The correct wire size for 100 amps is 3 AWG copper or 1 AWG aluminum, assuming your equipment terminals are rated for 75°C. Wire size for a 100-amp circuit refers to the specific American Wire Gauge (AWG) cross-sectional area required to carry 100 amperes of current without exceeding the thermal limits of the conductor's insulation or the connected breaker lugs. Choosing the right gauge changes the physical bend radius inside your panel, the voltage drop over distance, and the ultimate safety margin of the installation. People commonly confuse the wire's thermal ampacity with the breaker's magnetic trip curve, or they mix up the 60°C and 75°C NEC columns, which frequently leads to undersized, overheating feeders when using aluminum wire.

Safety & Code Caveat: Working on a 100-amp feeder involves lethal mains voltage. Always de-energize the upstream panel, lock out the breaker, and verify the busbars are dead with a properly functioning non-contact voltage tester and multimeter before touching any conductors. The guidance below reflects NEC-style best practices; your local Authority Having Jurisdiction (AHJ) has final authority on all inspections and permitting.

The NEC Ampacity Baseline (How the Numbers Work)

To size a wire, we look at NEC Table 310.16, which dictates the allowable ampacity of conductors based on material, size, and insulation temperature rating. The most critical mistake DIYers make is reading the 90°C column. While THHN wire insulation is rated for 90°C, nearly all standard residential breakers and panel lugs are only rated for 75°C (NEC 110.14(C)). You must size the wire based on the lowest temperature rating of any connected component.

NEC Table 310.16 Extract: Minimum Wire Size for 100 Amps
Material 60°C Column (NM-B / Romex) 75°C Column (THHN / THWN in Conduit)
Copper 3 AWG (100A) 3 AWG (100A)
Aluminum 1/0 AWG (100A) 1 AWG (100A)

Notice that for copper, 3 AWG hits exactly 100 amps in both columns. However, if you are pulling individual THHN conductors through conduit, you benefit from the 75°C column when using aluminum, allowing you to use 1 AWG aluminum instead of the thicker 1/0 AWG required for 60°C applications. For a comprehensive breakdown of conductor properties, refer to the National Fire Protection Association (NFPA) NEC guidelines.

Worked Numeric Example: 100A Subpanel Feeder

Ampacity tells you the wire won't melt, but it doesn't guarantee your equipment will run properly. We must also calculate voltage drop. The NEC recommends a maximum 3% voltage drop on feeders.

The Scenario: You are running a 100-amp subpanel to a detached garage. The one-way distance is 150 feet. You plan to pull 3 AWG copper THHN through PVC conduit. You expect a continuous load of 80 amps (e.g., an EV charger and a welder running simultaneously).

The Math:
We use the single-phase voltage drop formula: VD = (2 × K × I × L) / CM

  • K (Copper resistivity at 75°C) = 12.9
  • I (Current) = 80 amps
  • L (One-way length) = 150 feet
  • CM (Circular mils for 3 AWG) = 52,620

VD = (2 × 12.9 × 80 × 150) / 52,620
VD = 309,600 / 52,620 = 5.88 Volts

On a 240V circuit, a 5.88V drop represents a 2.45% voltage drop. This is well under the 3% feeder recommendation. If your run was 200 feet, the drop would hit 3.27%, and you would need to upsize to 2 AWG copper to maintain optimal performance. For deeper analysis on long-run calculations, EC&M's guide on voltage drop fundamentals is an excellent technical resource.

Where You Meet This In Practice

You will typically specify wire size for 100 amps in three specific residential and light-commercial scenarios:

  1. Subpanel Feeders: Supplying a 100-amp main-lug subpanel in a detached garage, workshop, or barn. This is the most common application, usually utilizing 4-wire setups (two hots, one neutral, one ground) in PVC or rigid metal conduit.
  2. Heavy Equipment & Welders: Large 240V stick/TIG welders or industrial air compressors often require dedicated 100-amp circuits. These require hard-wired connections or heavy-duty NEMA 14-100 / 14-50 receptacles (though 50A is more common for standard RV/EV plugs, custom 100A pin-and-sleeve setups exist in commercial spaces).
  3. Future-Proofed EV Charging: While most Level 2 home chargers run on 50A or 60A breakers, high-end dual-charger setups or future commercial-grade residential chargers may demand a 100A circuit, requiring this exact wire sizing.
Pro-Tip: Torque Matters. 100-amp lugs require significant mechanical force to ensure a solid connection. Modern NEC 110.14(D) requires using a calibrated torque screwdriver or torque wrench. A loose 1 AWG aluminum lug will arc, oxidize rapidly, and cause a thermal failure. Check the breaker label for the exact inch-pound (in-lb) specification—usually around 45-50 in-lb for this size.

Common Mistakes and Code Caveats

When pulling wire for a 100-amp service, avoid these frequent jobsite errors:

  • Using NM-B (Romex) for the 75°C column: NEC 334.80 strictly limits NM-B cable ampacity to the 60°C column, regardless of the wire's actual insulation rating. If you use 3 AWG NM-B copper, it is rated for 100A, but finding NM-B in 3 AWG is virtually impossible and prohibitively expensive. Stick to THHN/THWN-2 in conduit for 100A runs.
  • Skipping Anti-Oxidant Paste on Aluminum: If you choose 1 AWG aluminum to save money, you must apply an anti-oxidant compound (like Noalox) to the stripped wire ends before torquing them into the lugs. Aluminum oxidizes when exposed to air, creating a high-resistance layer that generates heat.
  • Ignoring Conduit Fill: 3 AWG and 1 AWG wires are thick and stiff. If you are pulling four conductors (two hots, neutral, ground) for a subpanel, you need at least a 1.25-inch PVC Schedule 80 conduit to stay within the NEC 40% conduit fill limit and avoid jamming the wires at bends.

Frequently Asked Questions

Can I use 4 AWG wire for a 100 amp breaker?

No, not for standard residential branch circuits or feeders. 4 AWG copper is rated for 85 amps at 75°C. While NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker, 85A does not qualify you for a 100A breaker. The wire's ampacity must be at least equal to the breaker size (or the calculated load, whichever governs). You must use 3 AWG copper minimum.

What size ground wire do I need for a 100 amp circuit?

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 to compensate for voltage drop over a long distance, you must proportionally upsize your ground wire as well.

Does the wire size for 100 amps change for 120V vs 240V?

The physical wire size (AWG) required to safely carry 100 amps does not change based on voltage; a 3 AWG copper wire will safely carry 100 amps whether the system is 12V, 120V, or 240V. However, the impact of voltage drop changes drastically. A 5-volt drop on a 240V circuit is a negligible 2%, but a 5-volt drop on a 120V circuit is over 4%, which can cause motors to overheat and lights to dim.

How much does 100 amp wire cost per foot?

Pricing fluctuates with commodity markets, but as of current 2026 estimates, individual 3 AWG copper THHN costs roughly $3.50 to $4.50 per foot. 1 AWG aluminum THHN is significantly cheaper, typically running $1.20 to $1.80 per foot. For a 150-foot subpanel run requiring four conductors, expect to spend around $2,400 for copper or roughly $900 for aluminum, not including conduit, fittings, and the panel itself.