What is the formula for voltage drop?
Single-phase: VD = 2 × Zeff × I × D ÷ 1000. Three-phase: VD = 1.732 × Zeff × I × D ÷ 1000. DC uses the single-phase form with resistance only. Zeff is the effective impedance per NEC Chapter 9 Table 9 (Note 2): Zeff = R × PF + X × √(1 − PF²), with R and X from Table 9 (XLPE 75°C basis, uncoated) and D the one-way run in feet.
Why does reactance matter for large conductors?
Above roughly 250 kcmil in steel conduit, inductive reactance is as large as or larger than resistance at typical power factors — for 500 kcmil copper, X ≈ 0.048 Ω/kft versus R ≈ 0.029. Resistance-only (K-factor) calculators understate the drop precisely in the feeder range behind transformers. This calculator uses the Chapter 9 Table 9 effective-Z method with a power factor input and a conduit material selection.
What voltage drop does the NEC allow?
The NEC states performance intent in Informational Notes, not enforceable limits. Common engineering practice: 3% maximum for a branch circuit and 5% total for feeder plus branch. The calculator checks both and labels each result against those references.
How does the temperature rating change voltage drop?
Conductor resistance rises with operating temperature — the field here is the conductor temperature, not the insulation's rating. Table 9's R is a 75°C value; it is corrected to 60°C or 90°C conductor temperature by (T + 234.5) ÷ 309.5 for copper and (T + 228.1) ÷ 303.1 for aluminum. Reactance is set by geometry and conduit material, so it barely changes with temperature. Pick the temperature that matches actual operation; when in doubt, 75°C is the conservative default.
What is the minimum conductor size for a run?
The smallest AWG/kcmil size that both carries the load current and keeps voltage drop within your limit — ampacity per NEC 310.16 at 75°C (termination basis, 110.14(C)) with 240.4(D) small-conductor caps (14 to 15A, 12 to 20A, 10 to 30A). Choose ‘Minimum conductor size’ above, set the limit (3% or 5% is typical), and the tool walks single conductors up to 600 kcmil; if no single size holds both, it recommends the smallest parallel combination of 250–600 kcmil conductors (2–6 sets, NEC 310.10(G)) that does.
How far can I run a given conductor?
Maximum one-way distance = limit × voltage × 1000 ÷ (phase factor × Zeff × current), with the phase factor 2 for single-phase/DC and 1.732 for three-phase, and Zeff = R × PF + X × sin(acos PF) per Table 9 Note 2. Choose ‘Maximum circuit distance’ and the tool solves it directly. Ampacity is a separate check — run the feeder sizing calculator too.
