnect. Do not use SE-U for subpanels.
Where You Meet This in Practice
You will typically specify and pull a 100 amp SE cable in three specific residential or light-commercial scenarios:
- Upgrading an Older Main Panel: Replacing a 60-amp fuse box in a mid-century home with a modern 100-amp main breaker panel. Here, you use 1/0 AWG Aluminum SE-U from the weatherhead to the main lugs.
- Detached Garage Subpanels: Supplying a 100-amp subpanel in a detached garage for welders, air compressors, and lighting. This requires 1 AWG Aluminum SE-R, plus a separate ground rod at the garage.
- EV Charger Infrastructure Prep: While a single Level 2 EV charger only needs 60 amps, installing a 100-amp feeder to a junction box allows for future-proofing dual-charger setups or integrating a smart load-management panel.
Worked Numeric Example: Voltage Drop on a 150-Foot Run
Ampacity tables tell you what size wire prevents a fire, but they don't tell you if your equipment will actually run. Let's calculate the voltage drop for a 100-amp subpanel feeder located 150 feet from the main panel, using 1/0 AWG Aluminum SE-R (upsized from the minimum 1 AWG to mitigate drop).
The Parameters:
- Load Current (I): 80 Amps (Assuming an 80% continuous load for safety margin)
- One-Way Length (L): 150 feet
- Conductor Material: Aluminum (K = 21.2 ohms per mil-foot at 75°C)
- Circular Mils (CM) for 1/0 AWG: 105,600 CM
- System Voltage: 240V
The Formula:
Voltage Drop (VD) = (2 × K × I × L) / CM
The Math:
VD = (2 × 21.2 × 80 × 150) / 105,600
VD = 508,800 / 105,600
VD = 4.81 Volts
The Result:
Percentage Drop = (4.81V / 240V) × 100 = 2.0%.
NEC 210.19(A)(Informational Note) recommends a maximum 3% drop for feeders. At 2.0%, this 1/0 AWG aluminum run is highly efficient and well within code recommendations. If we had used the minimum 1 AWG (83,690 CM), the drop would have been 6.07V (2.5%), which is still acceptable but leaves less margin for future load expansion.
Frequently Asked Questions
Can I use 100 amp SE cable underground in PVC conduit?
No. Standard SE (Service Entrance) cable is only rated for dry, above-ground locations. Even if you pull it through a sealed PVC conduit underground, condensation will eventually form inside the pipe and degrade the cable jacket. For underground runs, you must use individual THWN-2 conductors or direct-burial rated USE-2 (Underground Service Entrance) cable. Always check the manufacturer datasheet for the specific moisture rating of the jacket.
Does a 100 amp SE cable need a separate ground wire?
It depends entirely on the application. If the SE cable is feeding the main service disconnect (where neutral and ground are bonded), you can use SE-U style cable, which uses the concentric bare neutral wrap as the combined neutral/ground path. However, if you are feeding a subpanel, NEC 250.32 requires a separate Equipment Grounding Conductor (EGC). In this case, you must buy SE-R cable, which includes an insulated neutral and a dedicated bare ground wire, or pull a separate ground wire alongside the SE cable.
How much does 100 amp SE cable cost per foot?
As of early 2026, commodity copper and aluminum pricing fluctuates, but you can expect to pay between $3.50 and $5.00 per foot for 1/0-1/0-1/0-2 AWG Aluminum SE-R cable, and roughly $12.00 to $16.00 per foot for the equivalent 2-2-2-4 AWG Copper SE-R. Aluminum is the industry standard for 100-amp service entrances due to the massive cost savings, provided the terminations are properly torqued and treated with antioxidant paste.
