The standard 100 amp wire size copper for residential feeders and subpanels is 3 AWG when using 75°C rated terminations, though it must be upsized if the run exceeds 100 feet to mitigate voltage drop or if conduit derating applies. In a real circuit, this wire size dictates both the thermal limit (ampacity) before the insulation degrades and the electrical resistance over distance that can starve downstream equipment of voltage. People commonly confuse the 90°C wire insulation rating with the 75°C termination limit of the breaker, or they mistakenly apply aluminum wire sizing charts to copper conductors.
The Baseline: Sizing 100 Amp Copper Wire by the NEC
When sizing conductors, the National Electrical Code (NEC) requires you to look at two distinct temperature ratings: the insulation on the wire and the termination points (lugs) on your breakers and panels. Most modern 100-amp breakers and subpanel lugs are rated for 75°C. Even if you buy THHN wire with 90°C insulation, NEC 110.14(C) forces you to use the 75°C column for ampacity unless the equipment is explicitly marked otherwise.
Below is the standard ampacity chart for copper conductors based on NEC Table 310.16. Notice how the allowable current changes based on the temperature column you are legally permitted to use.
| Copper Wire Size (AWG) | 60°C Column (Older Gear) | 75°C Column (Standard Terminations) | 90°C Column (Wire Insulation / Derating) |
|---|---|---|---|
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
Where You Meet 100A Copper in Practice
You will typically encounter the need for a 100-amp copper feeder in a few specific residential and light-commercial scenarios:
- Detached Garage Subpanels: Powering a workshop with welders, air compressors, and standard 120V receptacles.
- Whole-Home Generator Interlocks: Feeding a 100A transfer switch or service entrance panel from a backup generator.
- Heavy EV Charging Hubs: Commercial or high-end residential setups running dual Level 2 chargers on a single 100A subpanel.
- Large HVAC Additions: Dedicated feeders for multi-zone mini-split systems or electric furnace conversions.
In these setups, what changes when you select the correct wire isn't just safety—it's performance. Undersized wire acts as a resistor, converting electrical energy into waste heat inside your walls and dropping the voltage available at the point of use.
The Voltage Drop Trap: A Real-World Scenario Walkthrough
Sizing for ampacity (heat) is only half the battle. If your run is long, you must size for voltage drop. The NEC recommends a maximum 3% voltage drop on feeders for reasonable efficiency.
The Setup
An installer runs a 100A feeder to a detached garage workshop located 200 feet from the main panel. They pull three 3 AWG THHN copper conductors (two hots, one neutral) and one 8 AWG ground through 1.25-inch PVC conduit. The subpanel powers a 240V, 50A plasma cutter and a 240V, 30A dust collection system.
The Numbers
When both machines run simultaneously, the continuous combined load is roughly 80 amps. Using the standard voltage drop formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=80A, D=200ft, and CM=52,620 for 3 AWG):
VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84 Volts
On a 240V circuit, a 7.84V drop equals 3.26%.
The Outcome
When the plasma cutter's high-frequency start fires while the dust collector motor is running, the voltage at the subpanel dips below 232V. The plasma cutter's internal low-voltage protection trips, shutting the machine down mid-cut and ruining the workpiece.
What Went Wrong
The installer sized strictly for the 100A breaker thermal limit (ampacity) but ignored the 200-foot distance. To fix this, the wire must be upsized to 2 AWG copper (CM = 66,360), which drops the voltage loss to 6.22V (2.59%), safely under the 3% threshold. Always use a reliable voltage drop calculator before pulling wire on runs over 100 feet.
Step-by-Step: Sizing Your 100 Amp Copper Feeder
Follow this sequence to ensure your copper feeder meets all NEC requirements and physical realities of your jobsite.
- Calculate the Continuous Load: If the load will run for 3 hours or more (like an EV charger), multiply the load by 125%. A 40A continuous EV charger requires wire sized for 50A, but if it's on a 100A breaker, you still size the wire to the breaker (100A).
- Check Termination Ratings: Verify the breaker and panel lugs are marked 75°C. If they are 60°C (common in very old panels), you must use 1 AWG copper.
- Count Current-Carrying Conductors (CCCs): For a standard 120/240V single-phase subpanel feeder, you have two ungrounded (hot) conductors and one grounded (neutral) conductor. Per NEC 310.15(C)(1), the neutral counts as a CCC. That means 3 CCCs in your conduit.
- Apply Conduit Derating: With 3 CCCs, you must apply an 80% derating factor to the 90°C column of your wire.
- 3 AWG THHN (90°C) = 115A × 0.80 = 92A. (Fails: 92A is less than the 100A breaker).
- 2 AWG THHN (90°C) = 130A × 0.80 = 104A. (Passes: 104A > 100A).
- Calculate Voltage Drop: Run the math for your specific distance. If the drop exceeds 3%, upsize the wire one more step.
- Size the Ground (EGC): Per NEC 250.122, a 100A circuit requires a minimum 8 AWG copper equipment grounding conductor. If you upsized your hots for voltage drop, you must proportionally upsize the ground as well.
Common Sizing Mistakes and Confusions
Even experienced DIYers and junior electricians trip over a few specific details when pulling 100A feeders.
Confusing Copper with Aluminum: Aluminum is cheaper and highly common for 100A service entrance cables (like SER cable). However, aluminum has higher resistance. While 3 AWG copper handles 100A, you must use 1 AWG aluminum to safely carry 100A at 75°C. Never use a copper chart for aluminum wire.
The 'Neutral Doesn't Count' Myth: Many builders assume the neutral wire doesn't generate heat and shouldn't be counted for conduit fill derating. On a multi-wire branch circuit or a subpanel feeder where the neutral carries unbalanced 120V return current, it absolutely generates heat. Failing to count it as a CCC leads to undersized wire and overheated conduits.
Ignoring the Equipment Grounding Conductor (EGC): A 100A subpanel requires a 4-wire setup (2 hots, 1 neutral, 1 ground). You cannot use the conduit itself as the ground for a detached structure, and you cannot use a 10 AWG ground wire just because it fits. Stick to the 8 AWG copper minimum.
FAQ: 100 Amp Copper Wire Sizing
Can I use 4 AWG copper for a 100 amp breaker?
No. 4 AWG copper is rated for a maximum of 85A at 75°C and 95A at 90°C. Because neither rating meets or exceeds 100A, using 4 AWG on a 100A breaker is a direct NEC violation and a fire hazard. The absolute minimum is 3 AWG, and 2 AWG is required if conduit derating applies.
What size ground wire do I need for a 100 amp copper feeder?
Per NEC Table 250.122, the minimum equipment grounding conductor for a 100A overcurrent protective device is 8 AWG copper. However, if you had to upsize your hot wires to 1 AWG to compensate for severe voltage drop over a long distance, you must proportionally upsize the ground wire to 6 AWG or 4 AWG depending on the exact circular mil ratio.
Does the neutral wire need to be the same size as the hot wires?
For a standard 100A subpanel feeder, yes. While NEC 220.61 allows for calculated neutral load reductions in specific, heavily documented commercial scenarios, residential inspectors almost universally require the neutral to be the same size as the ungrounded (hot) conductors—meaning you will pull three 3 AWG (or 2 AWG) wires and one 8 AWG ground.
