100 amp service wire aluminum refers to the specific gauge and insulation rating of aluminum conductors—typically 1/0 AWG or #1 AWG—used to safely deliver 100 amps of current to a residential main panel or subpanel without exceeding thermal limits. What this changes in a real installation is the physical conduit diameter required, the termination torque specifications, and the mandatory use of anti-oxidant compound to prevent high-resistance heating at the lugs. What people commonly confuse it with is assuming aluminum and copper share the exact same ampacity tables, or failing to distinguish between National Electrical Code (NEC) rules for main service entrance conductors versus subpanel feeders.
Aluminum is significantly lighter and cheaper than copper, making it the default choice for utility service drops and heavy feeder runs. However, aluminum expands and contracts more than copper under thermal cycling and is prone to galvanic corrosion. Getting the sizing and termination right is the difference between a safe, decades-long installation and a melted breaker lug.
The Sizing Matrix: Aluminum vs. Copper for 100A Circuits
The most critical mistake DIYers and junior apprentices make with 100 amp service wire aluminum is looking at the wrong column in NEC Table 310.16, or applying the residential service 83% rule to a subpanel feeder where it does not belong.
Under NEC Article 310.12(A), the conductors for a residential dwelling service only need to be rated for 83% of the service rating. For a 100A main service, 100A × 0.83 = 83A. Therefore, #2 AWG aluminum (rated 90A at 75°C) is legally permitted for the main service entrance. However, if you are running a 100A feeder to a detached garage subpanel, the 83% rule does not apply. You must size the wire to carry the full 100A, requiring a minimum of #1 AWG aluminum (110A at 75°C).
| Conductor Material | AWG Size | 75°C Ampacity | Permitted for 100A Main Service? | Permitted for 100A Subpanel Feeder? |
|---|---|---|---|---|
| Copper | #4 AWG | 85A | Yes (via 83% rule) | No |
| Copper | #3 AWG | 100A | Yes | Yes |
| Aluminum | #2 AWG | 90A | Yes (via 83% rule) | No |
| Aluminum | #1 AWG | 110A | Yes | Yes (Check Voltage Drop) |
| Aluminum | 1/0 AWG | 125A | Yes | Yes (Recommended for long runs) |
Worked Example: Sizing a 100A Detached Garage Feeder
Ampacity is only half the battle. If your run is long, voltage drop will dictate a larger wire size. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders, and under 5% total from the utility transformer to the furthest outlet.
The Scenario: You are wiring a 100A subpanel in a detached garage. The one-way distance from the main panel to the subpanel is 150 feet. The system is 240V single-phase. You want to use aluminum wire in PVC conduit.
The Math:
We use the single-phase voltage drop formula: VD = (2 × K × I × L) / CM
- K (Specific resistance for aluminum at 75°C) = 21.2
- I (Current) = 100A
- L (One-way length) = 150 ft
- CM (Circular Mils of the wire) = Looked up in NEC Chapter 9, Table 8
Attempt 1: #1 AWG Aluminum (CM = 83,690)
VD = (2 × 21.2 × 100 × 150) / 83,690
VD = 636,000 / 83,690 = 7.6 Volts
Percentage Drop = (7.6 / 240) × 100 = 3.16%
At 3.16%, #1 AWG slightly exceeds the recommended 3% feeder limit. While not a strict NEC violation for this specific feeder (unless local AHJ enforces the 3% informational note as mandatory), it is poor practice for a workshop that might run heavy inductive loads like welders or air compressors.
Attempt 2: 1/0 AWG Aluminum (CM = 105,600)
VD = 636,000 / 105,600 = 6.02 Volts
Percentage Drop = (6.02 / 240) × 100 = 2.51%
Where You Meet 100A Aluminum Wire in Practice
You will encounter 100 amp service wire aluminum primarily in three scenarios: upgrading an older 100A main service mast, running a heavy feeder to a detached structure (garage, barn, or ADU), or wiring a large shop addition. In all these cases, the physical installation requires specific techniques that differ from copper.
Anti-Oxidant Paste is Non-Negotiable
Aluminum rapidly forms a non-conductive oxide layer when exposed to air. If you terminate bare aluminum wire directly into a lug, this oxide layer increases electrical resistance, generating heat that can melt the breaker or start a fire. You must coat the stripped, un-insulated portion of the aluminum conductor with an anti-oxidant compound (commonly known by the brand name Noalox) before inserting it into the lug. The paste contains zinc dust that breaks through the oxide layer and seals out oxygen.
Torque Specifications Matter More Than with Copper
Aluminum exhibits 'cold flow'—it slowly deforms and compresses under constant mechanical pressure. If you under-torque an aluminum lug, the wire will eventually loosen as it thermally cycles, leading to arcing. If you over-torque it, you shear the strands or crack the lug. You must use a calibrated torque screwdriver or torque wrench set to the exact value printed on the breaker or panel label. For a typical 100A Square D or Eaton breaker, this is often around 250 to 300 inch-pounds, but always verify the manufacturer's spec sheet.
Conduit Fill and Bending Radius
Because 1/0 AWG aluminum requires a larger physical diameter than #3 AWG copper, your conduit size must increase. Four 1/0 AWG THWN-2 conductors (two hots, one neutral, one ground) require a minimum of 1.25-inch Schedule 40 PVC conduit to stay under the NEC 40% fill capacity rule. Furthermore, aluminum is stiffer; you will need a larger bending radius and may need to use a heat gun or mechanical bender to sweep conduit without kinking it.
Frequently Asked Questions
Can I connect aluminum wire directly to copper-only lugs?
No. If the breaker or lug is not explicitly rated for aluminum (marked 'AL' or 'AL/CU'), you cannot use it. Connecting aluminum to copper in the presence of moisture creates a galvanic battery effect, leading to rapid corrosion and high resistance. If you must transition, use a bi-metallic split bolt connector or a lug specifically rated for both materials, and apply anti-oxidant paste to the aluminum side.
Which insulation type is best for underground 100A aluminum feeders?
For direct burial or wet conduit runs, you need insulation rated for wet locations. XHHW-2 is generally preferred over THWN-2 for heavy aluminum feeders. XHHW-2 features a cross-linked polyethylene (XLPE) insulation that is thinner (allowing for easier pulling in conduit), highly resistant to moisture, and handles higher short-circuit temperatures without melting.
Do I need to derate 100 amp service wire aluminum if it runs through a hot attic?
Yes. NEC Table 310.15(B)(1) requires ampacity derating if the ambient temperature exceeds 86°F (30°C). If your attic reaches 122°F (50°C) in the summer, you must apply a 0.82 correction factor to the 90°C column of the wire. For 1/0 AWG aluminum (150A at 90°C), 150 × 0.82 = 123A. This still safely covers your 100A breaker, but if you were using #1 AWG (135A at 90°C), the derated capacity drops to 110A, leaving very little margin. Always check ambient temperatures for attic and rooftop conduit runs.
