The 120 amp wire size is the minimum conductor cross-section required to safely carry 120 amps of continuous or non-continuous current without exceeding the insulation's temperature rating at the terminations. Selecting the correct gauge changes more than just thermal safety; it dictates your physical installation parameters, including conduit diameter, bend radius, and the physical space required inside your panelboard lugs. Most DIYers and junior electricians confuse the breaker size with the wire ampacity, or they mistakenly size their conductors using the 90°C column of the NEC ampacity tables when their equipment terminations are only rated for 75°C.
The Core Rule: Sizing Wire for a 120A Load
To find the correct wire, we have to look at the intersection of the load, the breaker, and the termination temperature. Under NEC guidelines, specifically Article 110.14(C), you must size your wire based on the lowest temperature rating of any connected termination, device, or conductor. In almost all modern residential and light commercial panels, the breaker lugs are rated for 75°C. Therefore, you must use the 75°C column of NEC Table 310.16, regardless of whether your wire insulation (like THHN) is rated for 90°C.
Looking at the 75°C column, we get our baseline 1 AWG Copper (130A) and 1/0 AWG Aluminum (120A).
But what about the breaker? NEC Article 240.6 dictates standard breaker sizes, and there is no standard 120-amp breaker. The next standard size up is 125A. Under NEC 240.4(B), you are permitted to use the next standard size breaker (125A) to protect a wire whose ampacity does not match a standard breaker size, provided the actual calculated load does not exceed the wire's ampacity (120A).
Where You Meet a 120A Load in Practice
You won't find a single 120A receptacle in a standard home. A 120-amp circuit almost always serves as a feeder or a dedicated hardwired supply for heavy equipment. Here is where you will actually pull this wire:
- Subpanel Feeders: Feeding a 125A subpanel in a detached garage, workshop, or barn. This is the most common residential application.
- High-End EV Charging: Running a dedicated line for a dual-port Level 2 EV charger setup that can simultaneously deliver 48A to two vehicles (requiring a 120A circuit to handle the 96A continuous load, though continuous load rules would actually force you to size up to 125A wire in that specific EV scenario).
- Heavy Workshop Machinery: Large commercial CNC mills, heavy-duty rotary phase converters, or industrial welders that draw close to 100A under peak load.
- Commercial HVAC: Large rooftop package units or commercial compressors that require a 120A minimum circuit ampacity (MCA).
The Worked Example: 120A Subpanel Feeder at 100 Feet
Ampacity tells you the wire won't melt, but it doesn't tell you if your equipment will actually run correctly. Voltage drop is the silent killer of long feeder runs. Let's calculate the voltage drop for a 120A load on a 240V single-phase system over a 100-foot one-way run in PVC conduit.
The formula for single-phase voltage drop is: VD = (2 × K × I × D) / CM
- K = 12.9 for Copper, 21.2 for Aluminum (at 75°C)
- I = 120 Amps
- D = 100 Feet
- CM = Circular Mils of the wire
Scenario A: 1 AWG Copper THHN
CM for 1 AWG = 83,690.
VD = (2 × 12.9 × 120 × 100) / 83,690 = 3.69 Volts.
Percentage Drop = 3.69 / 240 = 1.54%.
Scenario B: 1/0 AWG Aluminum XHHW-2
CM for 1/0 AWG = 105,600.
VD = (2 × 21.2 × 120 × 100) / 105,600 = 4.81 Volts.
Percentage Drop = 4.81 / 240 = 2.00%.
Both scenarios fall well under the NEC's informational recommendation of a 3% maximum voltage drop for feeders (see Southwire's voltage drop guidelines for calculator verification). However, 1/0 Aluminum is roughly half the cost of 1 AWG Copper and significantly lighter, making it the pragmatic choice for this run.
Decision Tree: Picking Your Exact 120 Amp Wire Size
Use this matrix to lock in your exact material and breaker pairing based on your specific jobsite constraints.
| Condition / Constraint | Recommended Wire | Breaker Size | Min. Conduit (PVC) |
|---|---|---|---|
| Standard indoor run < 50ft, pulling through tight existing conduit | 1 AWG Copper THHN | 125A | 1 inch |
| Standard run 50-150ft, cost-conscious (Default Pick) | 1/0 AWG Aluminum XHHW-2 | 125A | 1 inch |
| High ambient temp (attic > 86°F/30°C) requiring derating | 1/0 AWG Copper THHN | 125A | 1.25 inch |
| Direct burial underground (no conduit) | 1/0 AWG Aluminum URD/MHF | 125A | N/A (Trench) |
Common Mistakes and Code Traps
Do I need to upsize the neutral wire on a 120A subpanel feeder?
Not necessarily to the same size as the hots. If your subpanel is purely 240V (like a dedicated pump house), you don't need a neutral at all. If it's a 120/240V subpanel, the neutral only needs to be sized for the maximum unbalanced 120V load. However, NEC 220.61 and 250.122 dictate that the neutral can never be smaller than the required equipment grounding conductor. For a 125A breaker, that means your neutral must be at least 6 AWG Copper or 4 AWG Aluminum, even if your calculated unbalanced load is tiny.
What size ground wire do I need for a 125A breaker?
Per NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 125A breaker is 6 AWG Copper or 4 AWG Aluminum. Do not rely on the conduit alone as your ground for a subpanel feeder; pull a dedicated insulated EGC. If you upsized your hot wires for voltage drop (e.g., jumping from 1/0 Al to 2/0 Al), NEC 250.122(B) requires you to proportionally upsize your ground wire as well.
Can I use a 125A breaker with 1/0 Aluminum wire if my load is exactly 120A?
Yes. This is a common point of confusion detailed in EC&M's conductor sizing guides. 1/0 Aluminum has an ampacity of exactly 120A in the 75°C column. Because 120A is not a standard breaker size, NEC 240.4(B) allows you to round up to the next standard breaker, which is 125A. This is perfectly legal and safe, provided your actual continuous and non-continuous load calculation does not exceed 120A.
Why can't I use 2 AWG Copper for a 120A load?
2 AWG Copper in the 75°C column is only rated for 115A. Even though 115A is close to 120A, the NEC does not allow you to exceed the conductor's ampacity. You must step up to 1 AWG Copper (130A) to safely carry the 120A load before applying the 125A breaker.
