Cable Check

What the check does, the formulas behind it, and what it deliberately leaves out.

1. What this check does

For a cable you have already chosen, RokBench computes the load current, compares the design current per set with the cable's allowable ampacity, and estimates the running and motor-starting voltage drop along the cable. It reports what does not hold up and why. It does not select a cable, although it may point to the smallest size in the same cable family that would pass.

2. Inputs used

Load as amperes, kW, HP or kVA; system voltage; single- or three-phase; power factor; efficiency (kW, HP).
Load class: motor, continuous or non-continuous.
Cable family and size, number of parallel sets, length, installation method and ampacity temperature column.
Optional: a combined ampacity correction factor, system frequency, motor starting method, starting current multiple and starting power factor, voltage drop limits.

3. Formulas and assumptions

Three-phase I = P / (√3 · V · PF · η) I = S / (√3 · V) Single-phase I = P / (V · PF · η) I = S / V HP P = HP × 745.7 W Design current per set Id = I × k / N k = 1.25 for motor and continuous loads, 1.0 otherwise; N = parallel sets Voltage drop ΔV = c · I · (L / 1000) · (R cos φ + X sin φ) / N c = √3 (three-phase, V line-to-line) or 2 (single-phase) R, X in Ω per 1000 ft per conductor; L in ft ΔV% = ΔV / V × 100 Starting same formula with I = multiple × I and φ from the starting PF

R, X and ampacity come from the cable library, which is built from manufacturer data sheets. The source is cited with every result. Rows still under source review are flagged.
R is the AC resistance at the temperature stated in the data (typically the conductor rating). No further temperature adjustment is made, which is conservative for lightly loaded cables.
X is scaled in proportion to frequency when the system frequency differs from the data.
The voltage drop formula is the common approximate form, accurate for the small angle between sending and receiving voltages found in feeder and branch circuits.
No correction for ambient temperature or grouping is applied unless you enter a correction factor.
If you do not set voltage drop limits, a default limit is applied and shown with the result. Set the limits your project criteria require.

4. What is NOT checked

Upstream voltage drop and source impedance. Starting drop is the cable drop only, not the voltage at the motor terminals.
Harmonics, phase imbalance, neutral current and cable heating from non-sinusoidal loads.
Short-circuit withstand, overcurrent protection and coordination.
Termination temperature limits beyond an informational note.
Conduit fill, tray fill, pulling tension and bending radius.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

NEC 2023 430.6(A)(1), 430.22 — motor full-load current and conductor sizing
NEC 2023 215.2(A)(1), 310.14, 310.15 — feeder sizing, ampacity and correction
NEC 2023 310.10(G) — conductors in parallel
NEC 2023 110.14(C) — termination temperature
NEC 2023 210.19(A) Informational Note No. 4, 215.2(A)(2) Informational Note No. 2 — voltage drop recommendations

6. Validation cases

Hand-calculated cases run on every change. Published textbook cases will be listed on the Validation page.

7. Known issues and changes

See the Changelog. Try the tool: Cable Check.