Voltage drop is the reduction in electrical potential across a conductor due to its inherent resistance, delivering less voltage to the load than the source provides. When you are wiring a detached garage, running a well pump, or installing driveway lighting, ignoring this phenomenon is the fastest way to burn out a motor or trip a breaker. This guide cuts through the theory and gives you the exact math, the physical consequences, and a concrete decision path to size your wire correctly on the first trip to the supply house.
The Core Concept (and What People Confuse It With)
Every wire has resistance. As current flows through that resistance, energy is lost as heat, reducing the voltage available at the far end. Think of it like water flowing through a long, narrow garden hose: the friction against the hose walls drops the water pressure by the time it reaches the spray nozzle.
What Voltage Drop Actually Changes in a Circuit
When voltage sags at the load, the physical behavior of your equipment changes drastically:
- Inductive Loads (Motors, Compressors): Torque drops significantly. To maintain mechanical output, the motor draws more current, which generates excess heat in the windings and drastically shortens the motor's lifespan.
- Switching Power Supplies (LED Drivers, PCs): Because power equals voltage times current ($P = V imes I$), a switch-mode power supply receiving 110V instead of 120V will draw more current to maintain its wattage output. This increased current draw exacerbates the voltage drop, creating a thermal runaway loop that can trip your breaker.
- Resistive Loads (Heaters, Incandescent Lights): They simply underperform. A 1500W space heater receiving 110V will only output about 1260W of heat.
The Most Common Confusion
Beginners frequently confuse conductor voltage drop with point-resistance voltage drop. If you measure 120V at the panel but only 105V at an outlet 10 feet away, that is almost certainly a loose terminal, a corroded wire nut, or a failing breaker contact—not wire length resistance. True length-based voltage drop scales linearly with distance; point-resistance drops happen abruptly at connections.
The Math: A Real-World Worked Example
Let's look at a standard scenario: You are wiring a 120V, 20-amp air compressor in a workshop. The one-way wire distance from the panel to the receptacle is 100 feet. The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits (which is 3.6V on a 120V system).
We use the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM
- K = 12.9 (approximate resistance constant for copper at 75°C)
- I = 20 Amps (the load)
- L = 100 feet (one-way length)
- CM = Circular Mils (the cross-sectional area of the wire, found in NEC Chapter 9, Table 8)
- 12 AWG (CM = 6,530): VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9V drop (6.6%). The compressor receives 112.1V. It will overheat and likely trip the breaker on startup.
- 10 AWG (CM = 10,380): VD = (2 × 12.9 × 20 × 100) / 10380 = 4.97V drop (4.1%). The compressor receives 115V. Better, but still outside the 3% NEC recommendation.
- 8 AWG (CM = 16,510): VD = (2 × 12.9 × 20 × 100) / 16510 = 3.12V drop (2.6%). The compressor receives 116.8V. This is under the 3% threshold.
Even though 12 AWG is legally rated for 20 amps regarding ampacity (heat dissipation), it fails the voltage drop test for this distance. You must upsize to 8 AWG to ensure proper equipment operation. Tools like the Southwire Voltage Drop Calculator can automate this, but knowing the manual math prevents you from blindly trusting app defaults that might assume a lower ambient temperature or a different load power factor.
Where You Meet This in Practice
You will rarely encounter problematic voltage drop in standard interior room wiring. It rears its head in specific, high-stakes installations:
- Detached Garage Subpanels: Running a 60-amp or 100-amp feeder 150 feet underground requires massive wire. A 2 AWG aluminum feeder might be fine for ampacity, but you often need to bump up to 1/0 AWG or 2/0 AWG aluminum to keep the drop under 3% when the table saw and space heater run simultaneously.
- Well Pumps: Submersible pumps are often located 200+ feet away from the pressure switch, plus another 200 feet down the well casing. Because they operate on 240V, the percentage drop is halved compared to 120V, but the extreme distance usually demands 10 AWG or 8 AWG instead of the standard 12 AWG.
- EV Chargers: A 48-amp continuous EV charger requires a 60-amp breaker and 4 AWG copper for standard runs. Push that run past 120 feet, and you must upsize to 3 AWG or 2 AWG to prevent the charger's internal contactors from chattering or failing to close due to low coil voltage.
Decision Tree: Picking the Right AWG for Your Run
Stop guessing. Use this decision matrix for standard 120V single-phase branch circuits to determine your minimum wire size. This table assumes copper wire, a continuous load at 80% of breaker capacity, and targets a strict <3% drop.
| Breaker Size | Max Load (Amps) | Distance (One-Way) | Required Wire Size (Copper) | Concrete Pick (Wire Type) |
|---|---|---|---|---|
| 15A | 12A | 0 - 50 ft | 14 AWG | 14/2 NM-B |
| 15A | 12A | 51 - 125 ft | 10 AWG | 10 AWG THHN in conduit |
| 20A | 16A | 0 - 40 ft | 12 AWG | 12/2 NM-B |
| 20A | 16A | 41 - 75 ft | 10 AWG | 10 AWG THHN in conduit |
| 20A | 16A | 76 - 125 ft | 8 AWG | 8 AWG THHN in conduit |
| 30A | 24A | 0 - 30 ft | 10 AWG | 10/2 NM-B |
| 30A | 24A | 31 - 90 ft | 6 AWG | 6 AWG THHN in conduit |
Frequently Asked Voltage Drop Questions
Does the NEC strictly mandate the 3% voltage drop limit?
No. For most residential branch circuits, the 3% limit (and 5% total for feeder plus branch) is found in the Informational Notes of the National Electrical Code (NFPA 70), specifically around 210.19(A). Informational notes are recommendations, not enforceable mandates, unless your local Authority Having Jurisdiction (AHJ) specifically adopts them into local law. However, ignoring them violates the core engineering requirement that equipment must operate safely at its rated voltage.
Should I calculate voltage drop using 120V or the actual measured voltage?
Always calculate using the nominal system voltage (120V or 240V). Utility transformers are tapped to deliver slightly higher voltage at the service entrance (often 122V to 125V) specifically to account for the voltage drop that will occur across the home's internal wiring. Designing for exactly 120V gives you a built-in safety buffer.
Does switching from copper to aluminum wire change the voltage drop?
Yes, significantly. Aluminum has a higher resistance than copper. The 'K' constant in the math formula changes from 12.9 (copper) to roughly 21.2 (aluminum). If you are running a long 240V feeder to a subpanel using aluminum (like 2-2-2-4 MHF), you must run the math with the aluminum constant, which usually forces you to upsize the wire by one or two AWG steps compared to a copper equivalent.
Can I just use a higher voltage to avoid voltage drop?
Yes, this is exactly why long-distance transmission lines use hundreds of thousands of volts, and why heavy shop equipment uses 240V instead of 120V. Running a 3000W load at 240V draws 12.5 amps, whereas running it at 120V draws 25 amps. Because the current is halved, the voltage drop is halved, allowing you to use much smaller, cheaper wire. If you have a choice between wiring a heavy tool for 120V or 240V, always choose 240V for long runs.
When planning any circuit over 50 feet, stop relying on standard ampacity charts. Calculate the drop, upsize the wire, and use THHN in conduit for long pulls. If your math dictates 8 AWG, buy 8 AWG—never compromise on wire size to save a few dollars, as the cost of replacing a burned-out compressor motor will dwarf the price of heavier copper.
