To keep voltage drop under the NEC-recommended 3% limit on a standard 120V branch circuit, a 12 AWG copper wire carrying its maximum 20A load can run exactly 45 feet one-way. If your run exceeds that distance, the voltage at the receptacle will sag below 116.4V, leading to dimming lights, tripping breaker thermal magnets, and shortened appliance lifespans. While the National Electrical Code (NEC) does not strictly mandate a 3% limit for branch circuits in most residential applications, NEC 210.19(A) Informational Note strongly recommends it for reasonable efficiency, and many local inspectors will flag longer runs that exceed a 5% total drop from the utility transformer to the furthest outlet.
This reference provides the exact maximum one-way distances based on the physics of conductor resistance outlined in NEC Chapter 9, Tables 8 and 9. Bookmark the quick-jump links below to find your specific wire gauge instantly.
- 14 AWG to 6 AWG Master Chart (120V Copper)
- Adjustments for 240V, 3-Phase, and Aluminum
- Temperature Derating and Terminal Limitations
The Master Voltage Drop Chart (120V, 1-Phase, Copper)
| Wire Gauge (AWG) | 15A Load | 20A Load | 30A Load | 40A Load | 50A Load |
|---|---|---|---|---|---|
| 14 AWG | 38 ft | — | — | — | — |
| 12 AWG | 60 ft | 45 ft | — | — | — |
| 10 AWG | 95 ft | 71 ft | 47 ft | — | — |
| 8 AWG | 151 ft | 113 ft | 75 ft | 56 ft | — |
| 6 AWG | 190 ft | 142 ft | 119 ft | 89 ft | 71 ft |
Source: Calculated using DC resistance values from NEC Chapter 9, Table 8 (K=12.9 for Copper) and the standard voltage drop formula: Distance = (V_drop × Circular Mils) / (2 × K × Current). Dashes (—) indicate ampacities that exceed the NEC thermal limits for that wire gauge.
Which Column Applies to Your Installation?
The table above is the baseline for standard 120V, single-phase, copper branch circuits. But jobsite conditions rarely stay perfectly baseline. Here is how you modify the chart's distances based on your specific installation parameters.
1. You are wiring a 240V circuit (Dryer, Range, EV Charger, Subpanel)
Because a 240V system allows for a larger absolute voltage drop (3% of 240V is 7.2V, compared to 3.6V for 120V), multiply the distances in the table by 2. For example, if you are pulling 8 AWG copper to a 40A EV charger on a 240V circuit, the chart shows 56 feet for 120V. Multiply that by 2, and your maximum one-way run for a 3% drop is 112 feet.
2. You are using Aluminum wire (SER, USE-2, or Al building wire)
Aluminum has higher electrical resistance than copper (K=21.2 vs K=12.9). If you are using aluminum conductors, multiply the copper distances by 0.61. Alternatively, the rule of thumb on the bench is to simply upsize the aluminum wire by two AWG sizes to match the copper equivalent (e.g., use 4 AWG Aluminum to replace 6 AWG Copper).
3. You are running 3-Phase power (Commercial/Workshop)
Three-phase systems are inherently more efficient over distance due to the phase angle geometry. To find your max distance on a 3-phase system, multiply the single-phase distance by 1.732 (the square root of 3). A 10 AWG copper wire carrying 30A on a 208V 3-phase system will easily push well over 100 feet while maintaining a tight voltage envelope.
Derating Factors and What the Chart Cannot Tell You
A voltage drop chart only solves half the problem: the physics of resistance over distance. It does not account for thermal derating or equipment terminal limits. If you ignore these, your wire might deliver perfect voltage while simultaneously melting its insulation inside the conduit.
How Derating Rows Modify the Base Value
When you pull more than three current-carrying conductors in a single raceway, or when ambient temperatures exceed 86°F (30°C), NEC 310.15 requires you to derate the wire's ampacity. This does not change the voltage drop physics, but it forces you to upsize the wire, which inherently improves your voltage drop.
| Ambient Temp (°F) | 75°C Wire (THWN/THW) | 90°C Wire (THHN/XHHW-2) |
|---|---|---|
| 87°F to 104°F (31-40°C) | 0.88 | 0.91 |
| 105°F to 113°F (41-45°C) | 0.82 | 0.87 |
| 114°F to 122°F (46-50°C) | 0.75 | 0.82 |
Example: You are running a 30A circuit through an attic that reaches 115°F. Using 90°C THHN wire, you multiply the base 40A ampacity by 0.82, giving you a derated ampacity of 32.8A. The wire survives thermally, and because you are using 8 AWG instead of 10 AWG for a 30A load, your voltage drop distance extends significantly beyond the chart baseline.
What the Voltage Drop Chart Cannot Tell You
Never treat a voltage drop chart as a standalone authority. It is blind to three critical jobsite realities:
- Terminal Temperature Limits (NEC 110.14(C)): Most residential breakers and receptacles are only rated for 60°C or 75°C terminations. Even if you use 90°C THHN wire to take advantage of derating columns, you must size the overcurrent protection based on the 60°C or 75°C ampacity column. The chart assumes 75°C; if your equipment is strictly 60°C rated, you must drop one wire size in your calculations.
- Motor Locked Rotor Amps (LRA): The chart calculates steady-state resistive drop. When a compressor or well pump starts, it draws 5x to 7x its running current for a fraction of a second. This causes an instantaneous voltage sag that can trip contactors or fry control boards. For motor circuits, always calculate voltage drop using the LRA found on the motor nameplate, not the Full Load Amps (FLA).
- Harmonic Loads: Modern LED drivers, variable frequency drives (VFDs), and switching power supplies generate triplen harmonics that add up on the neutral conductor. In commercial settings with heavy harmonic loads, the neutral can carry more current than the phase conductors, rendering standard voltage drop calculations dangerously optimistic.
For complex runs exceeding 100 feet, or when dealing with sensitive electronics and motor loads, bypass manual chart lookups and use a dedicated engineering tool like the Southwire Voltage Drop Calculator, which factors in AC reactance (XL) and power factor, not just simple DC resistance.
