Cable Reactance Calculator
For large conductors where reactance matters: enter size and length to get R, X, Z and the AC voltage drop — the refinement over DC-only drop math.
Last updated: 2026-09-15
How the calculation works
- Resistance comes from the NEC Chapter 9 Table 8 series; reactance uses the Table 9 approximate value for large conductors in non-magnetic conduit.
- Impedance combines both vectorially; voltage drop is current × impedance.
- For conductors below 1/0 AWG, resistance dominates and the plain voltage-drop calculator is sufficient.
Formula
Z = √(R² + X²) Vd = I × Z X ≈ 0.048 Ω/1000 ft (non-magnetic conduit, 60 Hz)
| Symbol | Meaning | Unit |
|---|---|---|
R | Conductor resistance | Ω |
X | Inductive reactance | Ω |
Z | Impedance | Ω |
Worked example
4/0 AWG, 200 ft, 150 A single-phase: R = 0.0243 Ω, X = 0.0192 Ω, Z = 0.0311 Ω → drop = 4.66 V. The DC-only calc would give 3.65 V — reactance adds 28% here.
Interpreting the result
As conductors get larger, their reactance stops being negligible: at 4/0 and above, X can rival R. That's why long large-conductor runs need the AC method, and why conductor spacing in conduit affects drop. Steel conduit adds reactance versus PVC or aluminum.
Assumptions
- 60 Hz, single-phase round-trip model, non-magnetic conduit.
- Uniform conductor spacing (typical random lay).
Limitations
- Three-phase systems divide the reactance path differently.
- Steel conduit raises X by 20–40%.
- Very large busway or parallel sets need engineering software.
Frequently asked questions
When does reactance matter for voltage drop?
Roughly from 1/0 AWG upward in long runs. Below that, resistance dominates and DC math is within a few percent.
Does conduit material affect voltage drop?
Yes — steel conduit adds inductive reactance (magnetic circuit around the conductors), increasing drop on large conductors. PVC and aluminum don't.