- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (Standard for residential breaker lugs)
- Ambient Temperature: 30°C (86°F)
- Installation: Not more than 3 current-carrying conductors in a single raceway
The Core Sizing Rules: Service Entrance vs. Subpanel Feeder
The most common mistake DIYers make when asking what size wire for 100 amp service is treating a main service entrance and a subpanel feeder as the same thing. The National Electrical Code (NEC) treats them differently because of load diversity.
A service entrance feeds your entire home. The NEC recognizes that you will rarely run every single appliance at maximum draw simultaneously. Therefore, NEC Article 310.12 allows you to size service entrance conductors at 83% of the service rating. A 100A service only requires wire rated for 83A.
A subpanel feeder does not get this 83% discount. If you are feeding a detached garage or a workshop subpanel with a 100A breaker, the wire must have a full 100A ampacity.
| Application | Copper AWG | Aluminum AWG | NEC Reference | Min. Ampacity Required |
|---|---|---|---|---|
| Main Service Entrance | #4 AWG | #2 AWG | 310.12(A) | 83A |
| Subpanel Feeder | #3 AWG | #1 AWG | 310.14 / 310.16 | 100A |
Why Not One Size Smaller? (The 83% Rule and Terminal Limits)
Why can't you use #6 AWG copper for a 100-amp service entrance? It comes down to terminal temperature limits and the strict math of the 83% rule.
Under the 83% rule, a 100A service requires a minimum ampacity of 83A (100 x 0.83). According to the Cerrowire standard ampacity charts based on NEC Table 310.16, #6 AWG copper in the 75°C column is only rated for 65A. It fails the 83A minimum requirement.
The 90°C Column Trap: Many beginners look at the 90°C column, see that #6 AWG copper is rated for 75A, and assume it's close enough. This violates NEC 110.14(C). Unless your breaker and lugs are explicitly rated for 90°C (almost none in residential panels are), you must use the 75°C column for termination limits. #4 AWG copper is rated for 85A at 75°C, safely clearing the 83A hurdle.
Aluminum has lower conductivity and higher thermal expansion than copper. A #4 AWG aluminum wire is only rated for 65A at 75°C. If you use the copper sizing rule for aluminum wire on a 100A feeder, the wire will overheat, the lugs will loosen over time due to thermal cycling, and you risk an arc fault or fire. Always use the aluminum-specific column.
Variables That Force a Size Upgrade
The baseline sizes above assume a perfect installation: a short run, standard ambient temperatures, and isolated conductors. Real-world jobsites rarely cooperate. Here is what forces you to upsize your wire.
1. Voltage Drop on Long Runs
The NEC recommends a maximum voltage drop of 3% for branch circuits and feeders to ensure equipment runs efficiently. Let's run the math on a 150-foot run of #4 AWG copper carrying a full 100A load on a 240V system.
- Formula: VD = (2 × K × I × D) / CM
- K (Copper): 12.9
- I (Current): 100A
- D (Distance): 150 ft
- CM (Circular Mils for #4 AWG): 41,740
- Calculation: (2 × 12.9 × 100 × 150) / 41,740 = 9.27 Volts
9.27V is 3.86% of 240V. This exceeds the 3% recommendation. For a 150-foot run, you must upgrade to #2 AWG copper (which drops the VD to 5.83V, or 2.4%) to maintain proper voltage at the panel.
2. Conductor Bundling and Derating
If you are pulling multiple circuits through the same conduit, the wires heat each other up. Under NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a raceway, you must derate the ampacity to 80%. If your #3 AWG copper feeder (100A) shares a conduit with another circuit, its derated ampacity drops to 80A. You would be forced to upsize to #1 AWG copper to maintain a 100A rating after derating.
| Site Condition | Impact on Wire | Required Action (Copper) |
|---|---|---|
| Run exceeds 100 feet | Voltage drop exceeds 3% | Upsize to #2 AWG |
| Ambient temp > 104°F (40°C) | Ampacity derates by 15% | Upsize to #3 AWG (Feeder) or #2 AWG |
| 4-6 wires in one conduit | Ampacity derates to 80% | Upsize to #1 AWG (Feeder) |
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline, your local Authority Having Jurisdiction (AHJ) or the utility company has the final say. You must consult an engineer or the AHJ in the following scenarios:
- Utility Drop Loops: Some utility companies require a specific minimum wire size (often #2 AWG aluminum or #4 AWG copper minimum) for the service drop from the pole to the weatherhead, regardless of the panel size.
- Continuous Loads: If your 100A subpanel is dedicated entirely to continuous loads (equipment running for 3 hours or more, like a massive server rack or commercial HVAC), NEC 210.20(A) requires the overcurrent device to be sized at 125% of the load. A 100A continuous load requires a 125A breaker and correspondingly larger wire.
- High-Temperature Attics: If you are routing service conductors through an uninsulated attic in a climate like Arizona or Texas, ambient temperatures can easily exceed 120°F. The severe derating requires professional load calculations to ensure the wire won't melt its insulation.
FAQ: Your 100-Amp Wire Questions Answered
Can I use #2 AWG aluminum for a 100-amp subpanel feeder?
No. #2 AWG aluminum is rated for 90A in the 75°C column. While 90A is sufficient for a 100A service entrance under the 83% rule (which requires 83A), a subpanel feeder does not get the 83% discount. A subpanel feeder must have a full 100A ampacity. For aluminum subpanel feeders on a 100A breaker, you must use #1 AWG aluminum (rated 100A) or 1/0 AWG aluminum (rated 120A) to account for voltage drop on longer runs.
Does the ground wire need to be the same size as the hot wires for 100 amps?
No, the equipment grounding conductor (EGC) is sized based on the breaker rating, not the hot wire size. According to NEC Table 250.122, a 100A overcurrent device requires a minimum #8 AWG copper or #6 AWG aluminum ground wire. However, if you upsized your hot wires to #2 AWG copper to mitigate voltage drop on a long run, NEC 250.122(B) requires you to proportionally upsize the ground wire as well to maintain a low-impedance fault path.
What size conduit do I need for three #4 AWG copper wires and a ground?
For a standard 100A service entrance using three #4 AWG THHN hot/neutral wires and one #8 AWG THHN ground wire, the total cross-sectional area of the wires is approximately 0.245 square inches. According to NEC Chapter 9, Table 1, conduit can only be filled to 40% capacity for three or more wires. A 1-inch PVC Schedule 80 conduit (which has an internal area of 0.688 sq in, allowing 0.275 sq in at 40%) is the standard, code-compliant choice that provides enough physical room to pull the wires without damaging the insulation.
