A light that dims when the air conditioner kicks on, a motor that struggles to reach full torque at the end of a long feeder, both often trace back to the same overlooked culprit. Voltage drop quietly undersells every circuit it touches, and getting the wire size right is the fix.
Every conductor resists current flow at least a little, and that resistance quietly steals voltage the further electricity has to travel. Voltage drop is simply that loss made visible, and understanding it is what separates a wire size picked by habit from one picked on purpose.
Voltage drop is the loss of electrical potential that happens as current pushes through the impedance of a wire on its way from source to load. It does not usually trip a breaker or blow a fuse, which is exactly why it goes unnoticed until equipment starts underperforming.
This guide covers what voltage drop actually is, the four factors that cause it, the formula behind every calculator built around it, and the National Electrical Code guidance that tells you how much drop is too much.
What Is Voltage Drop
Voltage drop is the amount of voltage lost across part or all of a circuit because of the conductor's impedance. The National Electrical Code does not treat it as a safety issue, but it does recommend limits, because a circuit that loses too much voltage delivers poor performance and can shorten equipment life.
A motor starved of voltage draws more current to do the same work, running hotter than it should. A lighting circuit with excess voltage drop simply looks dim, which is a subtle but real energy and performance cost over the life of an installation.
How Much Voltage Drop Is Acceptable
The NEC guidance most engineers design around allows a maximum of 3% voltage drop on the branch circuit alone, and a combined maximum of 5% across both the feeder and the branch circuit together.
On a 120 volt, 15 ampere circuit, that 5% limit means no more than a 6 volt drop at the farthest outlet under full load. It sounds like a small number until you are running a long cable run to a remote panel.
Voltage Drop Fundamentals: The Four Causes
Conductor Material
Copper conducts better than aluminum, so a copper conductor of the same size will always show less drop over the same length.
Wire Size
A larger diameter conductor has lower resistance, so increasing wire size is the most direct way to reduce the loss.
Wire Length
Resistance builds up along the whole run, so a shorter conductor will always show less drop than a longer one carrying the same load.
Current
The loss rises directly with the current flowing through the wire, so a heavily loaded circuit needs a larger conductor than a lightly loaded one of the same length.
The Voltage Drop Formula
At its core, voltage drop is simply Ohm's law applied to the conductor itself. Multiply the current the circuit carries by the total resistance of the conductor, and the result is the voltage lost along the way.
Where I is the circuit current in amperes and R is the total resistance of the conductor, out and back, in ohms.
In practice, most calculators express this using circular mil area and a resistivity constant K instead of a raw resistance value, since that lets you solve directly for wire size.
- K is the resistivity constant, about 12.9 for copper and 21.2 for aluminum at 75 degrees C
- I is the load current in amperes
- D is the one way circuit length in feet
- CM is the circular mil area of the chosen conductor
For a three phase circuit, the same idea applies with a factor of 1.732 in place of the 2, since the return path works differently across three conductors instead of two.
Watch: Voltage Drop and Cable Size Calculations
This walkthrough covers the same formula using worked examples, which helps if you prefer to see the numbers applied step by step.
Video: "Easy Voltage Drop Formula and Cable Size Calculations, All in One", via YouTube.
How to Calculate Voltage Drop Step by Step
Identify the circuit details
Note the conductor material, the one way circuit length, the load current, and the system voltage before doing any math.
Pick a trial wire size
Start with a wire size that already meets ampacity requirements for the load, then check whether it also meets the drop limits.
Apply the formula
Plug the length, current, and circular mil area of the trial wire into the formula to get the expected drop in volts.
Convert to a percentage
Divide the calculated drop by the system voltage to see whether the result falls within the 3% or 5% guidance for the circuit type.
Size up if needed
If the percentage is too high, move to the next larger standard wire size and recalculate, since a bigger conductor lowers resistance and therefore lowers the drop.
Voltage Drop vs Ampacity: Which Governs Wire Size
| Situation | What Typically Governs |
|---|---|
| Long cable runs | The drop usually governs, since resistance builds up over distance |
| Short, heavily loaded circuits | Ampacity usually governs, since heat buildup becomes the limiting factor |
| Any circuit | Both checks must pass, and the larger resulting wire size is the correct final selection |
Common Voltage Drop Mistakes
✔ Do
- Calculate the drop separately from ampacity, then compare both results
- Use the one way circuit length in the formula, not the round trip length, unless the formula variant already accounts for the return path
- Recheck the drop whenever a circuit is extended or a load is added
- Size conductors generously on long runs feeding motors or sensitive electronics
✘ Don't
- Assume a circuit is fine just because the breaker has not tripped
- Use the wrong K constant for the conductor material being installed
- Ignore the drop on long feeder runs just because ampacity checks out
- Forget that motor starting current briefly increases the drop well beyond running current
FAQs on Voltage Drop
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Read Full Article →What we learn today
- Voltage drop is the voltage lost across a conductor's impedance as current travels from source to load.
- NEC guidance commonly cited limits branch circuits to 3% voltage drop, with a combined 5% across feeder and branch circuit together.
- Four factors drive voltage drop: conductor material, wire size, wire length, and the current being carried.
- The formula VD equals I times R, or equivalently 2 times K times I times D divided by CM, connects all four factors directly to wire size.
- The loss and ampacity are separate checks, and the larger resulting wire size from either calculation is the correct final choice.