Voltage Drop Calculator
Calculate voltage drop % and the minimum copper or aluminum wire size to stay within the NEC 3% / 5% recommendation, single- or three-phase. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Wire Size & Ampacity Calculator
Find the minimum copper or aluminum wire size for a load using NEC Table 310.16, with 310.15 ambient and conductor-count derating and the 110.14(C) terminal limit. Free, no sign-up.
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Size a single- or three-phase transformer from load in kVA, kW, or secondary amps. Get the required kVA, the next standard size, primary and secondary full-load amps, and the NEC 450.3(B) primary OCPD. Free, no sign-up.
Explanation
Voltage drop is the loss of voltage along a conductor due to its resistance. The NEC does not make a drop limit mandatory, but it recommends keeping a branch circuit within 3% and the combined feeder plus branch within 5% (informational notes to 210.19 and 215.2). Excessive drop dims lighting, overheats motors, and trips electronics, so on long runs the wire is often sized by voltage drop rather than ampacity.
How the calculation works
This calculator uses the single-resistance “K” method, the standard approach for branch and feeder circuits at unity power factor. It covers DC, single-phase AC, and three-phase AC — DC two-wire and single-phase two-wire share a length factor of 2, while three-phase uses √3. For DC the K method is exact, since there is no reactance.
| Symbol | Meaning |
|---|---|
| 2 / √3 | Phase factor — 2 for single-phase, √3 (≈1.732) for three-phase |
| K | Resistivity, Ω·cmil/ft at 75 °C — 12.9 copper, 21.2 aluminum |
| I | Load current (A) |
| L | One-way circuit length (ft) |
| CM | Conductor area in circular mils (NEC Chapter 9, Table 8) |
The conductor areas come from the same NEC data as the AWG wire size chart. Because long runs and ampacity pull the size in different directions, size the conductor both ways and use the larger result — start with the wire size & ampacity calculator.
Recommended limits and solar
The 3% branch / 5% total guideline covers most general wiring. Some applications are stricter: for photovoltaic systems, NEC Article 690 and common design practice target about 2% on the DC string and feeder so production is not lost in the wire. For long EV, RV, well-pump, and subpanel runs, voltage drop usually governs and a larger conductor than ampacity alone would require is needed.
The 5% guideline lines up with ANSI C84.1, which sets the acceptable utilization voltage at about −5% of nominal (Range A) — roughly 114 V on a 120 V system or 228 V on 240 V. Alongside the drop, this calculator also reports the longest one-way run a given wire size can reach within your target, so you can answer “how far can I run this wire” directly. Low-voltage DC control and telecom systems use their own minimums (about 20.4 V for a 24 V system per IEC 61131-2, 40.5 V for 48 V per ETSI EN 300 132-2).
Notes and limitations
The K method assumes roughly unity power factor and ignores conductor reactance, which is accurate for branch circuits and smaller feeders. For large conductors (about 250 kcmil and up) or low power-factor motor loads, the exact AC method using the impedance in NEC Chapter 9, Table 9 gives a higher, more accurate drop. This tool also does not check ampacity — confirm the conductor separately and verify the final design against the current NEC and the authority having jurisdiction.