The correct 100 amp SER cable size is 3 AWG for copper or 1 AWG for aluminum, based on the 75°C ampacity column in the National Electrical Code (NEC). SER (Service Entrance Cable, Style R) is a multi-conductor cable assembly used to route high-amperage power from a main panel to a subpanel or heavy appliance, and sizing it correctly prevents overheating at the breaker lugs while maintaining acceptable voltage levels at the destination.

Choosing the right gauge changes three critical variables in your installation: the physical pull force required to route the stiff cable, the voltage drop over long distances, and the thermal headroom at your termination points. DIYers commonly confuse SER with SEU cable (which lacks the necessary insulated neutral and bare ground for modern 4-wire subpanels) and mistakenly size their wire using the 90°C insulation rating rather than the 75°C termination limit dictated by standard breaker lugs.

The Core Sizing Rule: Ampacity vs. Termination Limits

When sizing a 100 amp feeder, you must look at the weakest link in the thermal chain. Modern SER cable typically contains XHHW-2 or THHN insulated conductors, which carry a 90°C insulation rating. However, standard residential breakers and panelboard lugs are rated for 75°C. Under NEC 110.14(C), you must size the wire based on the lowest temperature rating of any connected component.

NEC 310.16 vs. 310.12 Distinction: If you are wiring a whole-house service entrance, NEC 310.12 allows you to use the 83% rule (permitting 4 AWG copper for a 100A main service). However, if you are running a feeder to a subpanel, the 83% rule does not apply. You must use standard ampacity tables (NEC 310.16), requiring a full 3 AWG Copper or 1 AWG Aluminum for a 100A breaker.

Using a 4 AWG copper wire on a 100A subpanel breaker violates code because 4 AWG is only rated for 85A at 75°C. While the wire's 90°C insulation won't melt, the breaker lugs will overheat, potentially causing thermal damage to the breaker internals or starting a fire inside the panel enclosure.

For a complete breakdown of cable ampacities and physical properties, refer to the Southwire Voltage Drop Calculator and Ampacity Charts, which align with NEC Table 310.16.

Worked Numeric Example: Voltage Drop at Distance

Ampacity tells you the wire won't melt, but it doesn't guarantee your tools will run properly at the far end. The NEC recommends a maximum voltage drop of 3% for branch circuits and feeders. Let us calculate the voltage drop for a 100A subpanel fed by 3 AWG copper SER cable.

Scenario Parameters:

  • Load: 80A continuous (a realistic maximum for a 100A breaker, applying the 125% continuous load rule)
  • Voltage: 240V single-phase
  • Distance: 150 feet (one-way)
  • Wire: 3 AWG Copper (52,620 Circular Mils)
  • K-Factor: 12.9 (standard for copper at 75°C)

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

Calculation for 150 feet:
VD = (2 × 12.9 × 80 × 150) / 52,620
VD = 309,600 / 52,620 = 5.88 Volts
Percentage = (5.88 / 240) × 100 = 2.45%

At 150 feet, 3 AWG copper keeps you well under the 3% limit. But what if your detached garage is 200 feet away?

Calculation for 200 feet:
VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84 Volts (3.26%)

At 200 feet, you exceed the 3% recommendation. You must upsize to 2 AWG Copper (66,360 Circular Mils). Recalculating with 2 AWG yields a drop of 6.22V (2.59%), bringing you back into compliance. Always run the math for your specific run length using a trusted tool like the NFPA National Electrical Code resources or manufacturer calculators.

Where You Meet This in Practice

You will typically pull 100 amp SER cable in three specific residential or light-commercial scenarios:

  1. Detached Garage Subpanels: A 100A subpanel is the modern standard for a detached garage that will house a welder, air compressor, or EV charger. SER is ideal here because it includes the four required wires (two hots, one neutral, one ground) in a single jacket.
  2. Hardwired EV Chargers: Level 2 EV chargers like the ChargePoint Home Flex or Tesla Wall Connector can be configured to pull up to 80A continuous. This requires a 100A breaker and 3 AWG copper SER cable routed directly to the charger's junction box.
  3. Workshop Additions: When adding a dedicated outbuilding for woodworking or metalworking machinery, a 100A feeder provides enough headroom for simultaneous use of dust collectors and 240V table saws without tripping the main feed.
Termination Torque Warning: When landing 3 AWG or 1 AWG wire into a 100A breaker, you must use a calibrated inch-pound torque screwdriver. NEC 110.14(D) requires terminations to be tightened to the manufacturer's specified torque. Under-torquing large-gauge wire causes high-resistance arcing at the lug, which is a leading cause of panel fires.

Frequently Asked Questions

Can I use 4 AWG copper SER cable on a 100 amp breaker for a subpanel?

No. While 4 AWG copper has a 90°C ampacity of 95A, its 75°C ampacity (which governs breaker terminations) is only 85A. Installing it on a 100A breaker for a subpanel feeder violates NEC 310.16 and creates a fire hazard at the lugs. You must use a minimum of 3 AWG copper. The only exception is if the cable serves as the main service entrance to the entire dwelling under the NEC 310.12 83% rule, but this does not apply to subpanel feeders.

Do I need to run 100 amp SER cable inside a conduit?

SER cable is designed to be run as a standalone cable assembly and does not require continuous conduit for standard indoor runs through studs or joists. However, if the cable is run underground, it must be replaced with individual THWN-2 wires inside a conduit (like PVC Schedule 80), as SER is not rated for direct burial or wet locations. Additionally, if the SER cable is run along a surface where it is subject to physical damage (like low on a garage wall), it must be protected by conduit or guard strips.

What is the difference between SER and SEU cable for a 100 amp feed?

Style R (SER) contains two insulated hot wires, one insulated neutral wire, and a bare ground wire wrapped around the assembly. Style U (SEU) contains two insulated hots and a bare concentric neutral/ground, but lacks a dedicated insulated neutral and separate ground. Modern NEC code requires a 4-wire setup (separated neutral and ground) for all subpanels. Therefore, SEU cable is illegal for subpanel feeders and can only be used for specific service entrance applications or 120/240V ranges/dryers where the code permits a 3-wire connection.

Can I use aluminum SER cable for a 100 amp subpanel indoors?

Yes, 1 AWG aluminum SER cable is perfectly legal and highly cost-effective for indoor 100A subpanel feeders. Aluminum is significantly cheaper and lighter than copper. The only trade-off is that aluminum wire is physically thicker and stiffer, making it harder to bend inside tight panel gutters. You must also ensure your breaker lugs are rated for aluminum (most modern 100A breakers are) and apply an antioxidant compound (like Noalox) to the stripped wire ends before torquing the lugs to prevent oxidation.