Voltage drop is the reduction in electrical potential along a conductor caused by the inherent resistance of the wire as current flows through it. When you are figuring voltage drop for a new circuit, you are essentially calculating how much of your source voltage gets wasted as heat in the wire before it reaches the load.

What Voltage Drop Actually Is (And Isn't)

In a real circuit, excessive voltage drop changes the operating characteristics of your load in measurable ways. For resistive loads like incandescent bulbs or space heaters, power output drops exponentially because power is proportional to the square of the voltage (P = V²/R). A 10% drop in voltage yields a 19% drop in heating or lighting output. For inductive loads like AC compressors, well pumps, or table saws, a voltage deficit forces the motor to draw higher amperage to maintain its mechanical power output. This excess current leads to overheating, degraded insulation, and premature winding failure. On the electronics bench, ESP32-based DIY controllers and smart home hubs will brownout and reboot if the 5V or 3.3V rail sags below their minimum threshold due to undersized supply wires.

DIYers and trade students commonly confuse uniform wire voltage drop with two other phenomena:

  • Voltage Sag: A temporary, utility-side dip caused by grid switching, heavy neighborhood loads, or large appliances starting up. This is the grid's problem, not your wire's problem.
  • High-Resistance Connections: A localized voltage drop at a loose terminal, backstabbed outlet, or corroded lug. This causes extreme, dangerous point-heating and is a fire hazard, whereas uniform wire drop distributes heat safely along the entire run.
The NEC 3% / 5% Rule: The National Electrical Code (NFPA 70) Article 210.19(A) Informational Note No. 4 recommends a maximum 3% voltage drop on the furthest branch circuit, and a maximum 5% total drop from the service entrance to the furthest outlet. While technically an 'Informational Note' and not a strict enforceable code article in all jurisdictions, it is the universal engineering standard for safe, efficient design.

The Math: A Worked Numeric Example

To figure voltage drop accurately, skip the rough 'K-factor' approximations and use the actual AC resistance values from NEC Chapter 9, Table 8. The formula for a single-phase circuit is:

VD = 2 × L × I × (R / 1000)

  • VD: Voltage Drop (in Volts)
  • L: One-way length of the wire (in feet)
  • I: Current (in Amps)
  • R: Resistance per 1,000 feet (from NEC Ch. 9 Table 8 for uncoated copper at 75°C)

The Scenario: You are wiring a 120V, 20A continuous load (like a heavy shop vac or window AC) in a detached workshop. The one-way wire distance from the panel to the outlet is 100 feet.

Attempt 1: 12 AWG Copper
NEC Table 8 lists 12 AWG solid copper resistance at 1.98 ohms/kft.
VD = 2 × 100 × 20 × (1.98 / 1000) = 7.92V drop.
Percentage: 7.92 / 120 = 6.6%. Fails the 3% recommendation.

Attempt 2: 10 AWG Copper
NEC Table 8 lists 10 AWG at 1.24 ohms/kft.
VD = 2 × 100 × 20 × (1.24 / 1000) = 4.96V drop.
Percentage: 4.96 / 120 = 4.1%. Still fails the 3% recommendation.

Attempt 3: 8 AWG Copper
NEC Table 8 lists 8 AWG at 0.778 ohms/kft.
VD = 2 × 100 × 20 × (0.778 / 1000) = 3.11V drop.
Percentage: 3.11 / 120 = 2.59%. Passes the 3% recommendation.

For this 100-foot run, you must upsize from the standard 12 AWG all the way to 8 AWG to maintain code-compliant efficiency and protect your motor loads.

Where You Meet This in Practice

You rarely need to calculate voltage drop for a 20-foot run to a bedroom outlet. The math becomes critical in these specific installations:

  • Subpanel Feeders to Detached Garages: A 100A or 60A subpanel located 150 feet from the main house will suffer massive drop if sized purely for ampacity. A 100A feeder requires 3 AWG copper for ampacity, but often demands 1 AWG or 1/0 AWG to keep the drop under 3% at full load.
  • Level 2 EV Chargers: As of 2026, most residential EV chargers pull 40A to 48A continuously. Running 6 AWG copper 80 feet for a 48A charger will result in a 4.5% drop, causing the charger's internal contactors to chatter or the vehicle to throttle charging speeds.
  • Low-Voltage Landscape Lighting: This is where voltage drop is most brutal. On a 12V AC system, a mere 1.5V drop (which is negligible on a 120V line) represents a 12.5% loss. This causes the furthest LED fixtures in the daisy chain to visibly dim. Landscape pros mitigate this by using 10 AWG or 8 AWG main trunk lines and stepping up to a 15V or 22V multi-tap transformer.
  • Deep Well Pumps: A 240V submersible pump sitting 300 feet down a well casing requires specialized 3-wire pump cable, often upsized to 10 AWG or 8 AWG even if the pump only draws 10A, simply to overcome the vertical distance.

Decision Tree: Sizing Wire to Kill Voltage Drop

Use this decision matrix to select your wire. Always calculate for the maximum continuous load, not the breaker size. The table terminates in specific, buyable part recommendations for standard North American installations.

Scenario Load / Voltage One-Way Distance Required Wire Size (Copper) Concrete Part Pick
Standard Indoor Branch 20A / 120V Under 50 ft 12 AWG Southwire 12 AWG NM-B (Romex) 250ft Coil
Long Indoor/Conduit Branch 20A / 120V 100 ft 8 AWG Southwire 8 AWG THHN/THWN-2 (pulled in 3/4" PVC)
Direct Burial Shed Feeder 60A / 240V 120 ft 4 AWG (Copper) or 2 AWG (Aluminum) Southwire 2-2-2-4 Dyke UF AL (Aluminum direct burial)
Hardwired EV Charger 48A / 240V 80 ft 4 AWG Cerrowire 4 AWG THHN (pulled in 1" PVC)
Pro-Tip for Upsized Branch Circuits: If you upsize to 8 AWG wire to fix voltage drop, but your 20A breaker and standard 20A receptacles only accept up to 10 AWG or 12 AWG wire, do not force the thick wire into the small lugs. Instead, use a wire nut or Wago 221 lever connector to pigtail a 12-inch piece of 12 AWG wire to the end of your 8 AWG run, and land the 12 AWG pigtail on the device.

FAQ: Common Voltage Drop Mistakes

Q: Does the breaker size change if I upsize the wire for voltage drop?
A: No. The breaker protects the wire from overcurrent, but it is perfectly safe (and common) to use a larger wire on a smaller breaker. A 20A breaker protecting 8 AWG wire is completely fine; the 8 AWG wire will simply run cooler than necessary, which is exactly what you want to minimize voltage drop.

Q: Can I just use aluminum wire to save money on long runs?
A: Yes, and for feeders over 100 feet, you usually should. Aluminum is significantly cheaper and lighter than copper. However, aluminum has higher resistance, so you must upsize it by one or two AWG sizes compared to copper to achieve the same voltage drop. Always use aluminum-rated lugs (marked CO/ALR or AL/CU) and apply anti-oxidant paste (like Noalox) to the terminations.

Q: My multimeter reads 122V at the panel but 114V at the outlet. Is this a voltage drop issue?no load, you do not have a wire sizing issue; you have a high-resistance connection (a loose neutral or a failing backstab) somewhere in the circuit. Turn off the breaker and inspect every splice.

When figuring voltage drop, never rely on 'it depends' guesswork. Run the math using the NEC Chapter 9 resistance values, enforce the 3% branch circuit limit, and buy the exact wire gauge the math demands. Your motors, microcontrollers, and utility meter will thank you.