The correct wire size for a 100 amp subpanel is 3 AWG copper or 1 AWG aluminum THHN/THWN-2, protected by a 100-amp 2-pole breaker. You must also pull an 8 AWG copper equipment grounding conductor. This assumes a 75°C termination rating, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (unless aluminum is explicitly stated)
- Insulation: THHN/THWN-2 (90°C wire, but we must use the 75°C ampacity column per NEC 110.14(C) because standard 100A breaker and panel lugs are rated for 75°C)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum of 3 current-carrying conductors (no derating required)
- System Voltage: 120/240V single-phase
Decoding the NEC Ampacity Table for 100A Feeders
When sizing a feeder, the National Electrical Code (NEC) requires you to match the wire's ampacity to the breaker protecting it. Many DIYers mistakenly look at the 90°C column on NEC Table 310.16 because the wire jacket says THHN (90°C). This is a critical error. You are legally bound to the temperature rating of the weakest link in the circuit, which is almost always the 75°C termination lugs on your breaker and subpanel.
| Wire Size (AWG) | Copper Ampacity (75°C) | Aluminum Ampacity (75°C) | Verdict for 100A Breaker |
|---|---|---|---|
| 4 AWG | 85A | 65A | Fails (Undersized) |
| 3 AWG | 100A | N/A (Al starts at 2) | Passes (Copper) |
| 2 AWG | 115A | 90A | Passes (Oversized Cu, Fails Al) |
| 1 AWG | 130A | 100A | Passes (Aluminum) |
Why this size and not one smaller? A 4 AWG copper wire is only rated for 85 amps at 75°C. While NEC 240.4(B) allows you to round up to the next standard breaker size if your wire ampacity falls between standard sizes, 100A is already a standard breaker size. If your calculated load can reach 100A, you cannot use an 85A wire and protect it with a 100A breaker. The wire must be rated for at least the breaker's rating.
When the Base Answer Changes: Voltage Drop & Derating
The 3 AWG copper / 1 AWG aluminum answer assumes a relatively short run. As distance increases, resistance causes voltage drop. The NEC recommends a maximum 3% voltage drop on feeders for reasonable efficiency. Let's run the math for a 100A load at 240V using 3 AWG copper (Circular Mil area = 52,620).
Voltage drop doesn't just make your lights dim; it causes motors in HVAC systems and well pumps to draw higher amperage to compensate for low voltage, leading to overheated windings and premature failure. Always calculate voltage drop for runs over 50 feet.
Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=100A, D=distance in feet).
- At 100 feet: Voltage drop is 4.9V. That is 2.04% of 240V. (Passes the 3% rule).
- At 150 feet: Voltage drop is 7.35V. That is 3.06% of 240V. (Fails the 3% rule).
If your run is 150 feet, you must upsize to 2 AWG copper (CM=66,360), which drops the voltage loss to 5.8V (2.4%). Furthermore, if you pull more than three current-carrying conductors in a single conduit (such as running two separate 240V circuits in the same pipe), NEC Chapter 9 Note 8 requires derating. Four to six conductors require an 80% derating factor. In that scenario, 3 AWG copper (100A × 0.80 = 80A) is no longer sufficient, and you must upsize to 1 AWG copper.
| Condition | Required Copper Size | Required Aluminum Size |
|---|---|---|
| Standard run (Under 100 ft, 3 conductors) | 3 AWG | 1 AWG |
| Long run (100 ft to 150 ft) | 2 AWG | 1/0 AWG |
| Very long run (150 ft to 200 ft) | 1 AWG | 2/0 AWG |
| Conduit bundling (4-6 current-carrying conductors) | 1 AWG | 2/0 AWG |
Grounding and Neutral Rules for Subpanels
A subpanel is not a main service panel. The most common code violation I see on jobsites is bonding the neutral and ground bars together in a subpanel. They must be strictly isolated. Because they are isolated, you are required to pull a 4-wire feeder: two hot legs, one grounded conductor (neutral), and one equipment grounding conductor (EGC).
Sizing the Equipment Grounding Conductor (EGC):
Per NEC 250.122, the EGC is sized based on the rating of the overcurrent device (the breaker), not the ungrounded conductors. For a 100-amp breaker, the minimum EGC is 8 AWG copper or 6 AWG aluminum. If you upsized your hot wires to 1 AWG copper for voltage drop over a 200-foot run, NEC 250.122(B) requires you to proportionally increase the ground wire size as well (bumping it to 4 AWG copper).
Sizing the Neutral:
While the NEC allows sizing the neutral smaller than the hot legs if you can prove the unbalanced load is low, standard residential practice is to pull a full-size neutral (3 AWG copper). Modern homes have heavy 120V single-phase loads (servers, space heaters, window ACs) that can heavily imbalance the legs. Pulling a full-size neutral prevents the neutral bus from overheating.
When to Call an Engineer or the AHJ
While the guidelines above cover 95% of residential and light commercial subpanel installs, you must involve a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- Continuous Loads: If the subpanel will serve continuous loads (on for 3 hours or more, like commercial lighting or EV chargers), the NEC requires the feeder to be sized at 125% of the continuous load. If your continuous load calculation hits 85 amps, your wire must be sized for 106.25 amps, pushing you out of 3 AWG and into 2 AWG copper.
- High Ambient Temperatures: If your conduit runs through an attic that regularly exceeds 86°F (30°C) or is installed on a hot roof, you must apply the temperature correction factors in Table 310.16. An attic at 110°F requires a 0.82 correction factor for 90°C wire, drastically reducing your ampacity.
- Utility Specific Rules: If this subpanel is being fed from a utility-owned transformer or meter main rather than your own main breaker panel, the utility company's specific 'Green Book' or service requirements will override general NEC guidelines.
Frequently Asked Questions
Can I use 2 AWG copper instead of 3 AWG for my 100 amp subpanel?
Yes, absolutely. 2 AWG copper is rated for 115A at 75°C, which safely exceeds the 100A requirement. In fact, many electricians prefer pulling 2 AWG because it is widely available, sometimes only marginally more expensive than 3 AWG, and provides a built-in buffer for voltage drop on longer runs. Just ensure the wire physically fits into the lugs of your 100A breaker and subpanel; some compact 100A breakers are only rated to accept up to 3 AWG or 2 AWG maximum.
What size wire do I need for a 100 amp subpanel 200 feet away?
For a 200-foot run carrying a full 100A load at 240V, 3 AWG copper will experience a 9.8V drop (4.08%), which violates the 3% recommendation. You must upsize to 1 AWG copper (yielding a 2.4% drop) or 2/0 AWG aluminum (yielding a 2.5% drop). Remember to proportionally upsize your equipment grounding conductor as well.
Is it safe to mix copper and aluminum wire on a 100 amp subpanel feeder?
You should never splice copper and aluminum wire inside a standard wire nut or conduit body due to galvanic corrosion, which causes high-resistance joints and fires. However, it is perfectly legal and safe to use aluminum wire for the long underground or outdoor run, and then transition to copper inside the main panel and subpanel using bi-metallic lugs or terminal blocks specifically rated for both Cu and Al (marked CU/AL). Ensure you use an anti-oxidant compound (like Noalox) on all aluminum terminations.
Do I need 4 wires or 3 wires for a 100 amp subpanel?
You must use 4 wires (two hots, one neutral, one ground). Prior to the 2008 NEC, 3-wire feeds to separate buildings were allowed under specific conditions with a ground rod acting as the ground path. This is no longer permitted for standard installations. The neutral and ground must be kept completely separate at the subpanel, meaning you must pull a dedicated equipment grounding conductor all the way back to the main panel's ground bus.
