The correct wire size for 120 amps is 1 AWG copper or 1/0 AWG aluminum when terminating on standard 75°C rated lugs, though continuous loads mandate upsizing to 1/0 AWG copper or 2/0 AWG aluminum to satisfy the 125% safety multiplier.
At this amperage, wire gauge dictates much more than just thermal safety; it fundamentally changes the physical installation realities of your circuit. Stepping up to 1 AWG or 1/0 AWG alters the minimum conduit diameter required, increases the bending radius inside the panel, and demands specific torque values on the lugs to prevent thermal expansion loosening and subsequent arc faults. The most common confusion among DIYers is referencing the 90°C ampacity column in NEC Table 310.16 to justify a smaller wire, forgetting that NEC 110.14(C) caps the final circuit capacity at the temperature rating of the termination lugs (usually 75°C). A secondary, massive point of confusion is searching for a "120-amp breaker," which does not actually exist in standard NEC 240.6 sizing.
The Core Sizing Rules: Decoding NEC Table 310.16
To size a conductor properly, you must cross-reference the load type (continuous vs. non-continuous) with the temperature rating of your weakest link—usually the breaker terminals or panel lugs. Most modern breakers and subpanels rated 100A and above are marked for 75°C terminations. Older equipment or specific NM-B cable setups may be restricted to the 60°C column.
| Wire Size (AWG/kcmil) | Copper (60°C) | Copper (75°C) | Copper (90°C) | Aluminum (75°C) |
|---|---|---|---|---|
| 2 AWG | 95A | 115A | 130A | 90A |
| 1 AWG | 110A | 130A | 145A | 100A |
| 1/0 AWG | 125A | 150A | 170A | 120A |
| 2/0 AWG | 145A | 175A | 195A | 135A |
For a standard non-continuous 120A load (where the peak draw lasts less than 3 hours), 1 AWG Copper (130A at 75°C) or 1/0 AWG Aluminum (120A at 75°C) is legally sufficient. However, if the load is continuous (like a commercial HVAC unit, a heavy-duty kiln, or a multi-vehicle EV charging station), NEC Article 210.20 requires the overcurrent device and conductors to be sized at 125% of the continuous load. That pushes your target ampacity to 150A, requiring 1/0 AWG Copper or 2/0 AWG Aluminum.
Worked Numeric Example: Sizing a 120A Subpanel Feeder
Let us walk through a real-world scenario: you are running a 240V feeder from a main residential panel to a detached garage subpanel to power a large workshop. Your calculated load is exactly 120 amps, and the one-way distance is 150 feet. Because a workshop with heavy machinery and dust collection can easily run for 3 hours or more, we must treat this as a continuous load.
Selected Conductor: 1/0 AWG Copper THHN (75°C rating = 150A)
Conduit Requirement: 1.5-inch PVC Schedule 80 (to maintain under 40% conduit fill for four conductors)
Next, we must verify voltage drop. The NEC recommends keeping feeder voltage drop under 3% for optimal efficiency. Using the standard voltage drop formula VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is 120A (the actual operating load, not the 150A sizing load), D is 150 feet, and CM (circular mils) for 1/0 AWG is 105,600:
VD = (2 × 12.9 × 120 × 150) / 105,600 = 4.39 Volts
On a 240V system, 4.39V represents a 1.8% voltage drop. This is well under the 3% recommended limit. If we had attempted to use 1 AWG copper (CM = 83,690) to save money, the voltage drop would have spiked to 2.7%—still technically under 3%, but leaving almost no margin for error if you add future loads. Tools like the Southwire Voltage Drop Calculator will confirm these exact figures when you input 1/0 Cu THHN in PVC conduit.
Where You Meet This in Practice
Handling 1 AWG and 1/0 AWG wire is where bench theory meets jobsite reality. You will typically encounter this wire size for 120 amps in the following applications:
- Subpanel Feeders: Supplying 100A to 125A subpanels in detached garages, barns, or large home additions.
- Heavy Machinery: Hardwiring large residential or light-commercial equipment like CNC routers, industrial air compressors, or pottery kilns.
- High-Capacity EV Charging: While most home Level 2 chargers max out at 60A (requiring a 60A breaker and 4 AWG wire), commercial multi-port EV pedestals frequently require 120A+ feeders.
- Service Entrance Conductors: Used as the main hot legs for smaller 120A service drops in older homes or modular builds.
Physical Installation Realities: 1/0 AWG THHN is incredibly stiff. You cannot bend it by hand into tight panel corners without risking damage to the insulation or the terminal lugs. You will need a minimum of 1.5-inch conduit (2-inch is highly recommended for easier pulling), and you must use a high-quality wire pulling compound. Furthermore, when terminating 1/0 AWG stranded wire, you must use a calibrated torque screwdriver. A typical 125A breaker lug requires between 35 to 45 inch-pounds of torque (always verify the exact spec printed on the breaker label). Under-torquing causes high-resistance hot spots; over-torquing strips the lug threads or crushes the copper strands.
FAQ: Wire Size for 120 Amps
What size breaker do I use for a 120-amp wire?
You must use a 125-amp breaker. Under NEC 240.6, standard overcurrent device sizes are 100A, 110A, 125A, and 150A. There is no standard 120-amp breaker manufactured for residential or commercial panelboards. If your calculated load is exactly 120 amps and it is non-continuous, you size the wire to handle at least 120A (1 AWG Copper) and protect it with the next standard size up, which is a 125A breaker. If the load is continuous, you must size the breaker at 125% of the load (150A) and use 1/0 AWG Copper wire.
Can I use 2 AWG wire for a 120-amp load?
No, not for standard installations. Looking at the 75°C column of NEC Table 310.16, 2 AWG copper is only rated for 115 amps, which is insufficient for a 120-amp load. You might notice that 2 AWG copper has a 90°C rating of 130 amps, leading some DIYers to assume it is safe to use. However, NEC 110.14(C) strictly forbids using the 90°C ampacity for final sizing unless both the wire and the termination lugs are explicitly rated for 90°C—and virtually no standard 125A breakers or panel lugs carry a 90°C rating. Stick to 1 AWG copper to remain code-compliant and safe.
Does the wire size for 120 amps change if I use aluminum instead of copper?
Yes, you must increase the wire gauge when switching to aluminum due to its higher electrical resistance. For a non-continuous 120A load, you must step up to 1/0 AWG Aluminum (rated exactly 120A at 75°C). For a continuous 120A load (requiring 150A capacity), you must use 2/0 AWG Aluminum. When using aluminum, it is mandatory to apply an anti-oxidant compound (like Noalox or Deox) to the stripped conductor before insertion, and you must be exceptionally careful with torque specs, as aluminum is softer than copper and prone to cold creep (loosening over time under pressure) if not torqued precisely to the manufacturer's specification.
