SER (Service Entrance Round) cable is a multi-conductor, jacketed assembly containing insulated phase conductors, an insulated neutral, and a bare concentric ground wire, designed to carry main power from a meter or main disconnect to a subpanel or large appliance. If you are feeding a standard 100-amp residential subpanel or service disconnect, the direct answer for your wire size is 2 AWG Copper or 1/0 AWG Aluminum SER cable, assuming standard 75°C termination ratings and dwelling service derating rules.
What 100 Amp SER Cable Changes in Your Installation
Choosing SER cable fundamentally changes the physical execution and grounding topology of your feeder run compared to pulling individual THHN/THWN wires through PVC or EMT conduit. In a standard conduit pull, you must individually route two hot wires, one insulated neutral, and one bare or green ground wire. SER cable consolidates this into a single, heavy, round-jacketed cable.
The most distinct feature of SER cable is its concentric neutral/ground. Instead of a single solid or stranded ground wire, the bare copper ground consists of multiple strands wrapped concentrically around the inner insulated conductors, sitting just beneath the outer gray PVC jacket. This design provides excellent fault-current capacity and physical shielding. Because the neutral is fully insulated (typically white or gray) and the ground is bare, SER cable is perfectly suited for feeding subpanels where the neutral and ground buses must remain strictly isolated—a requirement that older, unjacketed SEU cable cannot safely meet in modern installations.
The Math: Sizing 100 Amp SER Cable (Worked Example)
Sizing a 100-amp feeder requires navigating two distinct National Electrical Code (NEC) requirements: ampacity (heat dissipation) and voltage drop (performance over distance). Let us run the exact numbers for a 100-amp subpanel feed located 150 feet from the main panel, supplying a continuous load of 80 amps.
Step 1: Ampacity and the 83% Rule
For single-phase, 120/240V, 3-wire dwelling service entrance conductors, NEC Article 310.12 allows you to apply an 83% derating factor to the service rating.
- 100 Amps × 0.83 = 83 Amps minimum required ampacity.
Looking at the 75°C column of NEC Table 310.16 (the standard temperature rating for most modern breaker lugs):
- 2 AWG Copper: Rated for 115A (Passes, 115A > 83A)
- 1/0 AWG Aluminum: Rated for 120A (Passes, 120A > 83A)
Step 2: Voltage Drop Calculation
While the NEC does not strictly mandate a specific voltage drop for feeders, it strongly recommends keeping it under 3% for optimal performance. We will use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM.
Let us test 1/0 AWG Aluminum at an 80A continuous load over 150 feet:
- K (AC resistance constant for Aluminum) ≈ 21.2
- I (Current) = 80A
- D (Distance) = 150 ft
- CM (Circular Mils for 1/0 AWG) = 105,600
VD = (2 × 21.2 × 80 × 150) / 105,600 = 4.81 Volts
Percentage drop on a 240V line-to-line circuit: (4.81 / 240) × 100 = 2.0%. Because 2.0% is well below the 3% threshold, 1/0 AWG Aluminum is mathematically verified for this 150-foot run without needing to upsize to 2/0 AWG.
Where You Meet This in Practice
You will typically specify 100-amp SER cable in three specific scenarios:
- Detached Structure Subpanels: Feeding a 100A panel in a detached garage, workshop, or barn from the main house panel.
- Main Service Entrance: Routing power from the utility meter base to the main interior disconnect panel.
- Large Appliance Feeds: Supplying heavy commercial-grade equipment or large HVAC disconnects that require a dedicated 100A fused disconnect.
What People Commonly Confuse It With
Beginners and DIYers frequently confuse SER with other feeder cables, leading to code violations:
- SER vs. SEU (Service Entrance Unjacketed): SEU is flat, lacks an overall jacket, and has no insulated neutral (the neutral is the bare concentric strands). SEU is banned for new subpanel feeds because subpanels require a fully insulated neutral and a separate equipment ground. SER has both.
- SER vs. MHF (Mobile Home Feeder): MHF is rated for direct underground burial. SER is not rated for direct burial. If you need to run a 100A feed underground, you must either use MHF, or run your SER cable inside a continuous sweep of Schedule 80 PVC conduit.
- SER vs. THHN in Conduit: THHN requires pulling individual wires through a raceway. SER is a cable assembly that can be stapled directly to framing (where not subject to physical damage) without a continuous raceway, saving significant labor on interior runs.
Decision Path: Copper vs. Aluminum SER for 100 Amps
Choosing between copper and aluminum dictates your budget, the physical bending radius you must accommodate, and the termination hardware you need. Use this decision matrix to lock in your material choice.
| Criteria | 2 AWG Copper SER | 1/0 AWG Aluminum SER |
|---|---|---|
| Material Cost (per 100 ft) | ~$450 - $600 | ~$150 - $220 |
| Weight & Handling | Heavy, stiff, difficult to bend in tight panel gutters | Lighter, more pliable, easier to route |
| Termination Requirements | Standard lugs, no anti-oxidant paste required | Requires Al/Cu rated lugs and anti-oxidant paste (e.g., Noalox) |
| Panel Space | Smaller diameter, takes up less gutter space | Larger diameter, requires adequate gutter depth |
Installation Realities and Code Caveats
Buying the right cable is only half the job. Terminating 100-amp SER cable introduces specific mechanical requirements that, if ignored, lead to thermal failure at the lugs.
Bending Radius and Physical Protection
According to manufacturer specifications and NEC Article 339, the bending radius for SER cable cannot be less than five times the overall diameter of the cable. For 1/0 SER, this means you need sweeping bends, not sharp 90-degree kinks, which can damage the internal insulation and compromise the concentric ground. Furthermore, while SER can be run exposed along interior framing, it must be protected by conduit (like EMT or PVC) wherever it is subject to physical damage, such as running down a wall to a panel in an unfinished garage.
Torque Specifications: The Aluminum Killer
Aluminum expands and contracts more than copper under thermal cycling. If you do not torque the breaker and busbar lugs to the manufacturer's exact specification, the connection will loosen over time, creating a high-resistance fault that will melt the lug and start a fire. Most 100A breakers require between 45 and 50 inch-pounds of torque. Use a calibrated inch-pound torque screwdriver or torque wrench. Never 'guess' the tightness by hand. Always apply a thin layer of Noalox or similar anti-oxidant compound to the stripped aluminum conductors before insertion to prevent galvanic corrosion.
Frequently Asked Questions
Do I need to separate the neutral and ground in a subpanel fed by SER?
Yes. At the subpanel, the white insulated neutral wire lands on the isolated neutral busbar, and the bare concentric ground strands are bundled and landed on the equipment grounding busbar. The bonding screw or strap in the subpanel must be removed.
Can I use 100 Amp SER cable for a 125 Amp breaker?
No. While 1/0 Aluminum is rated for 120A in the 75°C column, the next standard breaker size up is 125A. You cannot use the 'next size up' breaker rule (NEC 240.4(B)) if the calculated load exceeds the conductor ampacity. For a 125A feed, you must step up to 1/0 Copper or 2/0 Aluminum.
How do I strip the concentric ground without nicking the inner wires?
Use a dedicated cable ripper or a specialized SER stripping tool. Score the gray outer jacket longitudinally, peel it back, and use diagonal cutters to carefully trim the bare ground strands. Do not use a standard utility knife to slice deeply into the jacket, as you will inevitably score the insulation of the current-carrying conductors beneath.
For comprehensive code compliance, always cross-reference your local Authority Having Jurisdiction (AHJ) amendments, as local inspectors may have specific preferences for feeder routing and physical protection methods that supersede general NEC guidance.
