The correct wire size for a 100 amp service is the minimum conductor gauge required to safely carry 100 amps of continuous electrical current from the source to the panel without exceeding the termination temperature rating. For a standard 100-amp subpanel feeder, NEC Table 310.16 (75°C column) dictates 3 AWG copper or 1 AWG aluminum. However, if you are installing a 100-amp main service entrance directly from the utility, local utility standards and NEC Article 310.12 often require upsizing to 2/0 AWG copper or 4/0 AWG aluminum.
Getting this distinction right changes everything in your installation: it dictates your physical conduit diameter, the terminal lug sizing on your main breaker, the anti-oxidant compounds you must use, and the thermal headroom of your entire electrical system. Undersizing leads to tripped breakers and fire hazards; oversizing means you are wasting money on copper and struggling to bend stiff wire into tight panel gutters.
The Core Concept: Ampacity and the 75°C Column
Ampacity is the maximum current a conductor can carry continuously under the conditions of use without exceeding its temperature rating. When sizing wire for a 100-amp circuit, the most common mistake DIYers and junior apprentices make is looking at the wrong column in the NEC ampacity tables.
Think of the wire like a multi-lane highway. The 90°C column is the absolute maximum traffic the asphalt can handle before physically melting, but the 75°C column is the posted speed limit because the off-ramps (the panel lugs) cannot safely handle the higher speed. If you push 90°C heat into a 75°C lug, the lug will fail long before the wire insulation melts.
For a 100-amp breaker, we need a wire that hits at least 100 amps in the 75°C column. Looking at standard ampacity charts, 3 AWG copper is rated exactly at 100A, and 1 AWG aluminum is rated exactly at 100A.
Where You Meet This in Practice: Service vs. Feeder
This is where internet forums give terrible advice. People commonly confuse a 'service entrance' with a 'feeder circuit', leading to massive over-purchasing or failed inspections.
| Installation Type | Governing NEC Rule | Minimum Copper | Minimum Aluminum | Why It Differs |
|---|---|---|---|---|
| Feeder (Main panel to 100A subpanel) | NEC 310.16 | 3 AWG | 1 AWG | Sized strictly to the breaker rating at 75°C. |
| Service Entrance (Utility drop to Main) | NEC 310.12 & Utility Specs | 2/0 AWG | 4/0 AWG | Utilities mandate larger wire to handle uncalculated continuous loads, voltage drop over long drops, and mechanical strength. |
If you are pulling wire from your main 200A panel out to a detached garage for a 100A subpanel, you are building a feeder. You can legally use 3 AWG copper THHN in conduit. If you try to pull 2/0 copper for a 100A feeder, you will waste hundreds of dollars and likely find the wire is too thick to bend into the subpanel's main breaker lugs.
Conversely, if the utility company is dropping a line from the pole to your meter for a 100A main service, their 'greenbook' (engineering standards) will almost certainly demand 2/0 copper or 4/0 aluminum, regardless of what the NEC minimums say. The utility has the final say on the service drop.
Real-World Scenario Walkthrough: The Melted Aluminum Lug
To understand why wire size and termination matter equally, let us look at a real-world failure.
The Numbers: 1/0 AWG aluminum at 75°C is rated for 120 amps. The breaker is 100 amps. The math is perfect. The wire is oversized, which should mean it runs cool.
What Went Wrong: The wire size was correct, but the termination was not. The homeowner used a standard ratchet to tighten the aluminum lug and did not apply anti-oxidant paste. Aluminum undergoes 'creep' (thermal expansion and contraction) and rapidly oxidizes when exposed to air. Without a calibrated inch-pound torque screwdriver, the lug was under-torqued. The loose connection created high electrical resistance right at the terminal. This localized heat exceeded the 75°C rating at the lug, independent of the wire's overall ampacity. This is why NEC 110.14(D) now strictly requires torque tools for all breaker terminations.
Step-by-Step Sizing and Termination Rules
When pulling your 100-amp wire, follow this sequence to ensure you pass inspection and maintain a safe system.
- Calculate Voltage Drop: For a 100A feeder under 100 feet, 3 AWG copper or 1 AWG aluminum is fine. If the run exceeds 100 feet, upsize one gauge (to 2 AWG Cu or 1/0 Al) to keep voltage drop below the recommended 3%.
- Select the Right Insulation: Use THHN/THWN-2 for conduit runs. If you are direct-burying, use UF-B or individual USE-2 conductors, and check the specific ampacity tables for direct burial, as earth thermal resistivity changes the math.
- Prep Aluminum Properly: If using aluminum wire, strip the insulation, immediately brush the exposed conductor with a wire brush, and coat it generously with Noalox (anti-oxidant paste) before inserting it into the lug. This prevents the oxidation that causes high-resistance fires.
- Torque to Spec: Look at the sticker inside the breaker or panel door. It will list the exact torque requirement (e.g., 250 in-lbs). Use a calibrated digital torque screwdriver or torque wrench to tighten the lug. Do not guess.
- Verify and Test: Tug firmly on the wire to ensure it is seated. Turn on the breaker and use a clamp meter to verify the load is balancing correctly across the legs if it is a 240V split-phase feed.
Frequently Asked Questions
Can I use 2 AWG copper for a 100 amp subpanel feeder?
Yes. 2 AWG copper is rated for 115 amps at 75°C, which exceeds the 100-amp requirement. While 3 AWG is the minimum, 2 AWG is highly recommended for runs over 75 feet to mitigate voltage drop, or simply because it is easier to find in big-box stores. Just ensure your breaker lugs are rated to accept the thicker 2 AWG wire.
Does the ground wire need to be the same size as the hot wires?
No. For a 100-amp feeder, NEC Table 250.122 requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor (EGC). However, if you upsized your hot wires for voltage drop (e.g., using 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to increase the ground wire proportionally.
Why do some electricians say I need 2/0 copper for a 100-amp panel?
They are likely referring to a main service entrance rather than a subpanel feeder. Utility companies often mandate 2/0 copper or 4/0 aluminum for the service drop from the transformer to the meter to ensure mechanical strength and handle high inrush currents from modern HVAC systems. Always check your local utility's service manual before buying service entrance cable (SER).
Sizing wire for a 100 amp service is not just about matching a number on a chart; it is about understanding the physical realities of your terminations, the legal distinction between a service and a feeder, and the thermal limits of your equipment. By respecting the 75°C column and torquing your lugs to manufacturer specifications, you build a system that will run safely for decades.
