Transformer Loading calculator
What the load costs upstream of the cable, in losses and heat.
Power & Utilities · Professional Microtool
Voltage Drop Calculator
Volt drop · Percentage · Longest run
Volt drop on a three-phase run, with the reactive term included: drop in volts and per cent, the loss it costs, and the longest run that stays inside your permitted drop.
Start from an example: 200 A over 150 m of 95 mm² copper at 0.9 power factor →
Full output
- —Drop in volts
- —Voltage at the load
- —Longest run within the limit
- —Conductor loss
- —Resistive share of the drop
- —Drop per metre
Voltage drop against power factor at constant current
At a fixed current the drop is worst not at the poorest power factor but near cos φ = R/√(R²+X²), because the resistive and reactive terms trade against each other. In a real installation a poorer power factor also raises the current for the same real power, which pushes the drop up again — this curve isolates the geometry, not the whole effect.
Keep it
Both carry the figures you entered, in the part of the address that is never sent to a server. Share only where that is appropriate. To keep a copy for a project file, print the page — it lays itself out as a document.
Need the whole cable schedule, not one run?
We can build the calculation across every circuit, with real conductor data at operating temperature, and keep it in step with the design as it changes.
Calculation basis 5 steps · view the method →
Effective R and X = per-km values ÷ conductors per phase—Drop = √3 × current × length × (R·cos φ + X·sin φ)—Percentage drop = drop ÷ nominal line voltage—Longest run = permitted drop × voltage ÷ drop per metre—Conductor loss = 3 × current² × effective resistance × length—
Assumptions & limitations Screening estimate for cable runs and preliminary selection · 8 assumptions →
This is a screening calculation for a balanced three-phase run at steady load. It excludes harmonics, motor starting, transient and fault conditions, conductor temperature correction, grouping and installation derating, and any local wiring rule about permitted drop. Use the applicable standard, the manufacturer's cable data at operating temperature, and a proper cable schedule before issuing a design.
- Balanced three-phase load at steady state; the neutral carries nothing.
- The power factor is assumed lagging; with a leading power factor the reactive term subtracts instead of adding, and the drop is smaller than shown.
- Resistance is taken as entered — at operating temperature it is typically 15–25% above the 20 °C value.
- Harmonic currents, motor starting and fault conditions are excluded.
- Grouping, ambient and installation-method derating are not applied.
- The load is treated as a single point at the end of the run, not distributed along it.
- Skin and proximity effects are assumed to be already inside the quoted AC resistance.
- The percentage is taken against nominal voltage rather than the actual sending-end voltage.
Worked examples
- 200 A over 150 m of 95 mm² copper at 0.9 power factor
Open with these figures → - The same run with two cables per phase
Open with these figures → - Unity power factor, where only resistance counts
Open with these figures →
About this tool
Volt drop is usually taught as resistance times current, which works until the power factor is poor and the reactance starts to dominate. This tool keeps both terms, so it shows why a larger conductor sometimes buys much less than expected — and reports the practical answer directly: the longest run that stays inside the limit. Written for electrical designers, plant engineers and anyone checking a cable schedule.
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