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.

Page last changed 2026-10-19 · 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

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#SourceTypeRevision / method
[1]Values computed in your browserderivedEvaluated locally from the formulas shown on the page. No data leaves the device.
[2]NFPA 70 NEC Table 310.16referenceNEC 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

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