For a 100 amp sub panel, you need #1 AWG aluminum wire protected by a 100-amp double-pole breaker. This assumes a standard 240V feeder using XHHW-2 insulation in the 75°C ampacity column. If your one-way run exceeds 130 feet, upgrade to 1/0 AWG aluminum to maintain acceptable voltage drop.
- Material: Aluminum (AA-8000 series) / Copper equivalent noted for comparison
- Temperature Column: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F)
- Conduit Type: PVC Schedule 80 or EMT, maximum 3 current-carrying conductors
The Ampacity Breakdown: Why #1 AWG and Not #2?
When sizing feeders, the National Electrical Code (NEC) requires you to look at the NEC 310.16 ampacity tables. A common mistake on the jobsite is grabbing #2 AWG aluminum wire because it is cheaper and easier to pull, assuming the "next size up" breaker rule will cover it. It won't.
Under NEC Table 310.16, #2 AWG aluminum wire is rated for exactly 90 amps in the 75°C column. Because 100 amps is a standard breaker size listed in NEC 240.6, you are not permitted to use the "next size up" rule (NEC 240.4(B)) to protect a 90A wire with a 100A breaker. The wire's ampacity must meet or exceed the breaker rating. Therefore, you must step up to #1 AWG aluminum, which is rated for exactly 100 amps at 75°C.
| Wire Size (AWG) | Material | 75°C Ampacity | 90°C Ampacity | Verdict for 100A Feeder |
|---|---|---|---|---|
| #2 AWG | Aluminum | 90A | 100A | FAIL (Terminals limit to 75°C) |
| #1 AWG | Aluminum | 100A | 115A | PASS (Minimum Code Compliant) |
| 1/0 AWG | Aluminum | 120A | 135A | UPGRADE (Best for long runs) |
| #3 AWG | Copper | 100A | 115A | PASS (Copper Equivalent) |
Note on Insulation: Always use XHHW-2 or THHN/THWN-2. While these insulations are rated for 90°C in the wire itself, NEC 110.14(C) dictates that you must size the wire based on the lowest temperature rating of any connected termination, device, or conductor. Standard subpanel lugs are rated for 75°C, locking you into the 75°C column.
Voltage Drop and Distance: The 130-Foot Threshold
Ampacity tells you what the wire can handle without melting; voltage drop tells you what the equipment at the end of the wire will actually receive. NEC 210.19 recommends keeping feeder voltage drop under 3%. For a 240V system, that means a maximum drop of 7.2 volts.
Using the Southwire Voltage Drop Calculator or standard manual formulas (VD = 2 x K x I x D / CM), we can map out exactly when #1 AWG aluminum stops being sufficient. Aluminum has a higher resistance than copper (K ≈ 21.2 for Al at 75°C vs 12.9 for Cu), making distance a critical factor.
| One-Way Distance | Wire Size (Aluminum) | Calculated Voltage Drop | Percentage Drop (240V) | Action Required |
|---|---|---|---|---|
| 50 feet | #1 AWG | 2.53V | 1.05% | Use #1 AWG |
| 100 feet | #1 AWG | 5.06V | 2.10% | Use #1 AWG |
| 130 feet | #1 AWG | 6.58V | 2.74% | Limit of #1 AWG |
| 150 feet | #1 AWG | 7.60V | 3.16% | FAIL (Upsize to 1/0 AWG) |
| 150 feet | 1/0 AWG | 5.30V | 2.20% | PASS (Use 1/0 AWG) |
What Changes the Sizing Answer?
The #1 AWG baseline assumes a perfect, straightforward installation. Real-world jobsite conditions frequently force you to upsize your aluminum feeder. Here is what changes the math:
1. Conductor Bundling (Derating)
If you are pulling two separate 100A feeders through the same PVC conduit to feed two different outbuildings, you now have six current-carrying conductors in the pipe. Under NEC 310.15(C)(1), 4 to 6 conductors require an 80% derating factor. Your #1 AWG aluminum (100A base) derates to 80A. You must jump all the way to 2/0 AWG aluminum (135A base x 0.80 = 108A) to legally protect the circuit with a 100A breaker.
2. High Ambient Temperatures
Running conduit through an unventilated attic in the summer or across a sun-baked roof pad changes the ambient temperature. If the ambient temperature hits 110°F (43°C), you apply an 82% correction factor to the 90°C column of the wire before checking termination limits. This thermal penalty often forces an upsize to 1/0 AWG even on shorter runs.
3. Aluminum vs. Copper Terminations
Never present aluminum and copper interchangeably. Aluminum expands and contracts at a different rate than copper and is prone to galvanic corrosion if they touch. Ensure your subpanel main breaker lugs and the upstream breaker lugs are explicitly rated AL/CU. Furthermore, you must apply an anti-oxidant compound (like Noalox) to the stripped aluminum conductors before torquing them to the manufacturer's exact inch-pound specifications to prevent high-resistance arcing over time.
When an Engineer or AHJ Must Confirm
While the NEC provides the baseline, local Authority Having Jurisdiction (AHJ) inspectors and electrical engineers step in when the load profile gets complicated.
- Continuous Loads: If your subpanel will supply a workshop where the lighting and a 50A electric heater will run simultaneously for 3 hours or more, NEC 210.20 requires you to multiply the continuous load by 125%. If your calculated continuous load exceeds 80A, a 100A breaker is no longer sufficient, and you must upsize the entire feeder.
- Mixed Temperature Ratings: If you are connecting a modern 75°C rated subpanel to an older legacy disconnect switch rated only for 60°C, the entire circuit must be sized using the 60°C column. #1 AWG aluminum drops to 75A in the 60°C column, requiring a massive upsize to 2/0 AWG. An inspector will flag this immediately.
- Utility Service vs. Feeder: Do not confuse a subpanel feeder with a main service entrance. The 83% sizing rule (NEC 310.12) allows smaller wires for main residential services, but it explicitly does not apply to feeders. If you try to use the 83% rule to justify #2 AWG aluminum on a 100A subpanel feeder, your inspector will fail the rough-in.
Frequently Asked Questions
Can I use 2-2-2-4 aluminum mobile home feeder cable for a 100 amp sub panel?
Technically, no. Pre-packaged 2-2-2-4 mobile home feeder consists of #2 AWG aluminum conductors. As established in the ampacity table, #2 AWG aluminum is only rated for 90 amps at 75°C. While it is incredibly common to see this cable sold as a "100A kit" at big box stores, that labeling relies on the 83% service rule or assumes a 90A breaker. For a true 100A subpanel feeder protected by a 100A breaker, you must buy individual #1 AWG XHHW-2 conductors or a custom 1-1-1-3 cable assembly.
Why do I need a separate ground bar in my 100 amp aluminum sub panel?
In a main service panel, the neutral and ground bars are bonded together. In a subpanel, they must be kept strictly separate to prevent neutral return current from traveling back along the grounding path, which creates a shock hazard and electromagnetic interference. When you install your subpanel, you must remove the green bonding screw or strap from the neutral bar, and install an accessory equipment grounding bar. For a 100A feeder, your equipment grounding conductor (EGC) must be a minimum of #6 AWG aluminum or #8 AWG copper per NEC 250.122.
What size conduit do I need for three #1 AWG aluminum wires and a ground?
For three #1 AWG XHHW-2 conductors and one #6 AWG aluminum ground, NEC Chapter 9 Table 5 dictates the cross-sectional area. XHHW-2 is highly recommended here because its thinner insulation profile saves space compared to THHN. You can comfortably fit this feeder in 1-inch PVC Schedule 80 or 1-inch EMT, keeping you well under the 40% conduit fill limit required for three or more wires. If you upgrade to 1/0 AWG for a long run, stick to 1-inch conduit, but ensure you use a high-quality pulling lubricant to avoid damaging the insulation on bends.
Does the 83% rule apply to my 100 amp sub panel feeder?
No. NEC 310.12 (formerly 310.15(B)(7)) allows residential service conductors to be sized at 83% of the service rating. This means a 100A main service can use wire rated for 83A. However, the NEC explicitly restricts this allowance to the main service drop or lateral. Subpanel feeders are governed by standard ampacity rules under NEC 310.16. You must size the feeder wire to carry the full 100A without relying on the 83% discount.
