To wire a 100 amp sub panel, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A double-pole breaker at the main panel. This assumes standard residential conditions. Always separate the neutral and ground bars in the subpanel, and run a dedicated 8 AWG copper equipment grounding conductor.
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C (Standard for residential panel terminations per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Raceway/Conduit: THHN/THWN-2 individual conductors in EMT/PVC conduit, or 4-wire SER cable
- System Voltage: 120/240V Single-Phase
Note: Sizing answers without stated assumptions are dangerous. If your ambient temperature exceeds 86°F or you are using a different insulation type, you must recalculate.
The Core Sizing Decision: Wire, Breaker, and Ampacity
Sizing a feeder for a subpanel requires matching the wire's ampacity to the overcurrent protective device (the breaker). According to NEC Table 310.16, 3 AWG copper THHN/THWN-2 in the 75°C column is rated for exactly 100A. This makes it the absolute minimum legal size for a 100A breaker.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity | Max Breaker Size |
|---|---|---|---|
| 4 AWG | Copper | 85A | 90A |
| 3 AWG | Copper | 100A | 100A |
| 2 AWG | Copper | 115A | 115A (Use 110A/100A) |
| 1 AWG | Aluminum | 100A | 100A |
Why this size and not one smaller?
You might wonder if you can use 4 AWG copper (85A) and rely on the NEC 240.4(B) "next size up" rule to protect it with a 100A breaker. You cannot. The "next size up" rule applies to specific branch circuits where standard breaker sizes don't match the calculated load, but it does not apply to calculated feeder loads. If your subpanel load calculation reaches 100A, the wire must be rated for 100A. Using 4 AWG on a 100A breaker risks melting the insulation and starting a fire before the breaker's thermal trip mechanism engages.
Variables That Force a Wire Size Upgrade
While 3 AWG copper is the baseline, real-world jobsite conditions frequently force you to upsize to 2 AWG or even 1 AWG. Here is the decision matrix for when to bump your wire size.
| Condition | Impact on 3 AWG Copper | Required Action |
|---|---|---|
| Long Distance Runs | Voltage drop exceeds 3% limit | Upsize to 2 AWG (or 1 AWG for >200ft) |
| Conduit Bundling | Ampacity derated by 80% or less | Upsize to 1 AWG or 1/0 AWG |
| High Ambient Temp | Attic/roof temp exceeds 86°F (30°C) | Apply Table 310.15(B)(1) correction factors; likely upsize |
The Voltage Drop Check
The NEC recommends a maximum 3% voltage drop on feeders. Let's run the math for a 150-foot run carrying a continuous 80A load on a 240V circuit using 3 AWG copper.
Using the formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, and CM=52,620 for 3 AWG):
- VD = (2 × 12.9 × 80 × 150) / 52,620
- VD = 309,600 / 52,620 = 5.88 Volts
A 5.88V drop on a 240V system is a 2.45% drop. This passes the 3% rule. However, if your run extends to 200 feet, the drop hits 3.27%, which fails. At 200 feet, you must upgrade to 2 AWG copper to maintain code-compliant power quality.
Conduit Bundling and the Neutral Conductor
A common mistake is miscounting current-carrying conductors (CCCs). For a standard single-phase 240/120V feeder, the neutral only carries unbalanced current and is not counted as a CCC per NEC 310.15(C)(1). Therefore, pulling one feeder (2 hots + 1 neutral + 1 ground) means only 2 CCCs. No derating is required.
However, if you pull two separate 100A feeders in the same conduit (4 hots + 2 neutrals = 4 CCCs), you must apply an 80% derating factor. 3 AWG copper at 90°C is 115A. Multiplied by 0.80, it drops to 92A—which is insufficient for a 100A breaker. You would be forced to upsize to 1 AWG copper.
Termination, Torque, and Physical Installation
Sizing the wire correctly is only half the battle; terminating it properly prevents arc faults and melted lugs. Modern NEC 110.14(D) mandates the use of calibrated torque tools for terminations.
- Strip Length: Strip exactly the length specified on the breaker or panel label (usually 5/8" to 3/4" for 3 AWG). Do not nick the copper strands.
- Insertion: Ensure no bare copper is visible outside the lug, and no insulation is trapped inside the lug. Both create high-resistance hotspots.
- Torque: Check the manufacturer's spec sheet. For a Square D HOM2100 or QO2100 100A breaker, the torque spec for 3 AWG is typically around 45 to 50 inch-pounds. Use a digital torque screwdriver. Guessing by hand leads to loose connections that will thermally fail under a 60A+ continuous load.
The Critical Grounding and Bonding Rule
The most frequent code violation in subpanel installations is failing to isolate the neutral and ground.
In a subpanel, the neutral bar must be isolated (floating) from the panel enclosure and the ground bar. If your subpanel came with a green bonding screw or a metal bonding strap connecting the neutral bar to the chassis, you must remove it. Neutral and ground must only be bonded at the main service disconnect. Bonding them at a subpanel creates parallel neutral paths, energizing the grounding system and creating a severe shock hazard.
Equipment Grounding Conductor (EGC) Sizing: Per NEC 250.122, a 100A overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. Do not rely on the conduit alone as the ground path for a detached structure; pull a dedicated wire.
When an Engineer or AHJ Must Confirm
While this guide covers standard residential and light-commercial applications, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer review your plans if:
- Your feeder run exceeds 250 feet (requiring complex voltage drop and fault-current calculations).
- The installation environment has ambient temperatures consistently above 113°F (45°C), such as specific industrial boiler rooms or unventilated desert attics.
- The subpanel will supply continuous industrial loads (like large HVAC compressors or EV fast-chargers) that exceed 80A for 3 hours or more, requiring 125% continuous load derating on the feeder.
Always defer to your local inspector, as municipal amendments to the National Electrical Code can supersede standard tables.
Frequently Asked Questions
Can I use a 100A main breaker panel as a subpanel?
Yes, and it is a very common, cost-effective practice. A "main breaker" panel simply has a built-in disconnect. You can feed the main lugs of the subpanel from your 100A feeder, and the 100A main breaker in the subpanel will act as a local disconnect switch. Just remember to remove the bonding screw/strap so the neutral floats, and ensure the feeder breaker at the main panel is also 100A (or smaller) to protect the bus bars.
What size ground wire do I need for a 100 amp sub panel?
According to NEC Table 250.122, the minimum equipment grounding conductor for a 100A breaker is 8 AWG copper or 6 AWG aluminum. If you have upsized your hot wires for voltage drop (e.g., using 1 AWG copper instead of 3 AWG for a long run), NEC 250.122(B) requires you to proportionally increase the ground wire size as well to maintain the same fault-current clearing capability.
Do I need a disconnect switch for a detached garage subpanel?
Yes. NEC 225.32 requires a disconnecting means for outbuildings. The easiest and most code-compliant way to satisfy this is to use a "main breaker" style panel for your subpanel. The main breaker inside the detached garage serves as the required local disconnect, allowing you to kill all power to the building without walking back to the main house panel.
