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.
Open calculatorLast 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.
Worked example
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.
AC voltage drop of a 200 ft run carrying 150 A, by conductor size
| Size | Resistance (Ω) | Reactance (Ω) | Impedance (Ω) | AC drop (V) | Drop at 480 V |
|---|---|---|---|---|---|
| 1/0 AWG | 0.0488 | 0.0192 | 0.0524 | 7.87 | 1.64 % |
| 2/0 AWG | 0.0387 | 0.0192 | 0.0432 | 6.48 | 1.35 % |
| 4/0 AWG | 0.0243 | 0.0192 | 0.0310 | 4.65 | 0.97 % |
| 250 kcmil | 0.0206 | 0.0192 | 0.0282 | 4.22 | 0.88 % |
| 500 kcmil | 0.0103 | 0.0192 | 0.0218 | 3.27 | 0.68 % |
Resistance and reactance are for the round trip. The percentage is relative to a 480 V supply.
Worked scenario: 1/0 AWG, 200 ft, 150 A
The round-trip resistance is 0.0488 Ω and the reactance 0.0192 Ω, giving an impedance of 0.0524 Ω and an AC drop of 7.87 V.
Why reactance matters for larger conductors
In small conductors resistance dominates the voltage drop. As conductors get larger, resistance falls but the reactance of the circuit does not fall at the same rate, so it makes up a bigger share of the drop and a resistance-only calculation understates it.
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.