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Voltage Drop & Wire Run Length Guide
Voltage drop is easy to overlook because it doesn't trip a breaker or show up as an obvious fault — it just quietly wastes energy as heat along the wire and starves the load at the far end of the voltage it actually needs. A wire that comfortably passes an ampacity check can still be the wrong size once the run length is factored in, and the fix is always the same: a larger conductor, not a different circuit protection setting.
Last updated: August 29, 2026
Voltage drop is one of the few electrical problems that can exist on a circuit indefinitely without ever tripping a breaker — the current stays within the wire's ampacity rating while the load still quietly receives less voltage than it needs. Catching it means running a separate check, not just trusting that an adequately-rated wire is automatically fine over any distance.
This guide covers what drives voltage drop, the standard 3%/5% planning targets, real-world symptoms of an undersized circuit, and a worked example.
The Core Relationship
Voltage drop follows directly from Ohm's law — current through resistance produces a voltage loss:
Four factors control the result:
| Factor | Effect | Practical Note |
|---|---|---|
| Current | Higher current increases drop proportionally | Reduce load on the circuit, or split it, if drop is marginal |
| Run length | Drop is directly proportional to one-way distance | Measure the actual routed path, not straight-line distance |
| Conductor material | Aluminum has higher resistance than copper for the same gauge | Aluminum runs may need a larger gauge than copper for the same drop target |
| Wire gauge | Larger cross-section has lower resistance, less drop | The standard fix for excessive drop — size up the conductor |
A commonly used planning target is 3% for a branch circuit alone, 5% for the combined feeder-plus-branch-circuit run — these are widely referenced guidance figures, not a universal fixed requirement for every circuit.
Why System Voltage Changes the Picture
Since drop is calculated as a percentage of source voltage, the same wire and load produces a very different percentage result depending on the circuit's voltage:
| System Voltage | Effect on Percentage Drop |
|---|---|
| 120V | Higher percentage drop for the same absolute voltage loss and current — long 120V runs are the most common voltage-drop-driven upsizing case |
| 240V (split-phase) | Half the percentage drop of an equivalent 120V run at the same power delivered, since current is halved for the same wattage |
| 277V/480V (commercial) | Even lower percentage drop per unit power delivered — one reason larger commercial buildings favor higher distribution voltages |
Worked Example — Long Detached-Garage Run
120V, 15A Load, 150 ft One-Way Run, 12 AWG Copper
Illustrative example
| Step | Result |
|---|---|
| 12 AWG copper voltage drop at 150 ft, 15A | Roughly 5-6% — exceeds the 3% branch-circuit target |
| Sizing up to 10 AWG | Meaningfully reduces drop, may still be marginal at 150 ft |
| Sizing up to 8 AWG | Comfortably brings drop within the 3% target at this length |
Note that 8 AWG is larger than what a 15A load's ampacity alone would ever require — this is a voltage-drop-driven upsize, not an ampacity-driven one, and it's a genuinely common outcome on long outbuilding runs.
Common Mistakes
Checking Only Ampacity, Never Voltage Drop
A wire gauge that comfortably passes the ampacity/heat check can still fail voltage drop on a long run — the two checks are independent, and both must pass. Skipping the voltage drop check on a long run is one of the most common oversights in DIY and even some professional wiring.
Measuring Straight-Line Distance Instead of the Actual Routed Path
The wire's real path — through walls, up and over framing, around obstacles — is typically longer than a straight-line distance between two points. Using straight-line distance understates the actual run length and therefore understates the calculated voltage drop.
Ignoring the Doubled Distance for a Round-Trip Circuit
Voltage drop calculations account for both the outgoing and return conductor, effectively doubling the one-way distance in the underlying formula for a standard single-phase circuit — using only the one-way distance in a formula that already expects it separately handled produces an incorrect result depending on which convention the specific calculator or formula uses.
Assuming a Circuit Breaker Trip Would Catch a Voltage Drop Problem
Excessive voltage drop does not trip a breaker — the current can be well within the wire's ampacity rating while voltage drop is still unacceptably high, since the two are separate physical phenomena. A circuit with a real voltage drop problem can run indefinitely without any protective device intervening.
Not Re-Checking Voltage Drop After a Load or Route Change
Adding load to an existing circuit, or extending/rerouting a run during a renovation, changes the voltage drop calculation even if the wire itself isn't touched — a circuit that passed originally can silently exceed the target after a change that wasn't re-evaluated for drop.
Relevant Standards and References
Voltage drop limits are typically recommended guidance rather than a strict universal requirement, but the specific percentages referenced vary by code.
| Region | Relevant Codes / Guidance |
|---|---|
| United States | NEC Informational Note to 210.19(A)(1) and 215.2(A)(3) recommend the 3%/5% branch-circuit/feeder-plus-branch guidance referenced throughout this guide |
| Europe / UK | BS 7671 recommends voltage drop limits (commonly cited around 3% for lighting, 5% for other uses from the origin of the installation) |
| India | IS 732 references permissible voltage drop limits for building wiring installations |
| Australia / New Zealand | AS/NZS 3000 sets voltage drop limits for electrical installations, commonly a 5% limit from the point of supply |
| General guidance | These percentages are widely-used planning targets, not a substitute for the specific numeric limit (where one is mandated) in the code edition actually adopted in your jurisdiction. |
Final Verdict
Voltage drop is a separate, must-pass check from ampacity — a wire that's safe from a heat/current perspective can still under-deliver voltage on a long run, and the standard fix is always a larger conductor.
- Always run a voltage drop check separately from the ampacity check — passing one doesn't mean the other passes too.
- Target roughly 3% for a branch circuit alone, 5% for a combined feeder-plus-branch run, as widely-used planning guidance.
- Measure the actual routed path length, not straight-line distance between two points.
- Watch for dimming lights, hot-running motors, and erratic electronics as real-world symptoms of excessive drop.
- 120V circuits are more voltage-drop-sensitive than 240V circuits carrying the same power, over the same length.
- Sizing up the conductor gauge is the standard, sufficient fix for the overwhelming majority of residential and small commercial circuits.
Related calculators
Use these calculators when you need to turn this reference information into project quantities:
- Voltage Drop Calculator
Calculate voltage drop percentage or the maximum run length for a given wire size.
- Wire Size Calculator
Find the correct wire gauge accounting for both ampacity and voltage drop.
Related resources
- Wire Gauge Selection Guide
Complete guide to selecting the correct wire gauge (AWG) — base ampacity by insulation rating, the derating factors that reduce it (conductor count, ambient temperature), the small-conductor breaker cap, and a worked example.