MV Cable Size Calculator

Inputs

Formula used

• Three-phase current (from kW): I = P / (sqrt(3) x V x pf x eta)
• Three-phase current (from kVA): I = S / (sqrt(3) x V)
• Single-phase current (from kW): I = P / (V x pf x eta)
• Voltage drop (three-phase, approx.): dV = sqrt(3) x I x (R cos(phi) + X sin(phi)) x L
• Voltage drop percent: dV% = (dV / V) x 100
• Adjusted ampacity: I_adj = I_base x k_temp x k_group x k_install

How to use this calculator

1. Select phase, system voltage, and load type (kW or kVA).
2. Enter load value, power factor, and efficiency (for kW).
3. Enter one-way route length and your allowed voltage drop.
4. Choose conductor material, installation method, and ambient temperature.
5. Set grouping circuits, parallel runs, and a practical design margin.
6. Press Calculate to see the recommended size and drop estimate.
7. Use CSV/PDF export for reviews, approvals, and procurement notes.

Professional guidance for MV cable sizing

1) Start with verified load data

Gather the maximum demand, duty cycle, and likely growth. For motors, use rated kW, realistic efficiency, and a measured or specified power factor. For feeders, kVA is often easier because it already includes reactive demand. This calculator converts kW or kVA into current using standard AC relationships. For example, 1500 kW at 11 kV, pf 0.90, and efficiency 0.95 is about 92 A before margin.

2) Choose the correct system voltage and phase

Medium-voltage distribution commonly falls around 6.6 kV, 11 kV, or 33 kV in many projects. Small voltage changes shift current significantly, so always confirm nominal voltage and the operational tolerance band. Select single-phase only for dedicated single-phase MV applications.

3) Apply practical derating factors

Cable ampacity depends on heat removal. Higher ambient temperature, grouping of loaded circuits, and enclosed routes reduce cooling. The tool combines temperature, grouping, and installation factors into one total derating multiplier and applies it to a typical base ampacity. Use site-specific thermal resistivity, duct spacing, and depth when finalizing designs. In high-resistivity soil or crowded duct banks, derating can dominate, so conservative inputs help avoid late redesign.

4) Control voltage drop on long runs

Long MV routes can experience noticeable drop even when ampacity is adequate. The calculator estimates drop using typical resistance and reactance per kilometer, plus your power factor. Document the installed route length, not just map distance. Tight limits (for example, 2–3%) often push the selection to larger conductors or parallel runs, especially for high-current feeders.

5) Document assumptions for review

Record the input basis, derating selections, and the selected size for internal checks, client approvals, and procurement. Export the results as CSV for calculation packs, and use the PDF summary for quick sharing. Always confirm the final cable with manufacturer datasheets, termination limits, and the governing standard.

FAQs

1) Is the recommended size final for construction?

No. It is a screening result using typical catalog values and simplified derating. Final sizing should follow the applicable standard, manufacturer ampacity tables, installation details, and project review requirements.

2) When should I enter kW instead of kVA?

Use kW when you know real power and have reasonable power factor and efficiency values. Use kVA for feeders or transformers where apparent power is specified and efficiency is not part of the current calculation.

3) Why do temperature and grouping reduce ampacity?

Higher ambient temperature and grouped circuits make it harder for heat to escape. The conductor runs hotter for the same current, so allowable current must be reduced to keep insulation temperature within limits.

4) What does “parallel runs” mean in this tool?

Parallel runs split the design current across identical cables installed in the same way. The calculator divides current per run and then checks ampacity and voltage drop for each run’s conductor size.

5) How is voltage drop estimated for MV cables?

The tool uses an AC approximation with resistance and reactance per kilometer and your power factor. It estimates drop as a percentage of system voltage, which is useful for comparing sizes during early design.

6) Why might I pass ampacity but fail voltage drop?

On long routes, resistance and reactance cause voltage reduction even if the cable can carry the current thermally. Larger conductors, improved power factor, or additional parallel runs are typical solutions.

7) Can I rely on the included cable catalog values?

They are typical, illustrative values for comparison only. Different constructions, screens, conductor stranding, installation methods, and manufacturer data can change ampacity and impedance. Always validate with approved product data.

Example data table