SER Cable: The Complete Guide to Service Entrance Round Cable
SER (Service Entrance cable, Round) is the cable type used for high-ampacity feeder applications in residential electrical systems. When you are adding a subpanel to a detached garage, workshop, or addition, feeding a generator transfer switch from the main panel, or running a large feeder circuit for heavy equipment, SER cable is often the most cost-effective and practical wiring method. Here is everything you need to know about SER cable including what is inside it, when to use it, how it compares to alternatives, and how to size it correctly.
What Is Inside SER Cable
SER cable contains two or more insulated current-carrying conductors plus a bare or insulated neutral, all assembled into a round cross-section and enclosed in a tough thermoplastic outer jacket. A typical residential SER configuration for a subpanel feed contains two hot conductors (usually aluminum, black and red insulation), one neutral conductor (usually aluminum, white insulation), and a bare aluminum or copper ground conductor, all within a single round jacket. The “R” in SER specifically designates the round cross-sectional shape, which distinguishes it from SEU (Service Entrance cable, U-configuration, which has a figure-eight cross section with the neutral wrapped around the outside).
Aluminum vs Copper SER
Most SER cable uses aluminum conductors rather than copper. Aluminum is significantly less expensive than copper for large-gauge applications, and for feeder and service entrance use where conductors are terminated at panels and breakers rather than at standard outlets and switches, aluminum is code-compliant and widely accepted. Aluminum conductors in SER must be terminated with connectors and lugs rated for aluminum conductors (marked AL/CU). Direct connection of aluminum conductors to terminals rated for copper only creates a fire hazard through galvanic corrosion and thermal expansion differences. All modern residential panel lugs accept aluminum conductors in the sizes used with SER cable.
Where SER Cable Is Used
Detached Garage and Outbuilding Subpanels
Running a subpanel to a detached garage, workshop, barn, or other outbuilding is one of the most common SER cable applications. A 60-amp subpanel requires 4 AWG SER. A 100-amp subpanel requires 2 AWG SER. A 125-amp subpanel requires 1 AWG SER. The SER cable is typically run in conduit from the main panel to the outbuilding (even though SER can sometimes be run without conduit for service entrance use, conduit protects the cable from physical damage for long outdoor runs and is the recommended approach for feeder runs to outbuildings).
Generator Transfer Switch Feeder
The feeder from the main electrical panel to a whole-house automatic transfer switch is commonly wired with SER cable. The transfer switch receives power from both the utility (through the existing service entrance) and the generator (through the generator output wiring), and the SER feeder connects these components. Generator feeders are sized to match the generator output: a 22kW generator producing approximately 90 amps requires a feeder cable sized for at least 100 amps, typically 2 AWG SER or THHN in conduit.
Main Service Entrance (SEU)
Service entrance cable is also used for the main service entrance conductors from the utility weatherhead down to the main electrical panel. However, for true service entrance applications (from the meter base to the panel), SEU (the U-configuration with the wrapped neutral) or individual service entrance conductors are more commonly specified than SER. Confirm with your utility company and electrician which cable type is approved for your specific service entrance configuration.
SER vs THHN in Conduit: Which to Use
SER cable and individual THHN conductors in conduit both deliver equivalent electrical performance for feeder applications. The choice between them comes down to installation context. SER is faster to install for short feeder runs where a single cable pull is simpler than pulling three or four individual THHN conductors. SER requires no conduit for the cable itself (though conduit is recommended for long outdoor runs for physical protection). THHN in conduit allows future conductor replacement or addition without disturbing the conduit installation. THHN in conduit is the standard for commercial applications. For residential feeder runs under 50 to 75 feet in a protected interior path, SER is often the more economical choice. For longer outdoor runs, conduit with THHN provides better long-term protection.
SER Cable Sizing Chart
For 60-amp subpanel feed: 4 AWG aluminum SER (4-4-4-6 SER configuration). For 100-amp subpanel feed: 2 AWG aluminum SER (2-2-2-4 SER configuration). For 125-amp subpanel feed: 1 AWG aluminum SER. For 150-amp subpanel feed: 1/0 AWG aluminum SER. For 200-amp main service: 4/0 AWG aluminum SER or equivalent. Confirm sizing with a licensed electrician based on the specific panel ratings, load calculations, and local code requirements for your installation.
IB Lighting carries SER cable in the most common residential sizes with nationwide shipping and free freight insurance. Browse our wire and cable selection or call (800) 674-9019 for availability, current pricing, and help sizing the right SER configuration for your subpanel or generator feeder project.