Voltage drop is the reduction in electrical potential between the power source and the load caused by the inherent resistance of the conductors. When current flows through a wire, that resistance converts some of the electrical energy into heat, meaning the load receives less voltage than the panel supplies. This changes the real-world performance of your circuit: lights dim, motors run hot and bog down, and switching power supplies may trigger low-voltage lockouts. If you are designing a branch circuit or a low-voltage DC system, understanding exactly why and how wires drop voltage is the difference between a reliable installation and a continuous troubleshooting headache.
The Physics of Voltage Drop (and Common Confusions)
At its core, voltage drop is just Ohm’s Law applied to the wire itself rather than the load. The formula is straightforward: Vdrop = I × Rwire. The current (I) multiplied by the total resistance of the wire (R) equals the voltage lost before it reaches your device. Think of it like water pressure losing energy to friction as it travels through a long, narrow pipe; the pump pushes 60 PSI, but the sprinkler head at the end of the yard only sees 45 PSI.
Before we look at the math, it is critical to clear up two common confusions that trip up DIYers and junior techs:
- Confusing drop with sag: Voltage drop is a steady-state, predictable loss caused by your wire's physical resistance. Voltage sag (or brownout) is a temporary, utility-side deficit caused by high grid demand or a failing transformer. You can fix voltage drop by upsizing your wire; you cannot fix a utility sag with thicker wire.
- Confusing drop with insulation rating: A wire labeled "600V THHN" does not mean it will deliver 600V perfectly over any distance. The 600V rating refers to the dielectric breakdown limit of the plastic insulation, not its resistive loss characteristics. A 600V-rated 14 AWG wire will still drop massive amounts of voltage on a 120V circuit if the run is long enough.
AWG Voltage Drop Reference Table (Copper, 120V Nominal)
To size a wire properly, you need to know its resistance per thousand feet (kft). The table below provides the round-trip voltage drop for common copper wire sizes at standard breaker currents. Note: Round-trip means the calculation includes both the hot and the neutral wire (e.g., a 100 ft run requires 200 ft of total wire).
| AWG Size | Resistance per 1,000 ft (Ω) | Round-Trip Drop at 10A (per 100 ft run) | Round-Trip Drop at 15A (per 100 ft run) | Round-Trip Drop at 20A (per 100 ft run) |
|---|---|---|---|---|
| 14 AWG | 3.14 Ω | 6.28V | 9.42V | Not Rated |
| 12 AWG | 1.98 Ω | 3.96V | 5.94V | 7.92V |
| 10 AWG | 1.24 Ω | 2.48V | 3.72V | 4.96V |
| 8 AWG | 0.778 Ω | 1.56V | 2.33V | 3.11V |
| 6 AWG | 0.491 Ω | 0.98V | 1.47V | 1.96V |
Source data derived from NFPA 70 (NEC) Chapter 9, Table 8 for uncoated copper conductors.
Worked Example: Sizing a 120V Branch Circuit
Let’s apply this to a real jobsite scenario. You are wiring a dedicated 120V circuit for a 12A continuous resistive load (like a baseboard heater or a server rack PDU) located 100 feet away from the subpanel.
The NEC Informational Note 4 to 210.19(A) recommends that branch circuit voltage drop be limited to 3% for reasonable efficiency. On a 120V nominal system, 3% of 120V is 3.6V. Therefore, your maximum allowable drop is 3.6V. Because the run is 100 feet, the total wire length (hot + neutral) is 200 feet.
Attempt 1: 14 AWG Copper
While 14 AWG is legally permitted on a 15A breaker, let's check the drop:
V_drop = 12A × (3.14 Ω / 1000 ft) × 200 ft = 7.53V
Result: 7.53V is a 6.2% drop. The heater will only see 112.4V. This fails the 3% recommendation and will cause the heater to output roughly 12% less heat (since power scales with the square of the voltage).
Attempt 2: 12 AWG Copper
Stepping up to the next standard size:
V_drop = 12A × (1.98 Ω / 1000 ft) × 200 ft = 4.75V
Result: 4.75V is a 3.9% drop. We are close, but this still exceeds the 3.6V maximum threshold.
Attempt 3: 10 AWG Copper
Stepping up one more size:
V_drop = 12A × (1.24 Ω / 1000 ft) × 200 ft = 2.97V
Result: 2.97V is a 2.4% drop. This is well under the 3.6V limit. The load will receive 117.03V, ensuring optimal performance and keeping the wire cool.
Where You Meet Voltage Drop in Practice
Voltage drop isn't just a theoretical exercise for passing electrical exams; it dictates hardware selection in several common DIY and professional scenarios.
1. Low-Voltage DC LED Strip Lighting
Because DC systems operate at low voltages (12V or 24V), the percentage drop happens incredibly fast. A mere 1.2V drop on a 12V LED strip is a 10% loss, which will cause visible color shifting (white LEDs turn pink/yellow) and severe dimming at the end of the strip. This is why installers use power injection (feeding power to both ends of the strip) or step up to 24V systems, which halves the current and subsequently halves the voltage drop for the same wattage.
2. Detached Garages and Well Pumps (240V)
Running a subpanel to a detached garage 200 feet away requires careful feeder sizing. While 240V systems are more forgiving than 120V (a 3% drop allows for 7.2V of loss), the high starting surge currents of well pumps or air compressors can cause momentary voltage drops that trip sensitive control boards. Using a tool like the Southwire Voltage Drop Calculator can help you model the exact impedance of your planned UF-B or direct burial feeder before you dig the trench.
3. Automotive and Marine 12V Systems
When wiring a 12V winch, windlass, or high-amperage inverter in a vehicle or boat, the distance from the battery to the load might only be 15 feet, but the current can exceed 150A. At 150A, even a short run of undersized wire will drop voltage so severely that the winch motor stalls. In these applications, you are often sizing wire based on voltage drop limits rather than the wire's thermal ampacity limit.
By treating the wire as a resistor in series with your load, you can predict exactly how your circuit will behave under load. Always verify your calculations with a true-RMS multimeter at the furthest receptacle or load terminal once the system is energized; if your measured voltage under load is more than 3% below your source voltage, it is time to pull thicker wire.
