Voltage drop is the reduction in electrical potential energy between the power source and the load, caused by the inherent resistance of the conductors. When you push current through a wire, that resistance converts a portion of the electrical energy into heat, meaning the voltage arriving at your outlet or appliance is measurably lower than what left the breaker panel. This changes how a real circuit performs: it causes incandescent lights to dim, AC compressor motors to draw higher amperage and overheat, and sensitive electronics to brown out or randomly reboot. People commonly confuse voltage drop with voltage sag (a temporary, utility-side dip in grid voltage) or a tripped breaker (an overcurrent protection event, whereas voltage drop occurs continuously even when the circuit is operating safely below its ampacity limit).
The Physics of the Drop and Conductor Resistance
Every conductor has resistance. While we treat wires as perfect paths in basic theory, real-world copper and aluminum impede electron flow. Think of water flowing through a long, narrow garden hose; the friction against the hose walls reduces the water pressure at the nozzle compared to the spigot, even though the same volume of water is flowing through the entire length.
The longer the wire and the smaller the gauge (higher AWG number), the higher the resistance. Because power loss is calculated as I²R (current squared times resistance), high-draw appliances on long, undersized runs waste significant energy as heat inside your walls or conduit. This is why the National Electrical Code (NEC) provides strict guidance on wire sizing beyond just the basic ampacity tables.
How to Determine Voltage Drop: The Math and a Worked Example
To calculate the exact voltage drop for a single-phase circuit, electricians use the standard approximate formula derived from Ohm's Law:
VD = (2 × K × I × D) / CM
- VD: Voltage Drop (in volts)
- 2: Multiplier for the complete circuit loop (hot and neutral/ground)
- K: Specific resistance of the conductor (12.9 for copper, 21.2 for aluminum at 75°C)
- I: Current in amperes (the actual load draw, not the breaker size)
- D: One-way distance from the panel to the load in feet
- CM: Circular mils of the conductor (a standard measure of wire cross-sectional area)
You are wiring a dedicated 120V outlet in a detached workshop for a heavy-duty table saw that draws a continuous 20A. The trench from your main panel to the workshop is 150 feet long, and you plan to use 12 AWG copper wire.
Variables: K = 12.9 | I = 20A | D = 150 ft | CM for 12 AWG = 6,530
Math: VD = (2 × 12.9 × 20 × 150) / 6,530
Math: VD = 77,400 / 6,530 = 11.85 Volts
The Result: Your 120V circuit will arrive at the saw at only 108.15V. This is a 9.8% voltage drop, which will cause the saw motor to overheat and potentially trip its internal thermal overload. To fix this, you must upsize the wire. Upgrading to 6 AWG copper (CM = 26,240) drops the VD to 2.94V (2.45%), safely within acceptable limits.
Where You Meet This in Practice
You rarely notice voltage drop on a 25-foot run to a bedroom outlet, but it becomes a critical design factor in several common residential scenarios:
- Detached Subpanels: Feeder runs to garages or barns often exceed 100 feet. If you size a 60A subpanel feeder using the bare minimum 6 AWG copper based solely on ampacity, a 50A load at 120 feet will result in severe voltage drop. You typically need to upsize to 4 AWG or 3 AWG.
- Level 2 EV Chargers: Modern EV chargers pull 32A to 48A continuously for hours. Because they are continuous loads, the wire must be derated by 125%, and the long runs typical in driveways make voltage drop the primary sizing constraint, not the breaker rating.
- Low-Voltage Landscape Lighting: A 12V halogen or LED system is hyper-sensitive. A mere 1.2V drop (10%) will visibly dim the furthest fixtures. Installers must use much thicker wire (like 10 AWG or 8 AWG) or center-feed the lighting zones to keep runs short.
NEC Guidelines and Wire Sizing Rules
While the NEC ampacity tables (like 310.16) dictate the maximum current a wire can carry before the insulation melts, voltage drop recommendations are largely found in Informational Notes, such as NEC 210.19(A). The NFPA National Electrical Code recommends a maximum of 3% voltage drop on a branch circuit, and a maximum combined 5% for the feeder and branch circuit together.
Below is a reference chart for a standard 120V, 20A load, showing the maximum one-way distance you can run copper wire before exceeding the 3% (3.6V) recommendation. For a deeper look at measurement techniques, Fluke's electrical testing guides offer excellent field procedures for verifying these calculations with a multimeter.
| Wire Size (AWG) | Circular Mils (CM) | Max One-Way Distance (20A Load @ 120V) |
|---|---|---|
| 14 AWG | 4,110 | 28 feet |
| 12 AWG | 6,530 | 45 feet |
| 10 AWG | 10,380 | 72 feet |
| 8 AWG | 16,510 | 115 feet |
| 6 AWG | 26,240 | 183 feet |
Frequently Asked Questions
How to determine voltage drop for a 240V circuit?
The formula remains exactly the same, but your baseline voltage changes to 240V. Because the voltage is doubled, the allowable 3% voltage drop is now 7.2V instead of 3.6V. This means a 240V circuit can be run roughly twice as far as a 120V circuit using the same wire gauge and amperage before hitting the 3% threshold. However, for a pure 240V load (like a baseboard heater with no neutral), you do not multiply the distance by 2 in the formula; the '2' in the numerator accounts for the two current-carrying conductors (L1 and L2) completing the loop.
What is the maximum allowable voltage drop per the NEC?
Technically, the NEC treats voltage drop recommendations in branch circuits as 'Informational Notes' rather than strictly enforceable code for standard residential dwellings, meaning an inspector might not fail you for a 6% drop. However, NEC 647.4(D) strictly enforces a 1.5% drop for sensitive electronic equipment, and many local jurisdictions adopt the 3% branch / 5% total rule as mandatory. Furthermore, equipment manufacturers often void warranties if their appliances are operated below 110V on a 120V nominal system.
Does voltage drop cause a breaker to trip?
No. A standard thermal-magnetic breaker trips based on current (amperage) and heat, not voltage. In fact, voltage drop can sometimes cause an indirect trip: if a motor receives low voltage, it will draw higher amperage to maintain its mechanical power output (Watts = Volts × Amps). This increased amperage can eventually trip the breaker or the motor's internal overload, but the voltage drop itself is not what the breaker detects.
How do I measure voltage drop with a multimeter?
Set your digital multimeter to AC Volts. Measure the voltage at the breaker terminals with the heavy load turned OFF. Then, turn the load ON and measure the voltage at the receptacle or appliance terminal. The difference between the two readings is your exact real-world voltage drop. For maximum accuracy, ensure your multimeter is True-RMS, as non-linear loads can distort the waveform and give inaccurate readings on cheaper averaging meters.
