Medium Voltage Feeder Calculator: What It Can Tell You

A medium voltage feeder calculator can estimate electrical quantities such as current, voltage drop, or cable losses from selected feeder data; it cannot, by itself, determine a safe cable or protection design. This page calculates the three-phase line current from real power, line-to-line voltage, and power factor, then explains which additional checks a real feeder design requires.

Formula

For a balanced three-phase load, line current is I = P / (√3 × V_LL × PF). In the calculator, the equivalent unit-adjusted expression is I (A) = P (kW) × 1000 / [√3 × V_LL (kV) × 1000 × PF] × 1000; this simplifies to P × 1000 / (√3 × V_LL × PF), with voltage entered in kV. P is real power in kilowatts, V_LL is line-to-line voltage in kilovolts, PF is the dimensionless power factor, and I is line current in amperes. Apparent power is S (kVA) = P (kW) / PF.

The calculator assumes a balanced three-phase system and a stated power factor. It does not infer power factor, account for harmonics, or establish a feeder’s allowable current. The input ranges require nonnegative power and voltage and a power factor from 0 through 1; zero voltage or zero power factor makes the current formula undefined, so those entries cannot produce a meaningful result.

Worked Example

Using the defaults—1,000 kW, 11 kV line-to-line, and power factor 0.90:

1. Convert real power to watts: 1,000 kW × 1,000 = 1,000,000 W.
2. Apply the three-phase relation: I = 1,000,000 / (√3 × 11,000 × 0.90).
3. Calculate the denominator: √3 × 11,000 × 0.90 ≈ 17,147.37.
4. Divide: I ≈ 58.32 A. Rounded to the calculator’s two decimal places, the line current is 58.32 A.
5. Apparent power is 1,000 / 0.90 = 1,111.11 kVA.

This example is a mathematical estimate at the stated operating point, not a recommendation for a conductor, transformer, switchgear, or protective-device rating.

What a Feeder Calculator Does

A feeder carries power from a source or substation to downstream loads. The phrase “medium voltage” is used for distribution systems between low and high voltage, but the exact boundaries vary by standard and region. Common system voltages include values such as 11 kV and 33 kV; always use the actual nominal or operating voltage specified for the project.

Different calculators answer different questions. A current calculator needs load and voltage assumptions. A voltage-drop calculator additionally needs conductor material, size, length, resistance and reactance, plus the load’s power factor. A loss calculator estimates energy dissipated in the conductor, often using current and resistance. A short-circuit or protection study requires still more system and equipment data. No single current formula replaces those analyses.

When to Use the Current Estimate

Use this calculation for a first-pass check of the approximate current associated with a known, balanced three-phase real-power load. It can help compare operating scenarios—for example, how current changes when load rises or when power factor improves—and provide a useful input to a more complete engineering study.

For fixed real power and voltage, a lower power factor means higher current. In the example, reducing PF from 0.90 to 0.80 while keeping 1,000 kW and 11 kV gives about 65.61 A. That increase matters because current affects conductor losses and equipment loading. It does not mean that correcting power factor alone proves a feeder is adequate; the equipment and network must be assessed as a system.

What a Complete Feeder Assessment Needs

Actual design depends on more than the load-current equation. A qualified electrical professional may need to evaluate:

• Load profile: maximum demand, diversity, operating cycles, motor starting, future expansion, and whether the load is continuous.
• Conductor data and route: conductor material and cross-section, circuit length, installation arrangement, joints, and relevant resistance and reactance.
• Thermal conditions: ambient temperature, soil conditions for buried cables, grouping, ventilation, and the manufacturer’s cable data.
• Voltage performance: permissible operating voltage range and voltage drop under normal and starting conditions.
• Fault and protection performance: available short-circuit current, protection coordination, interruption capability, grounding or earthing arrangement, and clearing times.
• System details: network configuration, transformer impedance, source strength, and whether the load is balanced or contains significant nonlinear equipment.

These checks draw on project specifications, applicable standards, utility requirements, and verified equipment information. The arithmetic here is not a substitute for any required study or review.

Common Errors and Limits

• Entering phase voltage instead of line voltage: This formula expects line-to-line voltage for a three-phase system. For an 11 kV system, enter 11 kV, not the phase-to-neutral value.
• Mixing units: Enter power in kW and voltage in kV as labeled. Substituting watts or volts without adjusting the formula changes the answer by factors of 1,000.
• Confusing kW and kVA: kW measures real power; kVA is apparent power. They are equal only at unity power factor under the stated assumptions.
• Treating nominal voltage as exact: Actual voltage can vary. The computed current changes with voltage, so use a voltage appropriate to the question being assessed.
• Assuming a balanced sinusoidal load: Unbalanced phases and harmonic currents may require phase-by-phase or more detailed analysis.

Rounding is for display; engineering calculations should retain suitable precision through intermediate steps. The simple equation estimates operating current only. It does not calculate voltage drop, cable ampacity, losses, fault current, or protection settings.

Frequently Asked Questions

How do you calculate medium voltage feeder current?
For a balanced three-phase load, divide real power by √3 times line-to-line voltage times power factor. Keep units consistent; this calculator accepts kW and kV and returns amperes.

Does the result tell me what cable size to use?
No. Cable selection requires thermal, installation, voltage-drop, short-circuit, protection, and applicable code or utility checks. Have a qualified professional assess the actual installation.

Why does lower power factor increase feeder current?
At the same real power and voltage, lower power factor means more apparent power is needed. The three-phase current therefore rises according to the formula.

Can I use this for a single-phase feeder?
No. The √3 relationship shown is for a balanced three-phase system. A single-phase calculation uses a different relationship and must match the circuit’s voltage and load assumptions.

Can this calculator estimate voltage drop?
No. Voltage drop depends on feeder length and conductor impedance as well as current and power factor. Use a method with the correct cable and installation data, and verify it against project requirements.

What if the load is unbalanced or has harmonics?
The estimate may not represent the actual phase currents. Use suitable measurements or a detailed analysis that accounts for the load characteristics and system configuration.

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in p | Real power | kW | 1000 | 0
in v | Line-to-line voltage | kV | 11 | 0
in pf | Power factor | - | 0.9 | 0 | 1
out i | Three-phase line current | A | p * 1000 / (sqrt(3) * v * 1000 * pf) * 1000 | 2
out s | Apparent power | kVA | p / pf | 2