Skip to content
ElectraKit
Open menu

Voltage Drop: Voltage Lost Over Distance

Voltage drop is the reduction in voltage between the source and the load caused by current flowing through conductor resistance or impedance over the circuit length.

Voltage drop gets confused with ampacity, breaker sizing and code failure. In this site's public calculator it is mainly a design-performance check: lost volts are estimated from operating current, conductor resistance, one-way length, nominal voltage and phase.

How the calculation works

The public calculator uses conductor resistance in ohms per 1000 feet, operating current, one-way length, nominal voltage and the phase multiplier.

Table 9 impedance is not described as a live public method unless that data path is verified and exposed by the calculator.

How to read the targets

The familiar branch-circuit and combined percentages come from informational notes, so an over-target result is reported as a caution rather than an automatic NEC violation.

That wording still leaves room for stricter project requirements from local amendments, equipment listings, utility rules or other applicable standards.

What changes after upsizing

Upsizing for drop can change terminations, raceway fill, pulling difficulty and equipment grounding conductor sizing.

After choosing a larger conductor, rerun the ampacity, breaker compatibility and conduit-fill checks so the fix does not create a different installation problem.

Where the Term Decides Something

Four points in the work where the figure is the answer to a question, not vocabulary.

  • Long branch circuits

    A receptacle, lighting, pump or EVSE branch circuit can pass ampacity while arriving several volts low at the load.

  • Feeder planning

    A feeder may consume part of the combined voltage-drop target, leaving less drop budget for downstream branch circuits.

  • Material comparison

    Copper and aluminum can both be valid choices, but their resistance differs enough that long feeders should be checked before conductors are bought.

  • EGC review after upsizing

    When ungrounded conductors are increased for voltage drop, the wire-type equipment grounding conductor may need proportional review.

Which Figure to Use

Reading the term correctly in practice.

  • Use operating current

    Voltage drop follows the current actually flowing, not the breaker handle. Use sustained load current unless the load really operates at the breaker rating.

  • Enter one-way length

    Use the routed conductor distance from source to load. The single-phase and three-phase multipliers account for the circuit path.

  • Set voltage and phase first

    The same amperes cause a larger percentage drop at 120 V than at 240 V, and three-phase uses a different multiplier than single-phase.

  • Treat targets conservatively

    The 3 percent and 5 percent figures are recommendations for most NEC work. Equipment instructions, energy codes or the AHJ can set a tighter project limit.

FAQs

Is exceeding 3 percent voltage drop an NEC violation?
For most NEC branch-circuit and feeder work, no. The 3 percent and 5 percent figures are informational-note recommendations, so the site reports a caution. The AHJ, equipment instructions, energy code or project specification can still make a tighter limit mandatory.
Should voltage drop be calculated from breaker size?
Usually no. Use the current that actually flows. A 50 A breaker serving a 32 A load does not create 50 A worth of voltage drop unless the load actually draws 50 A.
Do I enter round-trip distance?
No. Enter the one-way routed length. The calculation applies the single-phase return-path multiplier or the three-phase multiplier internally.
Can voltage drop require a larger conductor than ampacity?
Yes. Ampacity is a heat limit, while voltage drop is a performance limit tied to distance and resistance. Long runs can need a larger conductor even when the smaller conductor is thermally adequate.

Calculated against the NEC 2023 edition using Chapter 9 Table 8, 210.19(A) Informational Note, 215.2(A) Informational Note, 250.122(B). Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Mandatory voltage limits set by equipment instructions, utilities, project specifications or non-NEC standards.
  • Motor starting voltage, harmonic heating and detailed power-quality analysis.
  • Fault-current, short-circuit and coordination studies.

Data last verified