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Voltage Drop & Cable Size Calculation

Running and motor-starting voltage drop including upstream feeders, cable rating and breaker check, power loss and its yearly cost, minimum cable size and maximum run length — for DC, single-phase and three-phase circuits in copper or aluminium.

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Drop in this cable
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Total drop to load
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Voltage at load
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Overall check
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1Circuit
Supply system
V
 
2Load
 
%
From the motor nameplate or datasheet
3Motor starting
Values below are typical — overwrite with motor or starter data
× FLC
%
Typical guideline 10–15 % — check motor & contactor data
4Acceptance limit
%
%
Main + sub-main feeders feeding this circuit's panel
5Cable
Conductor
mm²
Actual cable route incl. risers & panel entry
°C
6Conductor data
Ω/km
 
Ω/km
 
7Cable rating & protection — optional
A
For your laying method, from the cable datasheet
Grouping × ambient × soil × depth
A
8Running cost of cable losses
h/yr
₹/kWh

Checks

Results

All standard sizes

Click a size to use it for this circuit.

Circuit schedule — all circuits in project

Preliminary engineering reference. Confirm the final cable selection against the cable manufacturer's data, IS 732 / IEC 60364 and the project specification.

Guide

How to check a cable for voltage drop

  1. Describe the circuit. Pick DC, 1-phase or 3-phase, the supply voltage and the load in A, kW, kVA or HP. For a motor, choose the starting method.
  2. Set the limit and the upstream drop. Choose the limit that applies — the IEC 60364-5-52 values or your project specification — and enter the drop already used by the main and sub-main feeders.
  3. Describe the cable. Copper or aluminium, size, real route length, parallel runs and insulation. Add the cable rating, derating factor and breaker size to check heating and protection too.
  4. Read the result. The strip shows the drop, the voltage at the load and an overall pass or fail. The Results tab lists every standard size, so you can see the smallest cable that works and what each size costs in losses.

Typical voltage drop limits

Installation suppliedLightingOther uses
From a public low-voltage network3 %5 %
From a private supply / own transformer6 %8 %
Motor terminals during starting guideline10–15 %

Running limits from IEC 60364-5-52 Annex G (informative), measured from the origin of the installation to the load. A project specification, if stricter, takes priority.

The formula used

ΔV = k · I · L · (RT cos φ + X sin φ) / n

  • k = √3 for three-phase (line-to-line), 2 for single-phase and DC
  • I = load current, A — L = one-way route length, km
  • RT = R20 × [1 + α (T − 20)] — conductor resistance at its operating temperature, Ω/km
  • X = cable reactance, Ω/km (zero on DC) — n = parallel runs
  • ΔV % = ΔV / V × 100 — loss = m · I² · RT · L / n

What people often forget

  • Upstream drop. The limit applies from the supply origin to the load. A final circuit fed from a sub-panel already carries the drop of the main and sub-main feeders — enter it, or a circuit can pass on its own and fail overall.
  • Motor starting. A DOL motor draws around six times its running current at a low power factor. A cable that is fine while running can drop enough at start to stall the motor or drop out contactors.
  • Hot conductor resistance. Copper at 90 °C has roughly 27 % more resistance than at 20 °C. Using the 20 °C value understates the drop.
  • Real route length. Use the actual cable route — vertical risers, tray detours and the length inside panels — not the straight-line distance on the layout.
  • Cable rating and derating. A cable that meets the voltage drop limit can still overheat. Grouping, ambient temperature and burial depth reduce its rating.
  • Unequal parallel runs. Parallel cables only share current equally when they are the same size, length and laying. Otherwise one cable carries more than its share.
  • Losses cost money. On long, heavily loaded feeders, one size up often pays for itself in energy saved. Compare the yearly loss cost in the size table.
  • Large conductors on AC. From about 240 mm² upward, AC resistance and reactance matter more — use the cable manufacturer's values.

Voltage drop — common questions

What is an acceptable voltage drop?
IEC 60364-5-52 Annex G (informative) suggests 3 % for lighting and 5 % for other uses when the installation is fed from a public LV network, and 6 % / 8 % when fed from the consumer's own transformer. These apply from the origin of the installation to the load. Many project specifications set tighter values — always use the project limit when one exists.
Why add the upstream drop?
The limit is for the whole path from the supply to the equipment. If the main feeder and sub-main already use 2 %, a final circuit on a 5 % limit has only 3 % left.
How much voltage drop is allowed during motor starting?
A common engineering guideline is 10–15 % at the motor terminals during starting, but the real limit depends on the motor's starting torque, the driven load and the contactor drop-out voltage. The upstream feeders also carry the starting current, so on long or weak supplies check the starting drop of the whole path. Check the motor and starter data, and the project specification.
Is the voltage for three-phase line-to-line or line-to-neutral?
Enter line-to-line voltage (e.g. 415 V) for three-phase and phase-to-neutral voltage (e.g. 230 V) for single-phase. The percentage drop is the same either way.
Is the motor kW the input or the output?
Motor nameplates give the rated shaft output. For a motor in kW or HP the calculator divides by the efficiency to get the electrical input, then by voltage and power factor to get the current. For general loads, kW is taken as the electrical input.
Does a small voltage drop mean the cable is the right size?
Not on its own. The cable must also carry the load current after derating, withstand the prospective short-circuit current and allow the protective device to disconnect an earth fault in time. Enter the cable rating and breaker size to check the first two items here; short-circuit and earth fault checks need a separate calculation.
Copper or aluminium?
Aluminium has about 1.6 times the resistance of copper of the same size, so it needs roughly one or two sizes larger for the same voltage drop. It is lighter and cheaper, which makes it common for large feeders; copper is preferred for smaller circuits, motors and where space in the panel is tight.
Where do the resistance values come from?
The built-in table holds the maximum DC resistance at 20 °C for Class 2 stranded plain copper and aluminium conductors per IEC 60228 (IS 8130 in India). Switch to "Manufacturer value" to use the datasheet value of your actual cable.

About this calculator

A free voltage drop calculator for electrical engineers, panel builders and contractors

Why We Included a Voltage Drop Calculator on the VERCHO Website

At VERCHO Engineering Solutions, our objective is not limited to supplying electrical and mechanical products. We also aim to provide practical engineering resources that are useful to electrical engineers, consultants, panel builders, contractors and maintenance teams.

Every feeder that leaves a panel has to answer one question before the cable is ordered: will the equipment at the far end still get enough voltage? Cables have resistance and, on AC circuits, reactance. As current flows along the run, part of the supply voltage is lost in the cable itself — so a motor 120 m from the MCC never sees the full 415 V the transformer delivers.

Our Voltage Drop Calculation Tool works out that loss for DC, single-phase and three-phase circuits, checks it against the limit, and shows the smallest standard cable that works — and what each size costs you in wasted energy every year.

Why Voltage Drop Matters

A little voltage drop is unavoidable. Too much causes problems that are often blamed on the equipment instead of the cable:

  • Motors — an induction motor’s torque falls roughly with the square of the voltage. Low voltage means more current, more heat and a shorter winding life, and at start-up it can mean a motor that stalls or will not accelerate its load.
  • Contactors and relays can chatter or drop out when the voltage dips during a large motor start.
  • Lighting dims and flickers; drives, PLCs and electronics trip on undervoltage.
  • Energy — every volt lost in the cable is power turned into heat, paid for on every bill for the life of the installation.

How Is This Relevant to VERCHO?

Every one of these feeders starts and ends in an enclosure. VERCHO designs and manufactures the electrical enclosures, control panels, junction boxes, gland plates, panel locks, hinges and fasteners that the cables run into. The cable size decides the gland size, the gland plate cut-outs, the terminal space and the bending room inside the panel — so getting the cable right early saves rework on the enclosure later. For panel builders, the circuit schedule from this tool is a quick check of every outgoing feeder before the panel layout is frozen.

A Worked Example

A 37 kW motor (efficiency 92 %, power factor 0.85) is started direct-on-line from an MCC at 415 V, 50 Hz. The route is 120 m of 70 mm² copper XLPE cable. The calculator gives:

ResultValue
Full-load current65.8 A
Voltage drop in the cable4.55 V · 1.10 %
Voltage at the motor410.4 V
Starting current (6 × FLC) and starting drop395 A · 3.54 %
Cable power loss533 W
Cost of losses (6,000 h/yr at ₹ 8/kWh)₹ 25,586 per year
Maximum length for a 5 % limit547 m

Now suppose the MCC is itself fed through a main and sub-main feeder that already use 2 % of the limit. A 16 mm² cable, which passes on its own at 4.26 %, now fails at 6.26 %. 25 mm² just passes at 4.75 %, with almost no margin left. That is why the calculator asks for the upstream drop.

The Smallest Cable Is Not Always the Cheapest

For the same motor, here is what each copper size costs in losses at 6,000 operating hours a year:

SizeTotal dropStarting dropLossLoss cost / year
16 mm²4.26 %10.31 %2,287 W₹ 1,09,789
25 mm²2.75 %7.10 %1,446 W₹ 69,406
35 mm²2.02 %5.57 %1,042 W₹ 50,026
50 mm²1.53 %4.53 %770 W₹ 36,946
70 mm²1.10 %3.54 %533 W₹ 25,586
95 mm²0.83 %2.97 %384 W₹ 18,425

16 mm² meets the drop limit, but wastes over ₹ 80,000 a year more than 70 mm² on this feeder. On long, heavily loaded circuits, one or two sizes up often pays for itself in a short time. Losses here are worked out at the same conductor temperature for every size; a larger cable actually runs cooler, so the real saving is slightly higher. The size table in the calculator shows this comparison for every standard size — click a size to use it.

What the Voltage Drop Calculator Works Out

  • Load current from A, kW, kVA or motor HP — for motors, kW and HP are treated as rated shaft output and divided by the efficiency.
  • Voltage drop in the cable, the total drop from the supply origin including upstream feeders, and the voltage at the load.
  • Motor starting drop for direct-on-line, star-delta, soft starter and VFD starting, with typical starting current and power factor that you can overwrite.
  • Cable rating check — the derated current rating against the load current, and the breaker or fuse rating between the two.
  • Cable power loss, energy lost per year and its cost at your tariff.
  • Minimum standard cable size and maximum run length for the limit.
  • A circuit schedule for a whole panel or project — save it as a project file, and download it as PDF or Excel.

Why Have We Included This Tool?

VERCHO is not only a supplier of industrial products; we aim to support engineers and project teams with practical engineering resources. This calculator joins our pump head, pipe size, tank volume and bolt torque tools, and our guides to switchgear panel construction and IP ratings.

The calculator is intended as a preliminary engineering reference and should not be considered a substitute for detailed design, cable manufacturer data, IS 732 / IEC 60364 requirements or project-specific engineering calculations.

Have a question about cable sizing or an enclosure for your feeders? Use the Ask a question button or write to info@verchogroup.com.

VERCHO — BUILT BEYOND.