For a 100A subpanel under 50 feet, use 3 AWG copper THHN or 1 AWG aluminum XHHW on a 100A double-pole breaker. If pulling NM-B cable, step up to 2 AWG copper because NM-B is restricted to the 60°C ampacity column.
- Material: Copper or Aluminum conductors (never mixed interchangeably).
- Temperature Column: 75°C for THHN/XHHW in conduit; 60°C for NM-B (Romex).
- Ambient Temperature: 30°C (86°F) baseline. Higher ambient requires derating.
- Conduit Type: PVC Schedule 80 or EMT for THHN/XHHW runs.
- Configuration: 4-wire feed (2 hots, 1 neutral, 1 equipment ground) as mandated by NEC 250.140.
The Core Ampacity Table for Subpanel Feeders
Before pulling any wire, you must match your breaker size to the correct ampacity column in NFPA NEC Table 310.16. The most common mistake DIYers make is looking at the 90°C column for THHN wire. Unless your breaker lugs are explicitly rated for 90°C (most residential breakers are not), you must use the 75°C column for conduit runs and the 60°C column for NM-B cable.
| Breaker Size | Copper THHN (75°C Column) | Copper NM-B (60°C Column) | Aluminum XHHW (75°C Column) |
|---|---|---|---|
| 60A | 6 AWG (65A) | 4 AWG (70A) | 4 AWG (65A) |
| 100A | 3 AWG (100A) | 2 AWG (115A) | 1 AWG (100A) |
| 125A | 1 AWG (130A) | 1/0 AWG (150A) | 1/0 AWG (120A)* |
| 200A | 2/0 AWG (175A)* | 4/0 AWG (195A)* | 4/0 AWG (180A)* |
* Asterisk denotes application of NEC 240.4(B) "Next Standard Size Up" rule, where the wire ampacity does not perfectly match a standard breaker size, allowing the next highest standard breaker to protect the wire.
Decoding the Rules: Why This Size and Not Smaller?
Wire sizing is dictated by thermal limits. If you push 95A through a 6 AWG copper wire rated for 65A, the insulation will melt and cause a fire long before a 100A breaker trips. You cannot use a smaller wire because the breaker's sole job is to protect the wire, not the appliance.
The NM-B "60°C Trap"
Why does a 100A subpanel require 2 AWG copper if you use NM-B (Romex), but only 3 AWG if you use THHN in conduit? NEC Article 334.80 strictly limits NM-B cable ampacity to the 60°C column, regardless of the fact that the wire's physical insulation might be rated for 90°C.
- 3 AWG Copper at 60°C: Rated for 85A. If you put this on a 100A breaker, a sustained 90A load will overheat the wire without tripping the breaker. This is a severe fire hazard and an automatic inspection failure.
- 2 AWG Copper at 60°C: Rated for 115A. This safely handles a 100A breaker.
The "Next Size Up" Rule (NEC 240.4(B))
Look at the 200A Aluminum row in the table above. 4/0 AWG Aluminum at 75°C is rated for 180A. There is no standard 180A breaker. NEC 240.4(B) allows you to round up to the next standard breaker size (200A) provided the calculated load does not exceed the wire's 180A ampacity. If your actual calculated load is 190A, you cannot use 4/0 Aluminum; you must step up to 250 kcmil.
Voltage Drop: When Distance Forces a Larger Wire
Ampacity tables assume a short run. When your main panel to sub panel wiring diagram spans a long distance, resistance in the wire causes voltage to drop at the destination. The NEC recommends a maximum 3% voltage drop for feeders to ensure motors and appliances operate efficiently without overheating.
Let's run a voltage drop check for our 100A subpanel located 150 feet away from the main panel, using 240V and 3 AWG Copper THHN.
K (Copper resistance constant) = 12.9
I (Current) = 100A
L (One-way length) = 150 ft
CM (Circular Mils for 3 AWG) = 52,620
The Math: (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts dropped.
Percentage: 7.35V / 240V = 3.06%.
At 3.06%, we are technically over the recommended 3% threshold for a feeder. While not an automatic code violation in all jurisdictions, it is poor practice. To fix this, we step up to 2 AWG Copper THHN (CM = 66,360). The new drop is 5.8V (2.4%), which is well within acceptable limits. If your run exceeds 150 feet, always calculate voltage drop before buying wire.
Variables That Alter Your Wiring Diagram
The table and math above assume a perfect, baseline installation. Real-world jobsite conditions frequently force you to change your wire size or materials. Use this decision tree to adjust your main panel to sub panel wiring diagram.
| Variable | Impact on Wire Size | NEC Reference & Action Required |
|---|---|---|
| Conductor Bundling | Must increase wire gauge (derate ampacity). | NEC 310.15(C)(1): If you pull more than 3 current-carrying conductors (CCCs) in one conduit, you must derate. E.g., 4-6 CCCs requires multiplying ampacity by 80%. |
| High Ambient Heat | Must increase wire gauge. | NEC Table 310.15(B)(1)(1): If conduit runs across a hot roof or through an attic exceeding 30°C (86°F), apply temperature correction factors. |
| Using Aluminum | Requires larger gauge than copper. | Aluminum expands/contracts more. You must use anti-oxidant paste (e.g., Noalox) and torque lugs to manufacturer specs to prevent arcing fires. |
| Detached Structure | Requires additional grounding rod. | NEC 250.32: A detached garage subpanel requires a local Grounding Electrode System (usually two ground rods), even though the equipment ground wire still runs back to the main panel. |
Executing the 4-Wire Diagram Correctly
The most critical aspect of your main panel to sub panel wiring diagram is the separation of neutral and ground. In the main panel, the neutral bar and ground bar are bonded together. In a subpanel, they must be completely isolated.
When wiring the subpanel:
- Ensure the subpanel has a separate, add-on equipment grounding bar.
- Remove the green bonding screw or strap from the subpanel's neutral bar. This is the #1 failure mode in subpanel installations.
- Route the two hot wires to the double-pole breaker.
- Route the white neutral wire exclusively to the isolated neutral bar.
- Route the bare/green equipment ground wire exclusively to the new grounding bar.
If you bond the neutral and ground in a subpanel, normal unbalanced neutral current will travel back to the main panel via the ground wire, energizing the conduit, the panel enclosure, and any bonded plumbing, creating a severe shock hazard.
Always consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if your feeder exceeds 400A, if you are wiring a multi-building campus, or if your local municipality has amended the NEC to ban aluminum feeders for residential use. Furthermore, OSHA electrical safety standards and local codes always supersede general internet guidance. Always de-energize the main breaker, lock it out, and verify with a tested multimeter before opening a panel.
