How to Choose a Voltage Drop Calculator
Last updated 2026-11-05
The right voltage drop tool depends on three things: where you are working, which wire data you need and whether you must keep a record. Pick the kind that fits all three, then check the result against the code in force where you work. No calculator replaces the code, a licensed electrician or the authority having jurisdiction (AHJ).
The five kinds of voltage drop tool
| Kind | Works with no signal | Wire data it uses | Shows the steps | Keeps a record | Suits |
|---|---|---|---|---|---|
| Browser calculator | Usually no | Usually built-in resistance values | Varies | Usually copy or print only | Quick checks at a desk or on a laptop |
| Manufacturer web calculator | Usually no | Usually that maker’s cable data | Varies | Varies | Work with that maker’s cable |
| Phone app | Usually yes | Usually built-in values | Varies | Varies | Quick checks on site |
| Spreadsheet | Yes, once the file is on the device | Whatever you enter | Yes, if the formulas are visible | Yes, as a saved file | Repeated jobs with your own formulas |
| Code-book tables by hand | Yes, on paper | NEC Chapter 9 Tables 8 and 9 | You work each step yourself | Only if you write it down | Checking a result against published values |
What to check before you trust a result
- Length: confirm whether the tool wants the one-way length or the round trip. The wrong one doubles or halves the answer.
- Supply: match the mode to the circuit. Single phase, three phase and DC use different factors.
- Metal: use copper or aluminum as installed. Aluminum has higher resistance at the same size.
- Conductor temperature: resistance rises as the wire warms, so check which temperature the tool assumes.
- Reactance: check whether the tool includes it. If it does not, results on larger AC conductors can read low.
- Ampacity: a voltage drop result does not show that the wire can carry the load. Check whether the tool tests ampacity as well.
Where BuildWire Calc fits
BuildWire Calc has a free voltage drop calculator that runs in your browser. It has three modes: the drop for a wire size, the smallest wire for a target drop, and the longest one-way run for a chosen size. It covers single phase AC, three phase AC and DC, copper or aluminum, and AWG / kcmil or mm² sizes. Resistance is calculated at the conductor temperature you enter, 75 °C by default, and reactance is added only if you type a value. It shows every step and warns past 3 % and 5 %. The smallest-wire mode also checks NEC 310.16 ampacity for AWG sizes at 30 °C with up to 3 conductors. You can copy the result, copy a link or print it, but the website does not save jobs, and it needs a connection the first time you open it.
The BuildWire Calc app is on the App Store for iPhone and iPad and on Google Play for Android. Voltage drop for single phase, three phase and DC is one of its nine free calculators, out of 36 in all. Every calculator works with no signal, and there is no account and there are no ads. Results update as you type, with the steps and warnings underneath. You can save a result to a job for a customer, quote or site, and copy the job as text. Pro unlocks all 36 calculators and PDF and CSV export. Jobs and history stay on your device. Results are estimates for planning.
When BuildWire Calc is the wrong choice
- You need a stamped engineering calculation or a whole-system study with short-circuit or harmonics analysis. BuildWire Calc does not do that.
- You need a specific cable’s published impedance. Use that maker’s data. You can type its reactance into the browser calculator.
- You work under a non-US code that sets its own method and tables. The ampacity check covers the NEC only.
- You need a metric cable current rating. mm² sizes are checked for drop only.
- You need the code text itself. Copyrighted code tables are not included.
Questions, briefly answered
Which voltage drop calculator should an electrician use?
Use the kind that fits where you are working, which wire data you need and whether you must keep a record. A phone app or a browser calculator is usually enough for a quick check on site. Use saved jobs or a spreadsheet when you must keep a record.
Is voltage drop limited by the NEC?
The informational notes to NEC 210.19(A) and 215.2(A) suggest no more than 3 % on one branch circuit or feeder, and 5 % in total from the service to the farthest outlet. These are recommendations, not enforceable limits.
Is a phone app accurate enough for voltage drop?
It can be, if its wire data and formula are right and your inputs are correct. For a run that matters, check the figures against NEC Chapter 9 Tables 8 and 9 in your own copy of the code.
Does a voltage drop calculator replace the code or an electrician?
No. A voltage drop calculator does not replace the code, a licensed electrician or the authority having jurisdiction (AHJ). Check every result against the code in force where you work.