Third-party sites, not ours. Check results against the NEC edition your jurisdiction enforces (NFPA maps) and your engineer or inspector.
- SparkRef: best all-in-one. Voltage drop with NEC 3%/5% guidance flagged, plus ampacity, conduit fill and box fill.
- OrbitalJump voltage drop calculator: a second opinion for voltage drop.
- SparkShift voltage drop: phone-friendly alternative.
Links checked 2026-10-16. Know a better free one? Tell us.