What Size Wire Gauge for a 100 Amp Subpanel? (Copper vs Aluminum)

Wire gauge for a 100-amp circuit is the specific physical cross-sectional area of a conductor—measured in American Wire Gauge (AWG)—required to safely carry 100 amps of current without exceeding the thermal limits of its insulation or terminations. For a standard 100-amp residential feeder, the National Electrical Code (NEC) mandates a minimum of 3 AWG copper or 1 AWG aluminum when terminations are rated for 75°C. However, due to voltage drop over distance and the 60°C termination rule for unmarked equipment, 2 AWG copper or 1/0 AWG aluminum are the practical, code-compliant standards used by electricians in the field.

Getting this right is not just about passing inspection; it is about preventing thermal degradation at the breaker lugs and ensuring your downstream equipment receives adequate voltage. Below is the exact sizing matrix, the physics of what gauge actually changes in your installation, and the field realities of pulling 100-amp feeders.

The 100 Amp Wire Gauge Sizing Matrix (NEC Table 310.16)

The NEC does not assign a single wire size to a breaker. Instead, it provides ampacity tables based on conductor material and temperature ratings. The table below maps standard AWG sizes to their allowable ampacities across the three primary temperature columns found in NEC Table 310.16.

What Wire Gauge Actually Changes in a 100A Circuit

Choosing a larger wire gauge does not change the amount of current the load draws; a 20kW heat strip will pull roughly 83 amps whether you feed it with 3 AWG or 1/0 AWG. What the wire gauge changes is the resistance per 1,000 feet, which directly dictates two critical installation variables: heat dissipation at the terminations and voltage drop at the load.

Think of voltage drop like traffic congestion on a narrow highway; the longer the road and the heavier the traffic (current), the more energy is lost as friction (heat) rather than reaching the destination. Upsizing the wire adds lanes, reducing resistance and keeping voltage stable.

Worked Numeric Example: Voltage Drop on a 100A Subpanel

Let us calculate the voltage drop for a 120/240V single-phase subpanel feeder supplying an 80-amp continuous load (like a large workshop welder and lighting) located 150 feet from the main panel.

• Formula: VD = (2 × K × I × D) / Circular Mills (CM)
• Constants: K (Copper) = 12.9 | K (Aluminum) = 21.2 | I = 80A | D = 150 ft

Scenario A: Minimum Code 3 AWG Copper (CM = 52,620)

• VD = (2 × 12.9 × 80 × 150) / 52,620
• VD = 309,600 / 52,620 = 5.88 Volts
• Percentage: 5.88V / 240V = 2.45% (Acceptable, under the 3% NEC recommendation for feeders).

Scenario B: Minimum Code 1 AWG Aluminum (CM = 83,690)

• VD = (2 × 21.2 × 80 × 150) / 83,690
• VD = 508,800 / 83,690 = 6.08 Volts
• Percentage: 6.08V / 240V = 2.53% (Acceptable).

Scenario C: The Edge Case (200-foot run with 1 AWG Aluminum)

If that same subpanel is 200 feet away, the voltage drop on 1 AWG Aluminum jumps to 8.10 Volts (3.37%). This exceeds the 3% feeder guideline in NEC 215.2(A)(FPN). To correct this, you must upsize to 1/0 AWG Aluminum (CM = 105,600), which drops the loss back down to 6.43V (2.6%). This is exactly why 1/0 Aluminum is the default choice for detached garage subpanels.

Where You Meet 100 Amp Feeder Sizing in Practice

You will rarely pull a 100-amp wire for a single appliance. In residential and light commercial environments, 100-amp wire gauge sizing is almost exclusively applied to feeder circuits that distribute power to secondary panels or massive dedicated loads.

• Detached Garage Subpanels: A 100A subpanel is the sweet spot for a standard 2-car detached garage running a table saw, air compressor, and LED lighting. 1/0-1/0-1/0-2 Aluminum SER (Service Entrance) cable is the standard material here due to its flexibility and cost-effectiveness compared to copper.
• Basement / Addition Subpanels: When finishing a basement with multiple high-draw appliances (electric dryer, HVAC air handler, bathroom heaters), a 100A feed from the main service panel prevents the need to run individual home runs back to the main service.
• Large Hot Tubs / Spas: While many spas run on 50A or 60A GFCI breakers, large commercial-grade or multi-pump luxury spas with inline electric heaters can require an 80A to 100A dedicated breaker, necessitating 100A-rated wet-location conductors (like individual THWN-2 wires in PVC conduit).
• Level 2 EV Charger Banks: While a single Tesla Wall Connector maxes out at 60A, installing a dual-charger setup with load-sharing or a commercial EV pedestal often requires a 100A feeder to the charging junction box.

Common Sizing Mistakes and Confusions

When sizing wire for a 100 amp breaker, DIYers and even junior apprentices frequently fall into three specific traps that result in failed inspections or, worse, melted lugs.

1. The 90°C Column Illusion

Modern THHN/THWN-2 wire insulation is rated for 90°C. Looking at the table above, 4 AWG Copper has a 90°C ampacity of 95A, and 2 AWG Aluminum hits exactly 100A at 90°C. You cannot use the 90°C column to size your breaker. The Copper Development Association and NEC 110.14(C) dictate that your ampacity is limited by the lowest temperature rating in the circuit. Since breakers and panel lugs are rarely rated above 75°C, the 90°C column is only used for derating purposes (e.g., when bundling more than three current-carrying conductors in a single conduit), never for the baseline ampacity.

2. Copper-Clad Aluminum (CCA) vs. Pure Aluminum

Never buy Copper-Clad Aluminum (CCA) wire for a 100-amp feeder. CCA is an aluminum core with a thin copper wash, primarily manufactured for telecommunications or cheap automotive audio kits. The NEC does not recognize CCA as a standard conductor material for building wiring, and it has a higher resistance and different thermal expansion rate than pure AA-8000 series aluminum. If you are using aluminum, buy XHHW-2 or SER cable explicitly marked as AA-8000 alloy.

3. Confusing Breaker Size with Wire Ampacity

NEC 240.4(B) allows you to round up to the next standard breaker size if your wire ampacity does not match a standard breaker (e.g., using a 400A breaker for 350A wire). However, 100 amps is a standard breaker size. Therefore, the wire must have a true, derated ampacity of at least 100A. You cannot use a wire rated for 95A and protect it with a 100A breaker.

FAQ: Quick Answers for 100A Installations

Do I need to pull a neutral wire for a 100A subpanel?
Yes. A subpanel requires a 4-wire feed (Hot, Hot, Neutral, Ground). The neutral and ground must remain isolated in the subpanel. For a 100A feeder, the neutral is typically sized the same as the hot wires (e.g., 1/0 AWG), though NEC 220.61 permits reducing the neutral size if the calculated neutral load is significantly lower than the line-to-line load. Most electricians just pull a 4-wire SER cable with a full-size neutral to avoid inspection headaches.

What torque spec should I use for 1/0 AWG aluminum lugs?
Never guess. NEC 110.14(D) requires terminations to be torqued to the manufacturer's specifications. For most 100A panelboard lugs accepting 1/0 AWG, the torque spec falls between 250 and 300 inch-pounds. Use a calibrated inch-pound torque screwdriver or torque wrench, and apply an anti-oxidant compound (like Noalox) to the aluminum wire before insertion to prevent galvanic corrosion and thermal creep.

Can I use NM-B (Romex) for a 100A subpanel?
No. NM-B cable is generally limited to 60°C ampacity ratings under NEC 334.80. To carry 100A at 60°C, you would need 1 AWG Copper NM-B, which is exceptionally stiff, difficult to strip, and rarely manufactured in standard retail coils. Furthermore, NM-B cannot be run to detached structures underground or exposed to physical damage. Use individual THWN-2 wires in conduit or SER cable for 100A feeds.