Copper Wire Size for 100 Amps: Sizing Rules, Voltage Drop, and Common Mistakes

The correct copper wire size for 100 amps is #3 AWG when terminated on standard 75°C lugs, acting as the physical pathway capable of safely dissipating the heat generated by 100 amperes of continuous electrical current without exceeding insulation temperature limits. In a real installation, stepping up to this specific gauge changes your physical routing constraints—dictating your minimum conduit fill diameter, the bending radius required at panel knockouts, and whether your voltage drop stays under the National Electrical Code (NEC) recommended 3% threshold at the far end of the run.

Where You Meet This in Practice

You will typically need to size copper wire for a 100-amp circuit in three specific residential and light-commercial scenarios:

1. Detached Garage Subpanels: A 100-amp feed is the modern standard for a detached garage running a welder, air compressor, and standard lighting circuits.
2. Hardwired Level 2 EV Chargers: While many EV chargers run on 40A or 50A circuits, high-speed residential chargers (like the ChargePoint Home Flex or Tesla Wall Connector configured for 80A output) require a 100-amp breaker and correspondingly sized wire.
3. Large Workshop Equipment: Heavy-duty CNC routers, large rotary phase converters, or industrial kilns often draw in the 75A to 80A continuous range, mandating a 100A breaker and properly sized feeders.

If you are wiring a 100-amp continuous load (defined by the NEC as a load expected to run for 3 hours or more, like an EV charger), you must apply a 125% multiplier. That means sizing the wire and breaker for 125 amps, which bumps your requirement up to #1 AWG copper.

The Temperature Column Trap (What People Commonly Confuse)

The most frequent mistake DIYers and junior apprentices make is looking at the wrong column on the ampacity chart. You will often see internet forums claiming you can use #4 AWG copper for 100 amps. This is dangerously incorrect for standard terminations.

Here is how the ampacity breaks down for copper wire across the relevant temperature columns, based on the Cerro Wire standard ampacity tables and NEC Table 310.16:

People confuse the 90°C column with the allowable ampacity. The 90°C column is only used for derating calculations (like adjusting for high ambient temperatures in a hot attic or bundling more than three current-carrying conductors in a single conduit). Your baseline ampacity must always land in the 75°C column. Therefore, #3 AWG copper is the absolute minimum for a standard 100A non-continuous circuit.

Real-World Scenario Walkthrough: The 200-Foot Garage Subpanel

To understand what happens when you only look at ampacity and ignore physics, let us look at a botched installation I was called in to troubleshoot.

The Setup: A homeowner ran a feeder to a detached garage 200 feet away from the main house panel. They installed a 100-amp breaker in the main panel and pulled four strands of #3 AWG copper THHN through 1-inch PVC conduit. They sized the wire perfectly for the 100-amp breaker based on the 75°C ampacity column.

The Numbers: The garage was wired with a split-phase 120V/240V subpanel. The homeowner plugged in a 120V, 80-amp draw welder on one leg of the panel. The one-way distance was 200 feet.

The Outcome: Every time the welder struck an arc, the garage lights dimmed severely, and the welder's internal low-voltage protection tripped, shutting the machine off after three seconds. The breaker never tripped, but the wire felt warm to the touch at the subpanel lugs.

What Went Wrong: The homeowner sized the copper wire for ampacity (heat dissipation) but completely ignored voltage drop. Over a 200-foot run, #3 AWG copper exhibits significant resistance. When the welder pulled 80 amps on a single 120-volt leg, the resistance of the wire choked the voltage down to roughly 111V at the receptacle. The welder requires a minimum of 114V to operate safely. To fix it, they had to abandon the #3 AWG and pull #1 AWG copper to keep the voltage drop under 3%.

Worked Numeric Example: Calculating the Voltage Drop

Let us run the exact math on that failed 200-foot scenario to prove why #3 AWG failed and what size you actually need for long runs. We use the standard single-phase voltage drop formula:

VD = (2 × K × I × L) / CM

• K = 12.9 (Ohms-cmil/ft for copper)
• I = 80 Amps (Actual continuous load on the 120V leg)
• L = 200 Feet (One-way distance)
• CM = 52,620 (Circular mils for #3 AWG copper, per NEC Chapter 9, Table 8)

Calculation for #3 AWG:
VD = (2 × 12.9 × 80 × 200) / 52,620
VD = 412,800 / 52,620
VD = 7.84 Volts

To find the percentage drop on a 120V circuit: (7.84 / 120) × 100 = 6.53% voltage drop. The NEC recommends a maximum of 3% for branch circuits and feeders to ensure efficient operation. A 6.53% drop is unacceptable and will damage motor-driven or sensitive electronic loads.

Recalculating with #1 AWG Copper (CM = 83,690):
VD = (2 × 12.9 × 80 × 200) / 83,690
VD = 412,800 / 83,690
VD = 4.93 Volts (4.1% drop) - Better, but still slightly over 3%.

Recalculating with 1/0 AWG Copper (CM = 105,600):
VD = 412,800 / 105,600
VD = 3.90 Volts (3.25% drop).

For a 200-foot run carrying an 80A 120V load, you actually need to step up to 1/0 AWG copper to get as close to the 3% ideal threshold as possible, despite the breaker only being 100 amps. This is why distance always overrides baseline ampacity charts on long runs.

FAQ: Common 100-Amp Copper Wiring Questions