100 amp service wiring is the physical conductor assembly—typically 3-wire or 4-wire cable—sized to safely deliver up to 100 amperes of current from the utility meter to a home's main breaker panel without exceeding thermal limits or voltage drop thresholds. This specific wiring tier dictates the physical conduit fill, the torque specifications on your meter base lugs, the minimum size of your grounding electrode conductor, and whether you can use standard 75°C rated terminals at the main disconnect.

Standard 100A residential service requires a minimum of #3 AWG Copper or #1 AWG Aluminum under NEC Article 310.12.

While 100-amp services are less common in new construction today—where 200-amp is the baseline for homes with central HVAC and EV chargers—they remain highly relevant for smaller homes, detached ADUs (Accessory Dwelling Units), large workshop subpanels fed directly from a meter, and older homes undergoing panel replacements without a full utility transformer upgrade. Getting the wire size, insulation type, and grounding bond correct is the difference between a safe, code-compliant installation and a fire hazard at the main lugs.

The Core Specs: Sizing 100 Amp Service Conductors

Sizing service entrance conductors is not just about looking at the standard ampacity tables. For single-phase residential dwellings, the National Electrical Code (NEC) provides a specific table—Article 310.12 (formerly 310.15(B)(7))—that allows for specific wire sizes based on the service rating, overriding some of the stricter general derating rules found in Table 310.16.

Below is the exact specification matrix for a standard 100-amp, 120/240V single-phase residential service. These values assume an ambient temperature of 30°C (86°F) and standard termination ratings.

Conductor Material Insulation Type Minimum AWG Size 75°C Ampacity Required GEC (Grounding)
Copper THHN / THWN-2 #3 AWG 100A #8 AWG Copper
Copper XHHW-2 #3 AWG 100A #8 AWG Copper
Aluminum THHN / THWN-2 #1 AWG 100A #8 AWG Copper
Aluminum XHHW-2 #1 AWG 100A #8 AWG Copper
Aluminum USE-2 (Direct Burial) #1 AWG 100A #8 AWG Copper
Bench Note on Aluminum: If you are pulling #1 AWG Aluminum through conduit, XHHW-2 is vastly superior to THHN. XHHW-2 has a thinner cross-linked polyethylene insulation profile, making the overall cable diameter smaller and significantly reducing pulling friction in 1.5-inch or 2-inch PVC sweeps.

Worked Example: Voltage Drop and Real-World Routing

Wire size tables assume a short run. In practice, the distance from the utility transformer to the meter base, and from the meter base to the main panel, can introduce significant voltage drop. The NEC recommends a maximum 3% voltage drop on the service feeder for optimal efficiency.

The Scenario: You are wiring a detached workshop with a 100A main breaker panel. The run from the meter base on the main house to the workshop panel is 85 feet. You are using #3 AWG Copper SER (Service Entrance Rated) cable routed through PVC conduit. The calculated continuous load on the panel is 80A.

The Math:
We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM

  • K (Specific resistance of Copper at 75°C) = 12.9
  • I (Current in Amps) = 80A
  • D (Distance in feet) = 85 ft
  • CM (Circular Mils for #3 AWG) = 52,620

VD = (2 × 12.9 × 80 × 85) / 52,620
VD = 175,440 / 52,620 = 3.33 Volts

To find the percentage drop on a 240V circuit:
% Drop = (3.33V / 240V) × 100 = 1.38%

The Verdict: At 1.38%, this is well under the 3% NEC recommendation. #3 AWG Copper is perfectly adequate for this 85-foot run. However, if this run were extended to 200 feet, the drop would exceed 3.2%, and you would need to step up to #2 AWG or #1 AWG Copper to maintain power quality, especially for sensitive electronics or motor starts in the shop.

Safety Caveat: Always de-energize the utility side before working on the meter base. The line-side lugs in a meter socket are always live unless the utility company pulls the meter or disconnects at the transformer. Local codes often require a licensed electrician and utility coordination for service entrance work.

Where You Meet 100A Service Wiring in Practice

Theory is clean; the jobsite is not. Here is where 100A service wiring dictates your physical workflow and material choices during an installation.

1. Meter Base and Panel Lugs

Almost all residential meter bases and main breaker lugs are rated for 75°C, not 90°C. Even if you buy THHN wire (rated for 90°C in the free air), you must terminate it based on the 75°C column. This is why #3 Copper is the hard minimum; #4 Copper has a 75°C ampacity of only 85A, which is insufficient for a 100A main breaker, regardless of what the 90°C column says.

2. Conduit Fill and Bending Radius

If you are pulling individual THHN/XHHW conductors instead of using bundled SER cable, you are pulling three current-carrying conductors (two hots, one neutral) plus a ground. For #1 AWG Aluminum, three conductors plus a #8 ground will comfortably fit inside 1.5-inch Schedule 40 PVC, but pulling it through a 90-degree sweep requires careful use of wire pulling lubricant to avoid tearing the insulation jacket.

3. Torque and Oxide Inhibitor

When terminating aluminum service conductors into the main breaker lugs, aluminum naturally forms a non-conductive oxide layer within minutes of being stripped. You must wire-brush the conductor, apply an antioxidant compound (like Noalox or Ideal Noalox), and torque the lug to the manufacturer's exact specification (usually printed on the breaker label, often between 40-60 in-lbs for this size). Use a calibrated torque screwdriver; guessing the torque leads to loose connections, which arc and melt the lug under heavy load.

Common Confusions: Service vs. Feeder and Grounding Rules

When DIYers and junior apprentices tackle 100A wiring, they frequently mix up service entrance rules with subpanel feeder rules. These mistakes can create parallel neutral paths and shock hazards.

Feature 100A Service Entrance (Meter to Main) 100A Feeder (Main to Subpanel)
Neutral/Ground Bond Bonded at the main disconnect (Neutral bar and Ground bar are connected). Strictly isolated (Neutral bar floats, Ground bar bonds to enclosure).
Wire Count Often 3-wire (2 Hots + Neutral) if meter base provides ground path, or 4-wire. Must be 4-wire (2 Hots + Neutral + Equipment Ground).
Sizing Rule NEC 310.12 (Dwelling Service allowances). NEC 310.16 (Standard ampacity and derating rules).
Grounding Electrode Requires connection to earth (ground rods, Ufer, or metal water pipe). No earth grounding required; relies on the equipment ground wire back to main.

The 'Next Size Up' Breaker Myth

A common point of confusion is NEC 240.4(B), the 'next size up' rule for overcurrent protection. Many assume they can use #4 AWG Copper (85A at 75°C) and protect it with a 100A breaker because 100A is the next standard breaker size up from 85A. This is false for service entrance conductors sized under 310.12. Article 310.12 explicitly mandates #3 AWG Copper as the minimum for a 100A service. You cannot use the next-size-up rule to bypass the specific dwelling service table.

The Grounding Electrode Conductor (GEC)

People often oversize or undersize the bare copper wire that runs from the panel's ground bar to the exterior ground rods. Per NEC Table 250.66, if your largest service entrance conductor is #3 AWG Copper or #1 AWG Aluminum, your GEC must be a minimum of #8 AWG Copper. However, if you are connecting to a continuous metal underground water pipe, the code requires a minimum of #4 AWG Copper for that specific segment to handle potential fault currents from the utility side. Always size the GEC to the most stringent requirement in your specific grounding electrode system.

By strictly adhering to the 75°C termination limits, respecting the physical differences between service and feeder bonding, and calculating voltage drop for longer runs, a 100-amp service installation will remain safe, cool, and code-compliant for decades.