How to Size Wire for 100 Amp Service: AWG, Ampacity, and NEC Rules

Sizing wire for a 100 amp service means selecting a conductor with an ampacity rating that safely carries 100 amps continuously without exceeding its temperature limits, typically requiring 3 AWG copper or 1 AWG aluminum under standard NEC guidelines. This single decision dictates the physical thickness of your feeder, the conduit fill capacity, the thermal headroom before insulation degrades, and whether your panel lugs will overheat under load. The most common point of confusion is mixing up a wire’s 90°C insulation rating with the 75°C termination limit of standard breaker lugs, which leads many DIYers to incorrectly assume that 4 AWG copper (rated 95A at 90°C) is safe to use on a 100A breaker. It is not.

The Core Rule: How to Size Wire for 100 Amp Service

When you are pulling a feeder for a 100A subpanel or service entrance, the National Electrical Code (NEC) requires you to look at the 75°C column of the ampacity tables, not the 90°C column. This is governed by NEC 110.14(C), which states that the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device. Since almost all standard residential breakers and panel lugs are rated for 75°C, your wire must have an ampacity of at least 100A in the 75°C column.

The Ampacity Table: Copper vs. Aluminum at 75°C

To understand why specific gauges are required, you need to look at the actual thermal limits of the metals. Aluminum is lighter and cheaper but has higher resistance, requiring a thicker gauge to carry the same current as copper. Below is the baseline data you need for standard THHN/THWN-2 or XHHW-2 conductors in a raceway or cable assembly.

Note: Data derived from standard NEC Table 310.16. Always verify with your local Authority Having Jurisdiction (AHJ), as local amendments can supersede baseline NEC tables.

Where You Meet This in Practice

You will typically encounter the need to size wire for a 100 amp service in three specific residential scenarios:

1. Detached Garage Subpanels: Modern garages often house welders, air compressors, and freezers. A 100A feeder provides enough headroom for simultaneous 240V tool usage without tripping the main.
2. EV Charger Future-Proofing: While most Level 2 chargers run on 60A circuits, installing a 100A subpanel in the garage allows you to add a second charger later without trenching new conduit from the main house panel.
3. Workshop or Barn Additions: When extending power to an outbuilding where lighting, heating, and power tools will be used concurrently, a 100A service is the standard minimum for functional usability.

Real-World Scenario: The Attic Derating Disaster

Theory is clean; jobsites are messy. Here is a walkthrough of a common failure mode when sizing wire for a 100 amp service through hostile environments.

The Setup: A homeowner decides to wire a 100A subpanel in a detached workshop. To save money, they buy 4 AWG copper SER (Service Entrance) cable, assuming the 90°C rating makes it safe for 100A. They route the cable through an uninsulated attic space in a southern climate before dropping it down the exterior wall.

The Numbers:

• Base ampacity of 4 AWG copper at 75°C = 85A.
• Attic ambient temperature in July reaches 120°F (49°C).
• According to NEC 310.15(B)(1), the temperature correction factor for 75°C wire at 46-50°C ambient is 0.82.
• Derated ampacity: 85A × 0.82 = 69.7A.

The Outcome: The homeowner turns on a 240V dust collector and a window AC unit, pulling about 75A. The 100A main breaker does not trip immediately because 75A is below its magnetic trip threshold. However, the wire is now carrying 75A while only rated for 69.7A in that specific thermal environment. The wire acts like a low-grade toaster. Over three weeks, the SER jacket becomes brittle and cracks, and the thermal transfer up the wire into the panel lugs causes discoloration on the aluminum bus bar.

What Went Wrong: The installer treated the wire's ampacity as a static number rather than a dynamic one. Think of ampacity like highway lanes: a 4-lane highway (4 AWG) handles 85 cars fine in cool weather, but when the asphalt melts in 120°F heat, the lanes narrow, and traffic jams (resistance and heat buildup) occur. The fix would have been using 1 AWG copper or 1/0 AWG aluminum, or routing the conduit underground where the earth maintains a stable 55°F ambient temperature.

Voltage Drop and Long-Run Adjustments

Ampacity tells you what size wire prevents a fire; voltage drop tells you what size wire actually delivers usable power. The NEC recommends (but does not strictly mandate for all feeders) a maximum voltage drop of 3% for branch circuits and feeders combined.

Worked Numeric Example:
Imagine you are running a 100A, 240V feeder to a detached garage that is 150 feet away from the main panel. You plan to use 3 AWG copper (the minimum code-allowed size).

• Formula: VD = (2 × K × I × D) / CM
• K (Copper resistance constant) = 12.9 ohms per mil-foot
• I (Current) = 100A
• D (Distance) = 150 feet
• CM (Circular Mils for 3 AWG) = 52,620

Calculation:
VD = (2 × 12.9 × 100 × 150) / 52,620
VD = 387,000 / 52,620 = 7.35 Volts

Percentage: (7.35V / 240V) × 100 = 3.06%.

While 3.06% is incredibly close to the 3% guideline, it technically exceeds it. If your garage is 200 feet away, that drop jumps to 4.08%, which will cause 240V welders to run hot and trip their internal thermal overloads. For a 150-foot run at a full 100A continuous load, the professional move is to bump the wire size to 1 AWG Copper or 1/0 AWG Aluminum to drop the voltage loss well under 2.5%. You can verify these calculations on the fly using the Southwire Voltage Drop Calculator.