How to Add a 100 Amp Subpanel: Wire, Breaker, and Conduit Sizing Guide

To feed a 100A subpanel, use a 100A 2-pole breaker and #3 AWG copper THHN/THWN-2 wire (or #1 AWG aluminum XHHW-2) for the hot and neutral conductors, plus an #8 AWG copper equipment grounding conductor. This assumes a standard 240V residential feeder in a single conduit at 30°C ambient.

The Baseline Feeder Spec Sheet

Before pulling any wire, you need a concrete bill of materials based on the assumptions above. Here is the exact specification for a standard indoor, 100-ampere subpanel feeder operating at 240V/120V split-phase.

Why #3 AWG Copper? (And Why Not One Size Smaller)

The most common mistake DIYers make when learning how to add a 100 amp subpanel is sizing the wire based on the 90°C column in the NEC ampacity tables. If you look at the 90°C column, #4 AWG copper is rated for 95A, which seems close enough to 100A to use the next standard breaker size (100A) under NEC 240.4(B). This is incorrect and a fire hazard.

Under NFPA 70: National Electrical Code (NEC) Article 110.14(C), the ampacity of a conductor is determined by the lowest temperature rating of any connected termination, conductor, or device. Almost all standard 100A residential breakers and subpanel lugs are rated for 75°C.

Looking strictly at the 75°C column in NEC Table 310.16:

• #4 AWG Copper: 85 Amps (Too small for a 100A breaker)
• #3 AWG Copper: 100 Amps (Exact match for a 100A breaker)

You must use #3 AWG copper because it is the smallest wire that achieves a true 100A rating in the 75°C termination column. Using #4 AWG would mean your terminations are subjected to temperatures they were not tested for, leading to lug degradation, arcing, and eventual panel failure.

Voltage Drop: When Distance Changes the Math

The baseline spec above assumes a relatively short run. The NEC recommends a maximum voltage drop of 3% on feeders to ensure efficient operation of appliances and prevent motor burnout. For a 240V feeder, a 3% drop equals 7.2 Volts.

To maintain a 3% or lower voltage drop on a 100-foot run at 100A, you must upsize to #2 AWG Copper (which drops the voltage by roughly 3.1% at 100A, or well under 3% at a realistic 80A continuous load). Always verify your specific run length using a reliable tool like the Southwire Voltage Drop Calculator before purchasing wire.

Copper vs. Aluminum Decision Tree

Copper is the gold standard for conductivity and ease of termination, but it is expensive. Aluminum (specifically XHHW-2 or SER cable) is significantly cheaper but requires larger gauges and strict termination protocols to prevent oxidation and thermal expansion loosening. Use the decision matrix below to finalize your material pick.

Critical Aluminum Rule: If you choose aluminum, you must use lugs rated for Aluminum/Copper (Al/Cu). You must also apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductor before inserting it into the lug, and you must use a calibrated torque screwdriver to tighten the lug to the exact inch-pound specification printed on the panel label. Aluminum creeps under pressure; hand-tightening will result in a high-resistance connection that will melt the lug within a year.

Conduit Sizing and the Proportional Ground Rule

For a 100A feeder using #3 AWG copper THHN, you are pulling four wires (two hots, one neutral, one ground). While NEC Chapter 9 tables mathematically allow these four wires to fit inside a 1-inch EMT or PVC conduit, pulling four large wires through 1-inch conduit with any bends is a nightmare that often results in damaged wire insulation.

The Practical Pick: Use 1-1/4 inch conduit. The marginal cost difference between 1-inch and 1-1/4-inch PVC is pennies per foot, but the reduction in pulling friction and jamming risk is massive. Furthermore, 1-1/4 inch accommodates future upsizing if you later realize you need a larger subpanel.

When the AHJ or an Engineer Must Confirm

While the specs above cover 95% of residential 100A subpanel installations, certain conditions require you to pause and consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer.

• Main Service Capacity: Adding a 100A subpanel does not magically create 100A of new power. If your home has a 100A or 150A main service, adding a 100A subpanel requires an NEC Article 220 Load Calculation to prove the main breaker won't trip under peak demand. If the calculation fails, you need a service entrance upgrade (e.g., to 200A or 320A), which requires utility coordination and a licensed electrician.
• Local Amendments: Some municipalities have strict local amendments that override the base NEC. For example, certain jurisdictions in California and New York require all feeder conductors to be installed in Rigid Metal Conduit (RMC) rather than PVC, or they may mandate 4 AWG grounds regardless of voltage drop upsizing. Always pull a permit and verify local codes.
• Continuous Loads: If the subpanel will supply continuous loads (operating for 3 hours or more, like EV chargers or heavy HVAC), the 100A breaker must be derated to 80% (80A continuous). If your actual continuous load exceeds 80A, you must upsize the entire feeder to 125A specifications (#1 AWG Copper, 125A breaker).

By strictly following the 75°C termination rule, checking your voltage drop at the actual measured distance, and applying the proportional ground rule, your 100A subpanel feeder will be safe, efficient, and ready to pass inspection on the first try.