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How to Check Voltage Drop

Ampacity can pass and the lights still sag. Voltage drop scales with length and operating current, not with the breaker handle. This guide shows which current to enter, how to read the informational-note targets, and when distance forces an upsize.
Code basis
Ch. 9 Table 8; 210.19 / 215.2 IN
Branch target
3% informational note
Feeder + branch
5% informational note
Input that matters
Operating amps × length

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    Enter operating current, not breaker size

    Drop is proportional to the current that flows. A 60 A breaker on a 32 A load should be checked at about 32 A for sustained drop.

  2. Step 2

    Measure one-way circuit length

    Use the conductor route length, not the building footprint. Long verticals and detours count.

  3. Step 3

    Set voltage and phase correctly

    The same amp load at 120 V drops harder than at 240 V. Single-phase and three-phase formulas differ — match the circuit.

  4. Step 4

    Compute drop and compare to the target

    Compare against the 3 percent branch and 5 percent feeder-plus-branch recommendations unless the AHJ or equipment instructions set a tighter limit.

  5. Step 5

    Upsize only after ampacity still passes

    If drop governs, increase conductor size and re-check both drop and the ampacity/termination path so the new size is still protected correctly.

Use operating current

Voltage drop is proportional to the current actually flowing. A breaker rating is not the same as operating current.

Continuous-load sizing can increase conductor ampacity requirements, but the sustained voltage drop still follows the load current.

Recommendation, not universal violation

The familiar 3 percent branch-circuit and 5 percent combined figures come from informational notes.

Some AHJs, equipment instructions or other standards may enforce tighter limits, so the calculator reports a caution rather than a code failure.

Decision Guide

The choices that change the answer, and how to make them.

  • Informational notes are not automatic violations

    The familiar 3 percent and 5 percent figures are recommendations in informational notes for most circuits. Treat them as engineering targets unless the jurisdiction or equipment enforces them.

  • Continuous-load sizing is a different number

    Conductors may be sized at 125 percent of continuous load while voltage drop still follows the current that actually flows.

  • Aluminum and long feeders need the check early

    Material changes and outbuilding lengths move drop as much as ampacity. Decide copper vs aluminum with both checks open.

FAQs

Is the 3 percent voltage drop rule required by the NEC?
For most branch circuits it appears in an informational note, not as an enforceable requirement. Some AHJs, equipment instructions or other standards may still enforce a limit.
Should I calculate voltage drop from the breaker rating?
No. Use the operating current of the load. Breaker rating is for overcurrent protection and is often larger than the sustained current that creates drop.
When does distance usually force a larger wire?
On long 120 V runs and on long 240 V feeders to outbuildings, EVSE and subpanels. Ampacity can still look fine while drop exceeds the target.

Calculated against the NEC 2023 edition using Chapter 9 Table 8, 210.19(A) Informational Note, 215.2(A) Informational Note. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified