Voltage drop is the loss of electrical potential across a conductor due to its inherent resistance, reducing the voltage available at the load. When this drop occurs in a real circuit, it changes the actual voltage delivered to your equipment, which can cause AC motors to overheat, incandescent lights to dim, and sensitive electronics to brownout or fail prematurely. Think of it like friction in a long garden hose; the pump pushes 60 PSI, but by the time the water travels 100 feet through a narrow tube, you only get 45 PSI at the nozzle. While ampacity rules protect the wire from melting, voltage drop rules protect the equipment at the end of the wire from starving.
The NEC Stance: Mandatory Rule vs. Informational Note
A common misconception on the jobsite is that the National Electrical Code (NEC) strictly mandates a maximum voltage drop for all residential circuits. In reality, the NFPA 70 (NEC) treats voltage drop primarily as an Informational Note rather than an enforceable mandate for standard dwelling units.
NEC Article 210.19(A) Informational Note No. 4 recommends that branch circuit conductors be sized to prevent a maximum voltage drop of 3%, and that the combined feeder and branch circuit drop should not exceed 5%. While inspectors rarely fail a standard residential bedroom outlet for a 4% drop, they will enforce strict voltage drop limits for specific applications like fire pumps (NFPA 20), sensitive electronic equipment (Article 647), and commercial lighting systems.
To calculate this, you cannot rely on the standard ampacity tables in NEC 310.16. Instead, you must use the exact Circular Mil (CM) area and DC resistance values found in NEC Chapter 9, Table 8. The standard formula for single-phase voltage drop is:
VD = (2 × K × I × D) / CM
- VD: Voltage Drop (in volts)
- K: Conductor resistivity (12.9 ohms per mil-foot for copper at 75°C)
- I: Current (in amps)
- D: One-way distance of the run (in feet)
- CM: Circular mils of the conductor (from Chapter 9, Table 8)
Worked Example: Sizing a 20A Circuit at 100 Feet
Let’s look at a real-world scenario: you are wiring a dedicated 120V, 20A receptacle in a detached workshop exactly 100 feet from the main panel. Your target is to stay under the NEC recommended 3% drop (which is 3.6V on a 120V nominal system).
Attempt 1: Standard 12 AWG Copper (CM = 6,530)
VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V
Percentage: 7.9V / 120V = 6.6% drop. This fails the 3% recommendation and will cause noticeable dimming if you plug in high-draw halogen work lights or cause a 20A compressor motor to run hot.
Attempt 2: Upsizing to 10 AWG Copper (CM = 10,380)
VD = (2 × 12.9 × 20 × 100) / 10380 = 4.97V
Percentage: 4.97V / 120V = 4.1% drop. Better, but still fails the strict 3% branch circuit target.
Attempt 3: Upsizing to 8 AWG Copper (CM = 16,510)
VD = (2 × 12.9 × 20 × 100) / 16510 = 3.12V
Percentage: 3.12V / 120V = 2.6% drop. This passes. To meet the NEC informational guidance for this specific distance and load, you must pull 8 AWG THHN, not the 12 AWG you would normally use for a 20A breaker.
Where You Meet Voltage Drop in Practice
You rarely need to worry about voltage drop on a 30-foot run to a kitchen counter. The math only bites you when distance and continuous loads combine. Here is where you must run the numbers on the bench before pulling wire:
- Level 2 EV Chargers: A 48A continuous load on a 60A breaker running 120 feet to the end of a driveway. Using standard 6 AWG will result in a massive drop; you will likely need to upsize to 3 AWG or 2 AWG copper to keep the charger’s internal contactors from chattering or failing to close.
- Submersible Well Pumps: A 240V, 30A pump located 250 feet down a well and 100 feet horizontally from the pump house panel. The total one-way distance is 350 feet. Undersized wire here won't just dim lights; it will cause the pump motor to overheat and trip its internal thermal overload on every startup.
- Detached Garage Subpanels: Feeding a 100A subpanel 150 feet away. Even though 3 AWG copper is rated for 100A at 75°C, the voltage drop on a fully loaded panel will exceed 5%. Most electricians default to 1/0 AWG aluminum (XHHW-2) or 2 AWG copper for this specific feeder run.
Decision Path: Selecting the Right AWG for Long Runs
Use this decision tree to determine your wire size for 120V single-phase branch circuits. This table assumes copper conductors and a strict adherence to the NEC 3% maximum drop recommendation.
| Load (Amps) | One-Way Distance | Standard Ampacity Wire | Required Wire for <3% Drop | Concrete Pick (Buy This) |
|---|---|---|---|---|
| 15A | 50 ft | 14 AWG | 14 AWG (1.9% drop) | 14/2 NM-B |
| 15A | 125 ft | 14 AWG | 10 AWG (2.9% drop) | 10 AWG THHN in conduit |
| 20A | 60 ft | 12 AWG | 12 AWG (2.3% drop) | 12/2 NM-B |
| 20A | 100 ft | 12 AWG | 8 AWG (2.6% drop) | 8 AWG THHN in conduit |
| 20A | 150 ft | 12 AWG | 6 AWG (2.6% drop) | 6 AWG THHN in conduit |
Clearing Up Confusion: Drop vs. Derating vs. Sag
People frequently confuse voltage drop with two other electrical phenomena. Knowing the difference prevents you from buying the wrong wire or blaming the wrong problem.
1. Voltage Drop vs. Ampacity Derating
Voltage drop is about the length of the wire and the resistance starving the load. Ampacity derating (NEC 310.15(C)(1)) is about the heat generated when you bundle multiple current-carrying conductors in a single conduit. If you put four 12 AWG wires in a conduit, you must derate their ampacity to 80%. Derating protects the wire insulation from melting; voltage drop protects the tool at the end of the wire from stalling. You must calculate both for long conduit runs and use whichever result requires the thicker wire.
2. Voltage Drop vs. Voltage Sag
Voltage drop is a permanent, fixed loss caused by your wiring. Voltage sag (or dip) is a temporary, utility-side event caused when a massive load (like a neighbor's central AC compressor or an industrial welder down the street) kicks on and momentarily pulls the transformer voltage down. You cannot fix utility voltage sag by upsizing your internal house wire; that requires a whole-house voltage regulator or a UPS system.
3. Voltage Drop vs. Loose Connections
If you measure 120V at the breaker but only 105V at the receptacle on a short 20-foot run, you do not have a conductor voltage drop problem. You have a high-resistance connection. This is almost always caused by a loose terminal screw, a backstabbed push-in connector that has arced, or a corroded wire nut. Tighten your lugs to the manufacturer's specified inch-pound torque rating before blaming the wire gauge.
When planning any circuit run over 75 feet, stop relying on the standard breaker-to-wire-size cheat sheet. Run the math, apply the 3% rule, and default to upsizing to 8 AWG THHN for 20A loads at the 100-foot mark. Your equipment will run cooler, last longer, and operate exactly as the manufacturer engineered it to.
