To use a wire size distance chart effectively, you must cross-reference your breaker size with your one-way run length to keep voltage drop under 3% for branch circuits or 5% for feeders. For most modern residential installations, you will pull from the 75°C ampacity column, assuming copper conductors and an 80% continuous load. The direct answer for standard runs: a 20A circuit requires 12 AWG copper up to 56 feet, while a 50A circuit requires 6 AWG copper up to 91 feet before you must upsize to mitigate voltage drop.
How to Read This Wire Size Distance Chart
Before pulling wire, you need to understand the assumptions baked into any reliable chart. The data below is synthesized from NFPA 70 (NEC) Table 310.16 for ampacity and standard voltage drop formulas from Southwire's engineering resources.
NEC 110.14(C) dictates that for circuits rated 100A or less, you must use the 60°C column unless the equipment (breaker and terminals) is explicitly marked for 75°C. For circuits over 100A, use the 75°C column. Even though modern THHN wire is rated for 90°C, the termination points at your breaker and receptacle are almost always the weakest thermal link, capping your allowable ampacity at the lower temperature rating.
Chart Assumptions:
- Voltage: 120V single-phase (for the distance column).
- Voltage Drop Limit: 3% maximum (3.6V drop), which is the NEC-recommended best practice for branch circuits.
- Load Calculation: Distances are calculated at 80% of the breaker's maximum rating to account for continuous loads (e.g., a 20A breaker is calculated at a 16A continuous draw).
- Material: Copper (Cu) and Aluminum (Al) where applicable.
Comprehensive Wire Size Distance Chart (NEC Table 310.16 & Chapter 9)
Use the quick-jump IDs below to bookmark the most common residential circuit sizes. The distance column represents the maximum one-way length from the panel to the load before voltage drop exceeds 3% at 120V.
| AWG Size | Material | 60°C Ampacity | 75°C Ampacity | Standard Breaker | Max 120V Distance (3% Drop) |
|---|---|---|---|---|---|
| 14 AWG | Copper | 15A | 20A | 15A | 47 feet (at 12A load) |
| 12 AWG | Copper | 20A | 25A | 20A | 56 feet (at 16A load) |
| 10 AWG | Copper | 30A | 35A | 30A | 60 feet (at 24A load) |
| 8 AWG | Copper | 40A | 50A | 40A | 72 feet (at 32A load) |
| 6 AWG | Copper | 55A | 65A | 50A / 60A | 91 feet (at 40A load) |
| 4 AWG | Copper | 70A | 85A | 60A / 70A | 121 feet (at 48A load) |
| 3 AWG | Copper | 85A | 100A | 80A | 104 feet (at 64A load) |
| 2 AWG | Copper | 95A | 115A | 90A / 100A | 115 feet (at 80A load) |
| 1/0 AWG | Copper | 110A | 130A | 100A / 125A | 144 feet (at 100A load) |
| 2/0 AWG | Copper | 125A | 150A | 125A | 145 feet (at 100A load) |
| 4/0 AWG | Copper | 170A | 205A | 150A / 200A | 165 feet (at 160A load) |
| 2 AWG | Aluminum | 75A | 90A | 75A | 72 feet (at 60A load) |
| 4/0 AWG | Aluminum | 135A | 180A | 150A / 200A | 108 feet (at 160A load) |
Derating Factors and What This Chart Cannot Tell You
A printed chart is a baseline, not a final verdict. Real-world jobsite conditions force you to modify these base values through derating.
How Derating Rows Modify the Base Value
When you pull more than three current-carrying conductors through a single raceway (conduit), the wires heat each other up. NEC 310.15(C)(1) requires you to apply a multiplier to the 90°C column of your wire's ampacity.
Example: You are running two 20A circuits (4 current-carrying conductors) in one EMT conduit using 12 AWG THHN. The 90°C ampacity for 12 AWG THHN is 30A. Because you have 4-6 conductors, you apply an 80% derating factor (30A × 0.80 = 24A). Since 24A is still higher than the 20A breaker and the 20A termination limit, 12 AWG remains legal. However, if you pulled four circuits (8 wires, 70% derating), 30A × 0.70 = 21A, which is dangerously close to the limit, and you would need to upsize to 10 AWG.
What This Table Cannot Tell You
- Ambient Temperature Corrections: If your conduit runs across a hot attic or near a boiler, you must apply temperature correction factors from NEC Table 310.15(B)(1). A 120°F attic reduces your ampacity by a multiplier of 0.82.
- Actual Load vs. Breaker Size: This chart assumes an 80% continuous load. If your 20A circuit is only powering a 5A LED lighting array, your 12 AWG wire can run hundreds of feet without exceeding a 3% voltage drop. Always calculate distance based on the actual expected load, not just the breaker handle.
- Local AHJ Amendments: Some jurisdictions, like those enforcing California's Title 24, mandate stricter voltage drop limits (e.g., 1.5% for lighting) or require larger equipment grounding conductors when ungrounded conductors are upsized for voltage drop.
Frequently Asked Questions
What size wire do I need for a 50 amp breaker 100 feet away?
For a 50A breaker at 100 feet on a 120V circuit, 6 AWG copper falls slightly short of the 3% voltage drop limit at a full 40A continuous load (it maxes out at 91 feet). You must upsize to 4 AWG copper to keep the voltage drop under 3%. If you are wiring a 240V circuit (like a welder or EV charger), the voltage drop is halved relative to the source voltage, meaning 6 AWG copper is perfectly adequate for 100 feet at 240V.
Does a wire size distance chart apply to 240V circuits the same as 120V?
No. The physical wire doesn't change, but the math does. Voltage drop is a percentage of the source voltage. A 3% drop on a 120V circuit is 3.6 volts. A 3% drop on a 240V circuit is 7.2 volts. Because you have twice the allowable voltage drop margin, you can run 240V circuits roughly twice as far as 120V circuits using the exact same AWG wire and load current. For 240V distances, simply double the values in the 120V distance column above.
Can I use aluminum wire instead of copper for long subpanel runs?
Yes, and it is highly recommended for long feeder runs to save money. Aluminum is significantly cheaper than copper. However, aluminum has higher resistance, meaning you must upsize by roughly one to two AWG sizes compared to copper to achieve the same ampacity and voltage drop. For a 100A subpanel feeder, you would typically use 2 AWG copper or 1/0 AWG aluminum. Always use an antioxidant compound (like Noalox) on aluminum terminations and ensure your lugs are rated for aluminum (marked AL or CU/AL).
Why does my wire size distance chart show different numbers than the wire packaging?
Wire packaging often lists the maximum ampacity based on the wire's insulation rating (usually 90°C for THHN) in free air. However, the NEC restricts your final allowable ampacity based on the weakest point in the circuit, which is almost always the breaker or receptacle termination (rated 60°C or 75°C). Furthermore, packaging rarely accounts for voltage drop over distance, which is a separate calculation from thermal ampacity. Always trust the NEC termination limits and voltage drop math over the marketing text on a spool wrapper.
