Why voltage drop matters. Every conductor has resistance, and current flowing through it loses voltage along the run. Excessive drop causes dim lights, overheating conductors, poor motor starting, nuisance tripping, and wasted energy paid for on every bill.
NEC Recommended Limits
NEC 210.19(A) and 215.2(A) informational notes recommend a maximum of 3% on branch circuits and 5% combined for the feeder and branch circuit together. These are recommendations rather than mandatory rules, but they are near-universal design practice and some jurisdictions and specifications make them binding.
The Formula
This calculator uses the circular-mil (K-factor) method:
VD = 2 × K × I × L / CM (DC, single-phase)
VD = √3 × K × I × L / CM (three-phase)
where K = conductor resistivity (12.9 Ω·cmil/ft copper, 21.2 aluminum at 75°C), I = load current in amps, L = one-way run length in feet, and CM = conductor circular-mil area. Parallel conductors divide the effective resistance. For very large conductors and high power factors, reactance becomes significant — use NEC Chapter 9 Table 9 impedance values for precise feeder work.
Rules of Thumb
Doubling the run length doubles the drop. Doubling the current doubles the drop. Going up three AWG sizes roughly halves the resistance. Low-voltage systems (12/24/48V solar and battery circuits) are hit hardest — a 1V drop is 8.3% of a 12V system but only 0.4% of 240V, which is why battery cables are short and thick.
Ampacity Still Governs
Voltage drop sizing never overrides ampacity. A conductor must first carry the load safely per NEC 310.16 (this tool checks the 75°C column), then be upsized further if the run length demands it. Derate for ambient temperature above 30°C and for more than three current-carrying conductors in a raceway — common in Caribbean heat.
Branch Circuits vs Feeders
A branch circuit is the wiring between the final overcurrent device (the breaker or fuse) and the outlets, lights, or equipment it serves — for example, the run from a 20A breaker to the receptacles in a room, or from a breaker to an A/C condenser. It is the last leg of the system, which is why the NEC recommends holding it to 3% drop.
A feeder is the heavier wiring upstream of that: the conductors between the service equipment (or another source such as a generator or inverter) and a downstream distribution panel — for example, the run from a main panel to a sub-panel in an outbuilding. Because the feeder and branch circuit drops add together at the load, the NEC recommends the combined total stay within 5%. So if a feeder already drops 2.5%, the branch circuits it supplies should be sized for no more than about 2.5% themselves — use the Custom limit option for that.
Installation Factors That Affect Voltage Drop
The installation method doesn't change the formula directly, but it changes the conductor's operating temperature and (for AC) its reactance — and both affect the real-world drop:
Conduit & raceways. Conductors bundled in conduit run hotter, and resistance rises about 0.4% per °C for copper — a conductor at 75°C has roughly 20% more resistance than at 20°C (this tool's K values already assume 75°C, which is conservative for lightly loaded runs). The conduit material also matters on AC: steel (magnetic) conduit increases conductor reactance by roughly 15–20% compared with PVC or aluminum, which becomes noticeable on large conductors (4/0 and up) and low power factor loads. More than three current-carrying conductors in one raceway also forces an ampacity derate per NEC 310.15(C).
Direct burial. Underground conductors are cooled by the earth and typically run cooler than conduit in the sun, so the actual drop is often slightly better than calculated. However, burial depth, soil thermal resistivity, and grouping of ducts affect ampacity, and wet locations require suitable insulation (THWN-2, XHHW-2, or UF cable).
Free air & cable tray. Conductors in open air or ventilated tray shed heat best, run coolest, and therefore have the lowest resistance in service and the highest ampacity ratings (NEC 310.17 free-air values are considerably higher than the 310.16 raceway values this tool uses — another conservatism). In tray, maintain spacing per NEC 392; bundled tray fills lose that advantage. Widely spaced single conductors on AC do gain some reactance, but for typical spacings the temperature benefit dominates.
Other factors: high Caribbean ambient temperatures (derate above 30°C), sun-exposed rooftop conduit (NEC 310.15(B)(2) adder), load power factor (lower PF shifts more of the drop into reactance), harmonics from electronics loading the neutral, and loose or corroded terminations — a bad lug can drop more voltage than the entire cable run.
Disclaimer: This tool provides preliminary sizing estimates. Final conductor selection must account for termination ratings, derating factors, and local code amendments. Always have designs verified by a licensed electrical professional.