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NEC 210/215
Validated against the standard.
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC 210/215 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What is voltage drop?
Every conductor has resistance, so some voltage is lost between the panel and the load. If that loss is too large, motors run hot, LEDs flicker, and heaters underperform. The NEC recommends keeping voltage drop to 3% on a branch circuit and 5% total for feeder plus branch (an informational note under 210.19 and 215.2, not a hard rule — but most inspectors and specs expect it). This calculator sizes conductors to hit that target.
How it is calculated
For a single-phase circuit the drop is:
Vd = 2 x L x R x I / 1000
where L is the one-way run in feet, I is the current in amps, and R is the conductor resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 (or Table 9 for AC impedance). Three-phase uses 1.732 instead of 2.
Worked example
A 16 A, 120 V circuit on #10 copper, 100 ft one-way.
#10 Cu resistance = 1.21 ohm/1,000 ft (Table 8). Vd = 2 x 100 x 1.21/1000 x 16 = 3.87 V.
3.87 / 120 = 3.2% — just over the 3% target, so the app recommends stepping up to #8 copper, which brings it under 3%.
The calculator supports copper and aluminum, single- and three-phase, and lets you solve for either the drop on a known wire or the smallest wire that meets your percentage limit.
Related calculators
Pair this with the wire size calculator for ampacity and derating, and the conduit fill calculator once the conductor count is set.
Voltage drop FAQ
How much voltage drop does the NEC allow?
The NEC recommends a maximum of 3% on a branch circuit and 5% combined for the feeder and branch circuit. These appear as informational notes in 210.19 and 215.2 rather than mandatory limits, but they are the standard most jobs are held to.
Does it handle copper and aluminum?
Yes. Select the conductor material and the app uses the correct resistance values from NEC Chapter 9, so aluminum runs are sized accurately, not just approximated from copper.
Single-phase and three-phase?
Both. The calculator applies a factor of 2 for single-phase and 1.732 for three-phase, and you can enter any standard voltage.
Why does voltage drop matter if it is only a recommendation?
Excessive drop causes poor equipment performance, nuisance tripping, and overheating, and many specifications and local amendments make the 3%/5% limits mandatory. Sizing for it up front avoids callbacks.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Voltage Drop on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.