Sources and scope for this page: Formulas as shown on this page (standard engineering relationships, no single governing revision), covering every value this calculator produces. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-08 · based on n/a — standard engineering relationships, no single governing revision
Quick Answer
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.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Values computed in your browser | derived | Evaluated locally from the formulas shown on the page. No data leaves the device. |
| [2] | NFPA 70 NEC Table 310.16 | reference | NEC 2023 (NFPA 70-2023) — source |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| Formulas as shown on this page | n/a — standard engineering relationships, no single governing revision | every value this calculator produces |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| All outputs | Computed 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.
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