Engineering Reference

Voltage Drop Calculator

Compute voltage drop over a conductor run from current, length and conductor resistance, and check it against the usual percentage limits.

Data verified 2026-09-29 · based on n/a — standard engineering relationships, no single governing revision

Quick Answer

Voltage drop is 2 × length × current × resistance per unit length for a single-phase two-wire circuit. The factor of 2 accounts for the current going out and returning. Branch circuits are normally limited to 3% and the total to 5%.

Voltage Drop Calculator

The Formulas Used

Vdrop = 2 × L × I × R
where L is the one-way length, I the current, and R the conductor resistance per unit length. The 2 accounts for the return conductor.

Why Voltage Drop Matters More Than Ampacity

Ampacity is not usually what limits a long run — voltage drop is. The conductor may be rated to carry the current comfortably, but over a long distance the resistance in the run drops enough voltage to leave the load outside its operating range.

The practical limits are 3% on a branch circuit and 5% for the feeder plus branch combined. Motors are particularly sensitive: a voltage drop at the terminals means higher current for the same power, which raises winding temperature and shortens motor life.

Getting the Resistance

Use the conductor resistance from the AWG wire gauge chart — the table gives mΩ per metre and per foot for each size. Note that those values are for copper at 20 °C; resistance rises with temperature, so a conductor running hot has a higher drop than the table suggests.

Frequently Asked Questions

How do I calculate voltage drop?
Vdrop = 2 × L × I × R, where L is the one-way length, I is the current and R is the conductor resistance per unit length. The factor of 2 accounts for the outgoing and return conductors.
What is an acceptable voltage drop?
3% on a branch circuit and 5% for the feeder and branch combined, as a common design guideline. Motor circuits are sometimes held to tighter limits because of the effect on winding temperature.
Why is there a factor of 2 in the formula?
Because current flows out through one conductor and back through the other, so the resistance of both must be counted. This applies to single-phase two-wire circuits.
Does voltage drop depend on the load?
Yes — it is proportional to current, so a lightly loaded circuit of the same length drops less. This is why a conductor sized for ampacity alone may still be too small for a long run at full load.
Does temperature affect voltage drop?
Yes. Conductor resistance rises with temperature, so a circuit running hot drops more voltage than the same circuit at 20 °C. The resistance values in wire tables are usually quoted at 20 °C.

Related

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#SourceTypeRevision / method
[1]ASME B1.1 — Unified Inch Screw ThreadsstandardASME B1.1-2019 — source
[2]ASTM A615 — Deformed steel bars for concrete reinforcementstandardASTM A615/A615M-20 — source
[3]ASTM E140 — Hardness Conversion TablesstandardASTM E140-12b — source
[4]Values computed in your browserderivedEvaluated locally from the formulas shown on the page. No data leaves the device.
[5]ISO 4287 — Surface texture: Profile methodstandardISO 4287:1997 — source
[6]ISO 68-1 — Basic profilestandardISO 68-1:2023 — source
[7]NFPA 70 NEC Table 310.16standardNEC 2023 (NFPA 70-2023) — source

Data Sources

StandardRevisionWhat it covers on this page
Formulas as shown on this pagen/a — standard engineering relationships, no single governing revisionevery value this calculator produces

Cross-checked against:

Derived values — the following values on this page are calculated, not taken directly from the standard:

ValueHow it is derived
All outputsComputed in the browser from the formulas above. No data leaves the device.

Outputs are computed from the formulas shown. Verify against the governing standard for design or acceptance work.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

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