For a standard 120V branch circuit, wire size is dictated entirely by the breaker amperage and the insulation temperature rating, not the voltage itself. A 15-amp circuit requires a minimum of 14 AWG copper, a 20-amp circuit requires 12 AWG, and a 30-amp circuit requires 10 AWG. The 120V designation only becomes the primary sizing factor when calculating voltage drop over long distances.
Below is the definitive reference chart for 120V residential and light-commercial wiring, followed by the specific NEC rules that dictate which column you must use, how to adjust for conduit bundling, and the physical limits this chart cannot show you.
The 120V Wire Size Calculator Chart (NEC Table 310.16)
How to read this table: This data is extracted from NEC Table 310.16 (2020/2023 editions) for copper conductors. The 60°C column applies primarily to NM-B (Romex) cable and older devices. The 75°C column applies to individual THHN/THWN wires in conduit and modern terminals rated for 75°C. The Max Breaker column reflects the strict limits imposed by NEC 240.4(D) for small conductors, which overrides the base ampacity. Bookmark the bolded rows for the most common 120V branch circuits.
| AWG Size | 60°C Ampacity (NM-B / Romex) | 75°C Ampacity (THHN in Conduit) | Max Standard Breaker (NEC 240.4(D)) |
|---|---|---|---|
| 14 AWG | 15 Amps | 20 Amps | 15 Amps |
| 12 AWG | 20 Amps | 25 Amps | 20 Amps |
| 10 AWG | 30 Amps | 35 Amps | 30 Amps |
| 8 AWG | 40 Amps | 50 Amps | 40 Amps* |
| 6 AWG | 55 Amps | 65 Amps | 60 Amps |
| 4 AWG | 70 Amps | 85 Amps | 70 Amps |
| 3 AWG | 85 Amps | 100 Amps | 100 Amps |
*Note: 8 AWG does not fall under the strict small conductor rule of 240.4(D), but standard breaker sizing rules (NEC 240.4(B) next standard size up) typically limit it to a 40A or 50A breaker depending on the exact termination temperature ratings.
Which Column Applies to Your 120V Installation?
The most common mistake DIYers make when using a wire size calculator chart is looking at the 75°C column for NM-B cable and assuming they can use a larger breaker. Here is how to determine your exact column based on the physical materials in your hands:
- Use the 60°C Column if: You are running NM-B (Romex) cable through wall cavities, or if you are terminating wires into older breakers, receptacles, or switches that are not explicitly stamped with a 75°C rating. Even if the individual wires inside the NM-B sheath are rated for 90°C, NEC 334.80 mandates that the ampacity of NM-B cable must be determined using the 60°C column.
- Use the 75°C Column if: You are pulling individual THHN or THWN-2 conductors through EMT, PVC, or flexible metal conduit, and every single termination point (breaker lug, wire nut, receptacle terminal) is rated for 75°C. Most modern Square D Homeline, Siemens, and Eaton breakers feature 75°C rated lugs.
How Derating and Voltage Drop Modify the Base Chart
The chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. Real-world jobsite conditions require you to modify these base values.
1. Conduit Bundling (Derating)
When you pull multiple 120V circuits through the same conduit, the wires heat each other up. According to NEC 310.15(C)(1), you must apply a derating factor to the 75°C or 90°C base ampacity (not the 60°C column) when you have 4 or more current-carrying conductors.
Worked Example: You are pulling three 120V circuits (6 hot wires, 3 neutrals) through a single 3/4" EMT conduit. That is 9 current-carrying conductors. The derating factor for 7-9 conductors is 70%.
If you use 10 AWG THHN (Base 90°C ampacity is 40A): 40A × 0.70 = 28 Amps.
Because 28A is less than the standard 30A breaker size, you must either upsize your wire to 8 AWG or drop the breaker to 20A. The base chart alone would have let you fail this inspection.
2. Voltage Drop on Long 120V Runs
A 120V circuit is highly susceptible to voltage drop because the baseline voltage is low. A 120V circuit operating at 110V due to wire resistance will cause motors to overheat and LED drivers to flicker. The NEC recommends a maximum 3% voltage drop for branch circuits (3.6V on a 120V line).
Worked Example: You need to run a 120V, 15-amp dedicated circuit to a window AC unit 120 feet away from the panel.
Using the chart, 14 AWG is legal for 15A. But calculating voltage drop (VD = 2 × K × I × D / CM) for 14 AWG copper yields a drop of roughly 7.5V (6.2%), which is unacceptable. You must upsize to 10 AWG copper to keep the drop under 3V (2.5%), even though the breaker is only 15 amps.
What This Wire Size Chart Cannot Tell You
A static reference chart is a starting point, not a complete engineering tool. Keep these three blind spots in mind before buying your wire:
- Continuous vs. Non-Continuous Loads: If your 120V load will run for 3 hours or more continuously (like a space heater, server rack, or EV trickle charger), NEC 210.20(A) requires you to size the breaker and wire at 125% of the continuous load. A 12-amp continuous heater requires a wire and breaker sized for 15 amps minimum (12 × 1.25 = 15A).
- Ambient Temperature Corrections: If your conduit runs across an unventilated attic in Arizona where ambient temperatures reach 110°F (43°C), you must apply a temperature correction factor from NEC Table 310.15(B)(1). The base ampacity drops, potentially requiring a larger wire gauge than the chart indicates.
- Local AHJ Amendments: Some municipalities strictly prohibit 14 AWG wire entirely, mandating 12 AWG as the minimum for all 120V branch circuits to reduce fire risk and accommodate future load upgrades. Always verify with your local Authority Having Jurisdiction (AHJ).
Frequently Asked Questions
Can I use 14 AWG wire on a 120V 20-amp breaker?
No. While 14 AWG wire might physically fit into the terminal of a 20-amp breaker, doing so is a direct violation of NEC 240.4(D). The 14 AWG wire has a maximum allowable ampacity of 15 amps. If a fault or overload pushes 19 amps through the wire, the 20-amp breaker will not trip, but the 14 AWG wire will overheat, potentially melting the insulation and starting a fire inside the wall cavity. Always match 14 AWG to a 15A breaker, and 12 AWG to a 20A breaker.
What size wire do I need for a 120V 30-amp RV outlet?
For a standard 120V, 30-amp RV receptacle (NEMA TT-30R), you must use a minimum of 10 AWG copper wire protected by a 30-amp single-pole breaker. If the run from the panel to the RV pedestal exceeds 70 feet, you should upsize to 8 AWG copper to prevent voltage drop, which can damage sensitive RV air conditioning compressors and microwaves.
Does a 120V circuit require a different wire size than a 240V circuit for the same amperage?
For ampacity and breaker sizing, no. A 20-amp 120V circuit and a 20-amp 240V circuit both require a minimum of 12 AWG copper wire. The physical heat generated by 20 amps of current is identical regardless of the voltage pushing it. However, 240V circuits are much more forgiving regarding voltage drop over long distances. A 3% drop on a 240V line allows for 7.2 volts of loss, whereas a 120V line only allows for 3.6 volts of loss before performance degrades.
How do I calculate voltage drop for a long 120V wire run?
Use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM.
'K' is the resistivity of copper (12.9 ohms per circular mil foot). 'I' is the current in amps. 'D' is the one-way distance in feet. 'CM' is the circular mil area of the wire (14 AWG = 4110, 12 AWG = 6530, 10 AWG = 10380). If your calculated VD exceeds 3.6V (which is 3% of 120V), you must move to the next larger AWG size on the chart and recalculate until the drop is within acceptable limits.
