For a 100-amp subpanel or service feeder, use 1/0 AWG aluminum wire protected by a 100-amp breaker. This setup safely delivers the required current while providing headroom for voltage drop. Always terminate on 75°C rated lugs and follow local code.
Sizing feeder wire isn't just about matching the breaker number to the wire number. When you are pulling aluminum for a 100-amp service or subpanel, you are balancing NEC ampacity tables, voltage drop over distance, and the physical realities of aluminum terminations. Below is the exact benchmark data and decision framework you need to pull the right wire and pass inspection.
The Core Sizing Specs and Baseline Assumptions
Before looking at the tables, we must establish the baseline assumptions. Wire ampacity is not a fixed number in a vacuum; it changes based on insulation type, ambient temperature, and termination ratings. The sizing guidance below relies on the following strict parameters:
- Conductor Material: Aluminum (Specifically AA-8000 series alloy, as required by NEC 310.14)
- Insulation Type: XHHW-2 or THHN/THWN-2
- Temperature Column: 75°C (Per NEC 110.14(C) for equipment terminations rated 100A or less)
- Ambient Temperature: 30°C (86°F) or lower
- Raceway Conditions: Maximum of 3 current-carrying conductors in a single conduit
According to Southwire's official ampacity tables and NEC Table 310.16, here is how the common aluminum feeder sizes stack up in the 75°C column:
| AWG Size (Aluminum) | 75°C Ampacity | 90°C Ampacity (Wire rating only) | Suitable for 100A Breaker? |
|---|---|---|---|
| 2 AWG | 90 Amps | 100 Amps | No (Undersized for 100A continuous/standard feeder) |
| 1 AWG | 100 Amps | 115 Amps | Yes (Exact match, but zero margin for drop) |
| 1/0 AWG | 120 Amps | 135 Amps | Yes (Ideal, provides drop/heat headroom) |
Note: While the wire insulation itself is often rated for 90°C, the National Electrical Code (NEC) mandates that you must size the wire based on the lowest temperature rating of any connected component. Since most 100A breakers and subpanel lugs are rated for 75°C, you must use the 75°C column for your final ampacity decision.
Why 1/0 AWG Aluminum and Not 2 AWG or 1 AWG?
A common mistake on the jobsite is trying to squeeze 2 AWG aluminum onto a 100-amp breaker because 'it's close enough.' It isn't. 2 AWG aluminum is rated for 90 amps at 75°C. While NEC 240.4(B) allows you to round up to the next standard breaker size if the exact ampacity doesn't match a standard breaker, 90A rounding up to 100A is generally rejected by inspectors for a full 100A service feeder because it leaves no room for continuous load calculations.
So why jump past 1 AWG (which is exactly 100A) straight to 1/0 AWG (120A)?
| Scenario | Recommended Size | Reasoning |
|---|---|---|
| Short run (< 50 ft), non-continuous load | 1 AWG Aluminum | Meets exact 100A requirement at 75°C. Cost-effective for very short distances. |
| Standard run (50-100 ft), mixed loads | 1/0 AWG Aluminum | 120A rating mitigates voltage drop and handles 80A continuous loads (which require 125% sizing, equating to 100A minimum wire ampacity). |
| Long run (> 100 ft) or high ambient heat | 2/0 AWG Aluminum | Required to counteract voltage drop over distance or ampacity derating from high attic/conduit temperatures. |
Furthermore, 1/0 AWG is frequently more available at local electrical supply houses than 1 AWG, which is an odd size that many distributors skip in favor of moving straight from 2 AWG to 1/0 AWG for residential feeders.
Voltage Drop and Distance Derating
Ampacity tells you if the wire will melt. Voltage drop tells you if your appliances will actually work. The NEC recommends a maximum 3% voltage drop on branch circuits and feeders. Let's run the math for 1/0 aluminum wire on a 100-amp load.
Assuming a 240V circuit, 100A load, and a 100-foot one-way distance:
Formula: VD = (2 × K × I × L) / CM
• K (AC resistance for Al) ≈ 21.2
• I (Current) = 100A
• L (Length) = 100 ft
• CM (Circular Mils for 1/0 AWG) = 105,600
Result: VD = (2 × 21.2 × 100 × 100) / 105,600 = 4.01 Volts.
Percentage = (4.01 / 240) × 100 = 1.67%.
Verdict: 1.67% is well under the 3% NEC recommendation. 1/0 AWG is perfectly safe for a 100-foot run.
However, if your run exceeds 150 feet, that 1.67% climbs past 2.5%, and you should strongly consider upsizing to 2/0 AWG aluminum to ensure heavy loads like HVAC compressors don't experience brownouts on startup.
Aluminum-Specific Installation Realities
If you are pulling 1/0 aluminum, you must respect the physical properties of the metal. Aluminum expands and contracts at a different rate than brass or copper lugs, and it oxidizes rapidly when exposed to air. Ignoring these facts is the leading cause of melted lugs and subpanel fires.
- Use Anti-Oxidant Paste: You must apply a listed antioxidant compound (like Noalox or Penetrox) to the stripped aluminum conductor before inserting it into the lug. This prevents aluminum oxide—which is highly resistive and generates massive heat—from forming at the connection point.
- Torque to Spec: NEC 110.14(D) requires terminations to be tightened to the manufacturer's specified torque. You cannot 'hand-tighten' 1/0 wire. Use a calibrated torque screwdriver or torque wrench. Most 100A panel lugs require between 40 and 50 in-lbs of torque. Check the label inside your specific panel.
- Beware of Cold Creep: Aluminum 'creeps' or deforms under constant pressure over time, which can loosen connections. This is why AA-8000 series alloy is mandated by code, and why proper torque and antioxidant paste are non-negotiable.
Frequently Asked Questions
Can I use 1/0 aluminum wire for a 100 amp main service entrance?
Yes, 1/0 AWG aluminum is widely used for 100-amp main service entrance conductors, provided your utility company and local AHJ (Authority Having Jurisdiction) approve it. However, service entrance rules are stricter than subpanel feeder rules. The utility may require specific insulation types (like XHHW-2 in wet locations) or mandate that the wire be pulled through rigid metal conduit rather than PVC. Always confirm with your local inspector and utility rep before purchasing wire for the meter-to-panel run.
How does bundling affect 1 0 aluminum wire for 100 amp service?
If you pull more than three current-carrying conductors through a single conduit, NEC Table 310.15(C)(1) requires you to derate the wire's ampacity. For example, if you have 4 to 6 current-carrying conductors in a pipe, you must multiply the 1/0 AWG's 120A base ampacity by 80%. That drops your usable ampacity to 96A, which is no longer sufficient for a 100-amp breaker. If you must bundle multiple circuits in one conduit, you will need to upsize to 2/0 AWG or 3/0 AWG aluminum to compensate for the heat buildup.
Is 1/0 aluminum or 3 AWG copper better for a 100 amp feeder?
Electrically, 3 AWG copper (rated 100A at 75°C) and 1/0 AWG aluminum (rated 120A at 75°C) will both safely handle a 100-amp breaker. The decision comes down to cost and conduit space. Copper is significantly more expensive and harder to bend, making it difficult to pull through tight conduit sweeps. Aluminum is much cheaper, lighter, and easier to work with in larger gauges. The only scenario where 3 AWG copper wins is if your conduit fill is strictly limited, as the copper wire has a smaller physical diameter.
