Voltage drop over distance is the reduction in electrical potential that occurs when current flows through the inherent resistance of a wire. When you push electrons through copper or aluminum, the metal fights back, converting some of that electrical energy into heat. This changes how your equipment behaves on the jobsite or in your home: incandescent lights dim, AC motors run hotter and draw more current to compensate, and sensitive electronics like smart home hubs or ESP32 microcontrollers will brown out and reboot. Beginners frequently confuse voltage drop with overcurrent (which trips a breaker) or utility voltage sag (where the power company delivers 114V instead of 120V to your main panel). Voltage drop is strictly a local issue caused by your wire sizing and run length, and it is entirely within your control to fix.
What Voltage Drop Over Distance Actually Is (and Isn't)
At its core, this phenomenon is just Ohm's Law in action: V = I × R. The resistance (R) of a copper wire is tiny per foot, but over a 100-foot run, that fractional resistance accumulates into a measurable voltage loss. Think of it like friction in a long garden hose; the water pressure (voltage) at the nozzle is lower than at the spigot because the hose walls resist the flow.
The National Electrical Code (NEC) doesn't strictly mandate a specific voltage drop limit for most residential branch circuits in its enforceable text. However, Informational Note 4 in NEC 210.19(A) recommends a maximum of 3% drop on branch circuits and a 5% total drop from the service entrance to the furthest outlet to ensure reasonable efficiency. If you ignore this guidance, your equipment will operate outside its designed nominal voltage range, leading to premature failure.
The Math: A Worked Numeric Example
Let's run the numbers for a common DIY scenario: a 120V, 15A continuous load (like a high-end window AC unit or a large space heater) located 150 feet from the main panel.
To calculate this, we use the standard resistance values per 1,000 feet found in NEC Chapter 9, Table 8. The formula for single-phase voltage drop is:
VD = 2 × L × I × (R_per_1000ft / 1000)
The '2' accounts for the hot wire going out and the neutral wire returning. L is the one-way distance (150 ft), and I is the current (15A).
Scenario A: Using Minimum Code 14 AWG Copper
14 AWG copper has a resistance of 3.14 ohms/kft.
- VD = 2 × 150 × 15 × (3.14 / 1000)
- VD = 14.13V
- Percentage Drop: 14.13 / 120 = 11.77%
Result: The load only sees 105.8V. This is a massive 11.77% drop, far exceeding the 3% recommendation. The AC motor will overheat, and the compressor may fail to start.
Scenario B: Upsizing to 8 AWG Copper
8 AWG copper has a resistance of 0.778 ohms/kft.
- VD = 2 × 150 × 15 × (0.778 / 1000)
- VD = 3.50V
- Percentage Drop: 3.50 / 120 = 2.91%
Result: Meets the 3% NEC recommendation. The load sees a healthy 116.5V.
Where You Meet Voltage Drop in Practice
You rarely notice voltage drop on a 20-foot run to a bedroom outlet. It rears its head in specific, high-load or long-distance applications:
- EV Chargers (Level 2): Chargers pull 32A to 48A continuously for hours. A 100-foot run in a garage or driveway will cause severe voltage sag if undersized. Worse, if the voltage at the charger drops below 208V on a 240V nominal line, the car's internal systems will throttle the charging speed or the charger's internal contactor will chatter and fail.
- Subpanel Feeds: Running a 100A subpanel to a detached shed 200 feet away. If you use 2 AWG aluminum, you'll lose over 7V on the 240V legs under full load, causing lights in the shed to flicker when the table saw starts.
- Landscape Lighting: 12V AC halogen or LED runs are brutally unforgiving. A 5% drop on 12V is only 0.6V. You often have to use massive 10 AWG or 8 AWG wire just to run 50 watts of LEDs 50 feet from the transformer without the lights at the end of the run turning yellow or dimming.
Decision Tree: Sizing Wire for Long Runs
Use this decision matrix to pick your wire gauge. This table assumes copper THHN in conduit, a 3% maximum drop limit, and standard 75°C terminations.
| Application | Nominal Voltage | Max Continuous Load | One-Way Distance | Required AWG (Copper) |
|---|---|---|---|---|
| Window AC / Space Heater | 120V | 15A | 100 ft | 10 AWG |
| Window AC / Space Heater | 120V | 15A | 150 ft | 8 AWG |
| Level 2 EV Charger | 240V | 40A | 100 ft | 6 AWG |
| Level 2 EV Charger | 240V | 48A | 150 ft | 4 AWG |
| Shed Subpanel Feeder | 240V | 60A | 150 ft | 4 AWG |
| Landscape Lighting | 12V | 5A (60W) | 50 ft | 10 AWG |
Code Caveats and Common Mistakes
Did you forget the return path?
The most common math error DIYers make is forgetting the multiplier '2' in the formula. The distance (L) is the one-way physical length of the cable, but the current must travel out on the hot wire and return on the neutral (or second hot) wire. The total wire length the current fights through is always double the physical distance for single-phase circuits.
Are you ignoring conduit fill and temperature derating?
Voltage drop calculations only care about resistance, but your breaker and wire insulation care about heat. If you pull four current-carrying conductors (e.g., two hots and two neutrals for two multi-wire branch circuits) in a single conduit, NEC 310.15(C)(1) requires you to derate the wire's ampacity to 80%. A 10 AWG wire might have low enough resistance for your voltage drop math, but if it's derated due to conduit heat, it might not be legally allowed to carry the 30A you're pushing through it.
Did you mix up Aluminum and Copper K-factors?
If you switch to aluminum wire—which is highly recommended for long subpanel feeders due to cost—the resistance jumps significantly. The approximate K-factor (resistivity constant) for copper is 12.9, while for aluminum it is 21.2. As a rule of thumb, you must upsize aluminum by two AWG sizes compared to copper to achieve the exact same voltage drop performance (e.g., if the math calls for 4 AWG copper, use 2 AWG aluminum).
Stop guessing and buy the right wire on the first trip to the supply house. For any 120V branch circuit exceeding 75 feet, or any 240V circuit exceeding 125 feet, abandon the minimum code-allowed AWG and upsize by at least one step. Default to individual THHN wires in conduit rather than NM-B (Romex) for long runs; the 90°C rating of THHN gives you more thermal headroom in the conduit, even if you must terminate at the 75°C column at the breaker and receptacle.
