Enter project inputs
Use this form to estimate a practical medium-voltage cable size from current demand, installation derating, voltage drop, and fault duty.
Formula used
1) Load current
For three-phase systems:
I = P / (√3 × V × pf × η)
If apparent power is entered, use I = S / (√3 × V). For direct current input, the entered current becomes the design current.
2) Required base ampacity
First apply design margin:
Imargin = I × (1 + margin%)
Then correct for derating:
Ibase required = Imargin / (ambient × grouping × soil × other)
3) Short-circuit conductor area
The thermal short-circuit check uses:
A = Isc × √t / k
Where k depends on conductor material and insulation. The calculator divides this requirement across equal parallel runs.
4) Voltage drop
Three-phase drop uses:
ΔV = √3 × I × (R cosφ + X sinφ) × L
The calculator converts voltage drop to percent of system voltage and compares it to your allowed limit.
How to use this calculator
- Choose the system type and your preferred load input method.
- Enter system voltage, power factor, efficiency, and one-way cable route length.
- Select installation method, conductor material, and insulation type.
- Enter all derating factors exactly as decimal multipliers.
- Add a practical design margin and define the maximum voltage drop limit.
- Enter the system short-circuit level and clearing time.
- Set the maximum parallel runs to evaluate, then submit.
- Review the recommended size, chart, and detailed evaluation table before final engineering approval.
Example data table
| Case | Voltage | Load basis | Length | Material | Installation | Derating product | Fault level |
|---|---|---|---|---|---|---|---|
| Pump feeder | 11 kV | 2,500 kW | 850 m | Copper | Duct | 0.846 | 25 kA for 1.0 s |
| Crusher motor | 6.6 kV | 1,500 kVA | 420 m | Aluminum | Buried | 0.910 | 20 kA for 0.5 s |
| Substation intertie | 13.8 kV | 180 A | 1,250 m | Copper | Air | 0.880 | 31.5 kA for 1.0 s |
FAQs
1) What does this MV cable size calculator estimate?
It estimates a practical cable size by checking load current, design margin, derating factors, voltage drop, and short-circuit thermal duty. It is useful for early design, tender comparisons, and construction planning.
2) Why are derating factors important?
Real cable installations rarely match ideal test conditions. Ambient temperature, grouping, soil thermal performance, and other installation constraints reduce usable ampacity, so ignoring them can lead to undersized feeders.
3) Why is short-circuit duty checked separately?
A cable can pass normal current limits yet still fail under fault heating. The short-circuit check ensures the conductor cross-section can withstand the expected fault current for the clearing time.
4) Can voltage drop control the final cable size?
Yes. Long feeders often require larger conductors even when ampacity is acceptable. That is common on MV projects with distant motors, remote pumps, and long buried routes.
5) When should I consider parallel runs?
Parallel runs become useful when one cable cannot satisfy ampacity, voltage drop, or installation practicality. They may also improve pulling conditions and cable availability on larger projects.
6) Is copper always better than aluminum?
Not always. Copper usually gives higher ampacity and lower resistance for the same area, but aluminum may reduce material cost. The better choice depends on project budget, termination design, and space.
7) Are the cable values exact manufacturer data?
No. The built-in values are typical reference values for pre-sizing and option comparison. Always verify the final selection against the chosen manufacturer’s catalogue and project installation standard.
8) Is this enough for final issue-for-construction design?
It is a strong starting point, but final design still needs coordination with system studies, cable arrangements, soil data, fault levels, shielding, accessories, standards, and utility or owner requirements.