Medium Voltage Cable Calculator: What It Can Tell You

A medium voltage cable calculator can estimate electrical quantities such as voltage drop or conductor losses when you provide the circuit data. It cannot, by itself, select or certify a safe cable: that requires installation details, applicable standards, manufacturer data and review by a qualified electrical professional.

Formula

This calculator gives an approximate voltage drop and resistive conductor loss for a balanced, three-phase circuit. It uses line-to-line voltage and assumes the supplied resistance and reactance are per conductor per kilometre, at the relevant operating conditions.

For voltage drop, the formula is ΔV = √3 × I × L × (R cos φ + X sin φ), where ΔV is line-to-line drop in volts (V), I is line current in amperes (A), L is one-way route length in kilometres (km), R is conductor resistance in ohms per kilometre (Ω/km), X is conductor reactance in Ω/km, and φ is the load power-factor angle. Since cos φ = pf, sin φ = √(1 − pf²), the calculator uses those entered quantities directly. The percentage drop is 100 × ΔV / V, with V the nominal line-to-line voltage in volts.

The approximate three-phase resistive loss is P = 3 × I² × R × L, expressed in watts when the units above are used. The output converts watts to kilowatts (kW). This is conductor I²R loss only; it does not include dielectric loss, screen or armour losses, joints, or other equipment.

Worked Example

Using the block’s defaults: V = 11,000 V, I = 100 A, L = 500 m = 0.5 km, R = 0.2 Ω/km, X = 0.1 Ω/km and pf = 0.9.

• sin φ = √(1 − 0.9²) = √0.19 ≈ 0.43589.
• Voltage drop = √3 × 100 × 0.5 × (0.2 × 0.9 + 0.1 × 0.43589) ≈ 19.3 V.
• Percentage drop = 100 × 19.3 / 11,000 ≈ 0.18%.
• Resistive loss = 3 × 100² × 0.2 × 0.5 / 1,000 = 3.000 kW.

The calculator rounds these outputs to 19.3 V, 0.18%, and 3.000 kW. The example demonstrates the arithmetic, not a cable recommendation.

What a Medium Voltage Cable Calculator Does

“Medium voltage” commonly describes distribution voltages above low-voltage systems and below high-voltage transmission, but the precise boundaries vary by standard and jurisdiction. A calculator may be designed for one particular task: voltage drop, current capacity, short-circuit withstand, charging current, or another quantity. Those tasks need different inputs and formulas, so a generic result should never be mistaken for a complete cable design.

This page’s calculator addresses approximate voltage drop and conductor loss for a three-phase load. It is useful for a preliminary check or for understanding how length, load, and electrical characteristics affect the result. It does not calculate ampacity or determine whether insulation, screens, joints, terminations, or protective devices are suitable.

How to Choose Inputs

• Voltage: Enter nominal line-to-line system voltage, not phase-to-neutral voltage. For an 11 kV three-phase network, enter 11,000 V.
• Current: Use the expected line current for the operating condition being studied. If the current varies, evaluate relevant operating cases rather than assuming one value covers all conditions.
• Length: Enter the one-way route length in metres. The formula converts it to kilometres. Do not double the length for the outgoing and return paths: the √3 three-phase expression already represents the balanced three-phase relationship.
• Resistance and reactance: Obtain values for the actual conductor and cable arrangement from appropriate technical data. Confirm units, conductor material, temperature basis, frequency, and installation configuration. A catalogue value may not match operating resistance.
• Power factor: Enter a value from 0 to 1, such as 0.9. This is the magnitude of the load power factor; the formula assumes a lagging load for its reactance term. Leading power factor and unusual network conditions need a more specific model.

For US customary source data, convert before entering it: 1 mile is 1.609344 km, 1 foot is 0.3048 m, and 1 Ω/mile is approximately 0.621371 Ω/km. Keep each value’s unit attached during conversion; mixing ohms per mile with kilometres can produce a substantial error.

Interpreting Results and Important Limits

The voltage-drop output is an estimated difference between sending-end and receiving-end line voltage under the stated assumptions. The percentage is relative to the entered nominal voltage. A small calculated percentage does not prove a design is acceptable: allowable drop depends on system requirements, equipment, operating conditions, and governing rules.

The model assumes a balanced three-phase circuit with sinusoidal conditions and constant current, resistance, reactance, and power factor over the route. It is a simplified series-impedance calculation. It does not model voltage regulation, unbalanced loading, harmonics, transient events, parallel cable sharing, network source impedance, or detailed cable capacitance. For long cables or conditions where charging current matters, a more complete distributed-parameter or network study may be needed.

Resistance changes with conductor temperature, and AC resistance can differ from DC resistance because of frequency-dependent effects. Cable geometry, spacing, bonding, screens, armour, installation method, and nearby conductors can also affect impedance and losses. The calculator’s loss output excludes these additional effects, so it is not a total system-loss figure.

Common Mistakes and Design Checks

• Entering phase voltage in the line-voltage field for a three-phase calculation.
• Using total circuit length where the calculator expects one-way route length, or entering miles as kilometres.
• Using resistance data at the wrong temperature or in incompatible units.
• Treating the calculated voltage drop as proof of current-carrying capacity or short-circuit withstand.
• Relying on rounded inputs or outputs for a final engineering decision.

Actual cable selection must consider more than voltage drop: load and overload current, installation and ambient conditions, thermal resistivity of surrounding material, grouping, short-circuit duration, fault protection, earthing and bonding, insulation level, environmental exposure, and termination compatibility are among the relevant checks. Medium-voltage equipment can present severe shock and arc-flash hazards. Do not use this calculator to size or install energized equipment; have the design reviewed and work performed under appropriate professional procedures.

Frequently Asked Questions

Can a medium voltage cable calculator tell me what cable size to buy?
No. This calculator does not determine conductor size or certify a cable. Cable sizing requires ampacity, fault withstand, installation, insulation, protection, and applicable-standard checks using project-specific data.

Does cable length mean one-way length?
Yes. Enter the one-way route length in metres. The balanced three-phase voltage-drop formula already includes the phase relationship; do not double the length as if it were a single-phase outgoing-and-return circuit.

Why do I need both resistance and reactance?
Resistance contributes to the in-phase voltage drop and conductor heating; reactance contributes to the quadrature component of voltage drop. Their contributions depend on power factor, which is why the formula includes both.

Is the loss result the cable’s total energy loss?
No. It is an approximate instantaneous three-phase resistive conductor loss in kW at the entered current. It excludes other cable and system losses. Energy over time would also require a load profile and duration.

Can I use this for a single-phase cable?
Not directly. The voltage-drop formula shown is for balanced three-phase circuits. Single-phase calculations use a different circuit relationship and should account for the actual conductor path and system arrangement.

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in v | Line voltage | V | 11000 | 1 | 1000000
in i | Load current | A | 100 | 0
in l | One-way cable length | m | 500 | 0
in r | Conductor resistance | ohm/km | 0.2 | 0
in x | Conductor reactance | ohm/km | 0.1 | 0
in pf | Power factor | - | 0.9 | 0 | 1
out drop | Approximate voltage drop | V | sqrt(3) * i * (l / 1000) * (r * pf + x * sqrt(1 - pf ^ 2)) | 1
out pct | Voltage drop | % | 100 * drop / v | 2
out loss | Three-phase conductor loss | kW | 3 * i ^ 2 * r * (l / 1000) / 1000 | 3