For a standard 100 amp service, what size wire do you need? The minimum code-compliant size is 3 AWG copper or 1 AWG aluminum, protected by a 100-amp breaker. This assumes THHN/THWN-2 insulation rated at 75°C terminations. However, most electricians upsize to 2 AWG or 1 AWG copper to mitigate voltage drop over distance.
- Material: Copper or Aluminum (explicitly separated)
- Termination Rating: 75°C (Standard for modern breakers and lugs)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Single circuit in PVC or EMT conduit, or standard SER cable
- Load Type: Non-continuous (under 3 hours of maximum draw)
The Core Sizing Rules: Copper vs. Aluminum
When sizing conductors for a 100-amp feeder or service entrance, you must reference Cerrowire Ampacity Charts or NEC Table 310.16. The National Electrical Code (NEC) strictly separates copper and aluminum due to their different thermal expansion rates and conductivity profiles. Never treat them interchangeably.
| Material | Insulation Type | Minimum AWG Size | 75°C Ampacity | 90°C Ampacity |
|---|---|---|---|---|
| Copper | THHN/THWN-2 | 3 AWG | 100A | 115A |
| Copper (Upsized) | THHN/THWN-2 | 2 AWG | 115A | 130A |
| Aluminum | XHHW/THWN-2 | 1 AWG | 100A | 115A |
Why 3 AWG? The Physics and Code Logic
A common point of confusion on the jobsite is why we use 3 AWG copper (rated 100A) instead of 4 AWG copper (rated 85A at 75°C, but 95A at 90°C). The answer lies in NEC 110.14(C) - Temperature Limitations.
Almost all modern residential breakers, panel lugs, and disconnects are tested and rated for a maximum termination temperature of 75°C. Even though THHN wire insulation is rated for 90°C, you are legally required to size the wire based on the 75°C column of the ampacity table when terminating at a 75°C lug.
If you use 4 AWG copper, its 75°C ampacity is only 85A. Putting 100A through a 4 AWG wire connected to a 75°C breaker lug will cause the lug to overheat, potentially melting the insulation or causing a thermal failure at the termination point long before the 100A breaker trips. Therefore, 3 AWG (exactly 100A at 75°C) is the absolute minimum.
Variables That Force an Upsize: Voltage Drop & Derating
While 3 AWG copper is the code minimum, real-world physics often forces an upsize. The two primary culprits are voltage drop over distance and conduit derating.
The Voltage Drop Check (150 ft Run)
The NEC recommends a maximum voltage drop of 3% for feeders. Let's run the math on a 150-foot run of 3 AWG copper carrying a full 100A load at 240V.
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper): 12.9
- I (Current): 100A
- D (Distance): 150 ft
- Circular Mils (3 AWG): 52,620
Calculation: (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts dropped.
7.35V / 240V = 3.06% drop.
At 150 feet, 3 AWG fails the 3% recommendation. You must upsize to 2 AWG copper (66,360 Circular Mils), which drops the loss to 5.83V (2.42%), keeping your 240V appliances running efficiently and preventing motor burnout.
Conduit Bundling and Derating
If you pull multiple circuits through the same conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to derate the ampacity based on the number of current-carrying conductors.
| Scenario | Current-Carrying Conductors | Adjustment Factor | Required Wire Size (Copper) |
|---|---|---|---|
| Standard 240V Feeder (2 Hots, 1 Neutral, 1 Ground) | 3 (Ground doesn't count) | 100% (No derating) | 3 AWG (or 2 AWG for distance) |
| Two 240V circuits sharing a conduit (4 Hots) | 4 | 80% | 2 AWG (115A × 0.8 = 92A at 75°C termination limit)* |
| Three 240V circuits sharing a conduit (6 Hots) | 6 | 50% | 1/0 AWG (Requires massive upsize to maintain 100A) |
*Note: When derating, you use the 90°C column to find your base wire size, but the final derated ampacity cannot exceed the 75°C termination rating. For a true 100A load with 4 conductors, 1 AWG copper is the safest benchmark.
When to Call an Engineer or the AHJ
While the rules above cover 95% of residential subpanels and standard service entrances, specific scenarios require professional engineering or explicit AHJ approval:
- Continuous Loads: If your 100A load will run at maximum capacity for 3 hours or more (e.g., a large commercial EV charger or continuous heating elements), NEC 210.20(A) requires you to multiply the load by 125%. A 100A continuous load requires a 125A breaker and wire sized for 125A (1 AWG Copper).
- Utility Transformer Limits: If you are upgrading a service entrance, the utility company must confirm their transformer and service drop can handle the fault current and continuous draw. A 100A panel fed by an undersized utility drop will experience severe brownouts.
- Ambient Temperature Extremes: If your conduit runs through an attic in Arizona where ambient temperatures regularly exceed 110°F (43°C), you must apply temperature correction factors from NEC Table 310.15(B)(1), which will force an upsize regardless of distance.
Frequently Asked Questions
Can I use 2 AWG aluminum for a 100 amp service?
No. At the 75°C termination rating, 2 AWG aluminum is only rated for 90 amps. To safely and legally carry 100 amps with aluminum, you must step up to 1 AWG aluminum (rated exactly 100A at 75°C). Many electricians prefer 1/0 AWG aluminum for 100A services simply because it provides a buffer for voltage drop and is easier to seat into larger mechanical lugs.
What size ground wire do I need for a 100 amp feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100-amp overcurrent device is 8 AWG copper or 6 AWG aluminum. If you upsized your ungrounded (hot) conductors for voltage drop, you are technically required by NEC 250.122(B) to proportionally increase the size of your ground wire, though many local inspectors allow the standard table size if the upsize was purely for voltage drop and not ampacity.
Why do some online charts say 4 AWG copper is good for 100 amps?
Those charts are referencing the 90°C column for THHN insulation. While 4 AWG copper at 90°C is rated for 95A (and sometimes rounded up in non-NEC contexts), you cannot use this column for breaker terminations. Furthermore, 4 AWG at 75°C is only 85A. Always ignore charts that do not explicitly state which temperature column they are using.
Does the wire size change if I use SER cable instead of THHN in conduit?
The AWG size requirement remains the same, but the physical installation changes drastically. Service Entrance Round (SER) cable typically uses XHHW-2 or THHN conductors bundled with a bare concentric neutral/ground. Because SER is rated for 75°C in most residential applications, 3 AWG copper or 1 AWG aluminum is still the minimum. However, SER cable is stiff and difficult to pull through bends; it is best reserved for straight runs through framing cavities, whereas individual THHN in conduit is preferred for complex routing.
