Voltage Drop Calculator
Find the conductor size that keeps voltage drop under your target for a given load, length and system voltage. Uses the K-factor method with Chapter 9 Table 8 circular mils, for single-phase and three-phase circuits, copper or aluminum.
Sized for voltage drop only. Check ampacity separately in the wire size calculator · 310.16. Circular mils come from NEC Ch. 9 Table 8; the K constant is the industry value derived from that table’s dc resistance column, not a number the NEC prints.
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The JobVolt app runs the same verified engines offline, keeps every calculation in History, and turns them into inspector-ready PDF submittals on your letterhead.
How voltage drop is calculated
The calculator uses VD = (2 × K × I × L) / cmil for single phase, replacing 2 with √3 for three phase. K is the industry constant of 12.9 for copper and 21.2 for aluminum at 75°C. The NEC does not print K; it is derived from the dc resistance column of NEC Chapter 9 Table 8, which is also where the conductor circular-mil areas come from. The smallest conductor whose drop lands under your target is the answer, and the full arithmetic is shown so you can check it.
What target should I use?
3% on a branch circuit is the number the trade works to, and the NEC carries it as an informational note to 210.19. It is a recommendation, not a requirement, but inspectors and spec sheets treat it as the baseline. Energy codes in some states make it mandatory. Size for ampacity first, then check the drop.
Prefer the raw numbers? The Chapter 9 Table 8 circular mils is on the site too, rendered from the same verified dataset this calculator reads.