Voltage Drop Calculator

What is the Voltage Drop Calculator?

Every conductor has resistance, so the voltage arriving at a load is always lower than the voltage leaving the panel. This calculator works out how much lower. Give it the system (DC, single-phase or three-phase), the source voltage, the load current, the one-way run in feet and the conductor, and it returns volts dropped, drop as a percentage, the voltage actually present at the load, and the smallest conductor that would bring the run inside your target. Resistance comes from NEC Chapter 9 Table 8, which is the same table an electrician would open.

How to Use the Voltage Drop Calculator

1. 1Pick the system: DC and single-phase both use a factor of 2, three-phase uses the square root of 3
2. 2Enter the nominal source voltage, for example 120, 240, 277 or 480
3. 3Enter the load current in amperes, not the breaker size
4. 4Enter the one-way length of the run. The return path is already in the multiplier, so do not double it yourself
5. 5Choose copper or aluminum, and the conductor size
6. 6Set your drop target if 3% is not what you are working to, and read off the smallest size that meets it

Key features

Why Voltage Drop Matters

A load that is 8% low does not usually announce itself. Motors run hotter and produce less torque, because torque falls with the square of the voltage. Resistive heaters take longer and never quite reach temperature. LED drivers flicker or drop out at the bottom of their input range. Contactors chatter. None of these read as a wiring fault, so they get chased as equipment faults for a long time before anyone measures the voltage at the load with the load running. Sizing for drop at design time costs one conductor size and avoids all of it.

Common use cases

• Size a feeder to a detached garage or workshop at the far end of a property
• Check whether an existing circuit explains a motor that runs hot and low on torque
• Work out how far a 20 A branch circuit can run on 12 AWG before it breaks 3%
• Compare copper against aluminum for a long service run where cost matters
• Decide whether paralleling two smaller sets beats one larger conductor
• Verify a low-voltage DC run, where drop is proportionally far more punishing

The formula

• For DC and single-phase: volts dropped = 2 × I × R × L / 1000, where I is amperes, R is the conductor resistance in ohms per 1000 feet, and L is the one-way length in feet. The factor of 2 is there because the current travels out along one conductor and back along another, so the copper in the circuit is twice the run.
• For three-phase: volts dropped = 1.732 × I × R × L / 1000. The square root of 3 is not a physical doubling. It falls out of the phase relationship between line-to-line voltages, and it is why a three-phase run drops noticeably less than a single-phase run of the same length and current.
• Percentage drop is simply the volts dropped divided by the nominal source voltage. Note that the percentage, not the absolute volts, is what the guidance is written against: 6 volts is trivial on a 480 V feeder and severe on a 12 V DC run.

Why 3% is a recommendation, not a rule

• The familiar figures are 3% on a branch circuit and 5% total including the feeder. They appear in NEC 210.19(A) and 215.2(A), but they appear there as Informational Notes. Informational Notes are explanatory and are not enforceable code text, which means an inspector cannot fail a run on drop alone under the base NEC.
• That is not permission to ignore them. Many jurisdictions amend the code to make the figures mandatory, some load types have their own requirements elsewhere in the code, and the performance problems that drop causes are real regardless of what is enforceable. Treat 3% as a design target you should have a reason to miss, and check your local amendments rather than assuming the base code applies as written.

What this calculator does not model

• It uses DC resistance from Chapter 9 Table 8 and ignores reactance. For branch circuits and small feeders that is the ordinary method and the error is small. On large conductors, long runs, or loads with a power factor well below 1, the reactive term stops being negligible and Chapter 9 Table 9 is the correct basis instead. Table 9 also varies with the conduit material, because a steel raceway raises reactance in a way PVC does not.
• It uses the 75 °C resistance values. A conductor running hot is more resistive than that and will drop slightly more; one running cool will drop slightly less. Table 8 is stated at 75 °C and that is the figure the code book carries, so it is the figure used here.
• It does not size the conductor for ampacity. Drop and ampacity are separate limits and neither substitutes for the other. A conductor can be thermally fine and still drop far too much, which is the usual outcome on a long run. Use the wire size calculator to satisfy both at once.

Copper against aluminum

• Aluminum is about 61% more resistive than copper at the same size, so an aluminum conductor drops that much more over the same run. The usual rule of thumb is to go up two sizes when substituting aluminum for copper, and the numbers here bear that out: 4/0 aluminum at 0.1 ohms per 1000 feet sits between 2/0 copper at 0.0967 and 1/0 copper at 0.122.
• Aluminum is still often the right answer on long feeders and services, where it is substantially cheaper per amp delivered even after the size increase. Where it is usually the wrong answer is on branch circuits, both because the size penalty bites harder at small sizes and because terminations need more care.

Voltage drop at 20 A over a 100 foot run, 120 V single-phase

Stranded copper, computed from NEC Chapter 9 Table 8. Note where the 3% target falls: at this current and length, 12 AWG is already well past it, and the first size that comfortably clears is 8 AWG.

Drop scales linearly with both current and length, so the same 12 AWG run at 10 A, or at 50 feet, drops half as much and lands at 3.30%.

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