For a standard residential 100 amp sub panel, the minimum code-compliant feeder is 3 AWG copper or 1 AWG aluminum, protected by a 100A 2-pole breaker. This assumes THHN/THWN-2 conductors, 75°C terminations, and a run under 100 feet.
- Material: Copper (Aluminum alternatives noted separately)
- Temperature Column: 75°C (Standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit: EMT or PVC, 3 current-carrying conductors + ground (no derating required)
- Voltage: 240V/120V split-phase residential
Safety Note: Always de-energize the main panel, verify dead with a tested multimeter, and follow NFPA 70 (NEC) guidelines. Your local AHJ has final authority.
The Core Specs: Wire, Breaker, and Conduit Sizing
When interpreting a residential 100 amp sub panel wiring diagram, you are looking at a 4-wire feeder system. The National Electrical Code (NEC) strictly requires two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor (EGC) for any subpanel installed after the 1990s.
| Component | Specification | NEC Reference |
|---|---|---|
| Feeder Breaker | 100A, 2-Pole | 240.4(B) / 310.16 |
| Hot Conductors (2x) | 3 AWG Copper or 1 AWG Aluminum | 310.16 (75°C col) |
| Neutral Conductor (1x) | 3 AWG Copper or 1 AWG Aluminum | 220.61 / 310.16 |
| Equipment Ground (1x) | 8 AWG Copper or 6 AWG Aluminum | 250.122 |
| Minimum Conduit Size | 1-inch PVC or EMT | Chapter 9, Table 1 |
Why 3 AWG Copper? (And the Continuous Load Trap)
A common mistake DIYers make is looking at the 90°C column of NEC Table 310.16, seeing that 4 AWG THHN is rated for 95A, and assuming it is close enough to 100A. This is a code violation and a fire hazard. You must use the 75°C column because virtually all residential breakers and panel lugs are rated for 75°C terminations. In the 75°C column, 4 AWG copper is only rated for 85A. 3 AWG copper is rated for exactly 100A.
The Continuous Load Trap
There is a critical distinction between a '100A rated panel' and a '100A continuous load'. By NEC Article 215.2(A)(1), conductors must be sized at 125% of any continuous load (loads expected to run for 3 hours or more).
If you are installing a subpanel with a 100A busbar rating, the maximum continuous load you can legally place on it is 80A. Therefore, a 100A breaker and 3 AWG wire are perfectly correct. However, if your actual calculated continuous load (like a massive EV charger plus a heat pump) equals 100A, you must multiply by 1.25. You would then need a 125A breaker and 1 AWG copper wire, and you would need a subpanel rated for at least 125A.
Voltage Drop and Distance Adjustments
The NEC mandates minimum wire sizes for safety (preventing fires), but it only recommends voltage drop limits (Informational Note in 310.15(B)). For a feeder, you want to keep voltage drop under 3% to ensure your 240V appliances actually receive at least 232V.
Let us run the math on 3 AWG copper carrying 80A (a realistic continuous load for a 100A panel). The resistance of 3 AWG copper is roughly 0.245 ohms per 1,000 feet. At 100 feet, the voltage drop is about 3.92V (1.6%). At 150 feet, it jumps to 5.88V (2.45%). Both are well under the 3% threshold. But if you push past 160 feet, you need to upsize.
| Run Distance | Copper Size | Aluminum Size | Voltage Drop (@ 80A Load) |
|---|---|---|---|
| Under 50 ft | 3 AWG | 1 AWG | < 1.0% (Pass) |
| 50 - 110 ft | 3 AWG | 1 AWG | 1.0% - 2.2% (Pass) |
| 110 - 160 ft | 2 AWG | 1/0 AWG | 1.8% - 2.6% (Pass) |
| 160 - 220 ft | 1 AWG | 2/0 AWG | 2.0% - 2.8% (Pass) |
What changes the answer? If you are pulling this feeder through an attic in Phoenix where ambient temperatures reach 50°C (122°F), you must apply a temperature correction factor from NEC Table 310.15(B)(1)(1). At 50°C, the 75°C column ampacity is multiplied by 0.75. Your 3 AWG wire (100A) derates to 75A. You would be forced to upsize to 1 AWG copper just to maintain your 100A rating, regardless of distance.
Grounding, Bonding, and the Wiring Diagram Essentials
The most frequent point of failure in subpanel installations is improper bonding. In your main service panel, the neutral and ground bars are bonded together. In a subpanel, they must be strictly isolated.
- Neutral Bar: Must be floating (isolated from the metal panel enclosure). Ensure the green bonding strap or screw that comes pre-installed in new panels is removed or left out.
- Ground Bar: Must be bonded directly to the metal panel enclosure. All equipment grounding conductors from branch circuits land here, along with your 8 AWG feeder ground.
- Torque: Do not just tighten lugs until they feel tight. Use a calibrated torque screwdriver. For 3 AWG copper in a standard Square D or Eaton lug, the required torque is typically between 40 and 45 in-lbs, but always verify the sticker inside your specific panel door. Loose connections cause high resistance, arcing, and melted lugs.
- Aluminum Prep: If you opt for 1 AWG aluminum to save money, you must score the wire, apply an anti-oxidant compound like Noalox, and torque to the manufacturer's exact specification. Aluminum creeps under pressure and will loosen over time if not prepped correctly.
For more on safe grounding practices and avoiding shock hazards, refer to the OSHA electrical safety guidelines and your local inspector's requirements.
Frequently Asked Questions
Can I use a 100A main breaker in the subpanel if the feeder breaker is 100A?
Yes, you can install a subpanel that has a 100A main breaker, even if the feeder breaker back at the main panel is also 100A. The breaker in the subpanel acts merely as a local disconnect switch, which is required by NEC 225.32 if the subpanel is in a detached structure (like a garage or shed). Having two 100A breakers in series does not cause a coordination issue for residential loads, as the upstream breaker will trip on a true short circuit. Just ensure the subpanel's busbar rating is at least 100A.
What changes if I bundle this feeder with other circuits in the same conduit?
If you pull your 4-wire subpanel feeder into a conduit that already contains other current-carrying conductors (like a 20A circuit to a detached garage), you trigger NEC 310.15(C)(1) bundling derating rules. The neutral in a split-phase 240/120V system only carries unbalanced current, so it is often not counted as a current-carrying conductor (CCC), but the grounds never count. If you have 4 to 6 CCCs in the pipe, you must derate the ampacity to 80%. Your 3 AWG copper (100A) drops to 80A, meaning you must upsize to 2 AWG copper to maintain your 100A feeder rating. Keep subpanel feeders in their own dedicated conduit whenever possible.
When do I need an engineer or AHJ to confirm my subpanel sizing?
You should pull a permit and have the local Authority Having Jurisdiction (AHJ) review your plans anytime you are installing a subpanel in a detached structure (which requires a separate grounding electrode system per NEC 250.32), or if your total service load calculation is approaching your main service rating (e.g., adding a 100A subpanel to an older 150A main service that already has heavy HVAC and electric range loads). If you are dealing with complex continuous loads, solar backfeeding (NEC 705.12), or battery storage systems, a licensed electrical engineer or master electrician should stamp the load calculations.
