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Cable sizing: current, voltage drop and coordination with the protection
Calculate the design current of a three-phase, single-phase or DC circuit, the cable voltage drop with resistance at operating temperature and reactance, the minimum section by the drop limit and the Ib ≤ In ≤ Iz coordination with the breaker.
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Where it helps at work
- Check whether the cable of a motor far from the MCC will deliver enough voltage at start and in operation.
- Assess a load increase on an existing circuit without changing the cable.
- Compare copper and aluminium, or parallel cables, for a long feeder.
- Document the voltage drop check of an installation change.
What you enter
What you get
Worked example: feeding a 15 kW motor
A 15 kW (shaft) three-phase motor, 380 V, power factor 0.85 and 90 % efficiency, is 50 m from the panel. The planned cable is copper, 6 mm², PVC insulation (conductor at 70 °C), with 0.08 mΩ/m reactance. The maximum drop allowed in the circuit is 4 %. Does the cable meet it?
| System | Three-phase |
|---|---|
| Line-to-line voltage | 380 V |
| Shaft power | 15 kW |
| Power factor | 0.85 |
| Efficiency | 90 % |
| Length (one way) | 50 m |
| Conductor | Copper · 6 mm² · PVC 70 °C |
| Drop limit | 4 % |
| Design current Ib | 29.79 A |
|---|---|
| Voltage drop | 7.649 V |
| Voltage drop (%) | 2.01 % |
| Minimum section by voltage drop | 4 mm² |
| Conductor losses | 457.7 W |
How to read it
The design current is 29.79 A. With 6 mm² the drop is 7.649 V, or 2.01 % of the voltage, below the 4 % limit. By voltage drop alone, the smallest section would be 4 mm², but that is not enough to choose the cable: the current-carrying capacity Iz for the installation method comes from the IEC 60364-5-52 (NBR 5410) table or the manufacturer. Entering the breaker In and that Iz, the tool checks Ib ≤ In ≤ Iz. Conductor losses add up to 457.7 W.
Limits and cautions
- It does not calculate the current-carrying capacity Iz: it depends on the installation method, grouping, ambient temperature and insulation, and comes from IEC 60364-5-52 (NBR 5410) or the manufacturer.
- Short circuit, shock protection, neutral and protective conductor sections, harmonics and motor starting drop are not included.
- The reactance is a typical value you enter; check it in the cable catalogue.
- It does not replace the electrical design signed by a qualified professional.
Technical basis
Design current and voltage drop with temperature-corrected resistance (IEC 60228) and reactance; coordination with the protection per IEC 60364 / NBR 5410.
- ABNT NBR 5410 — instalações elétricas de baixa tensão
- IEC 60364-5-52 — seleção e instalação de linhas elétricas
- IEC 60228 — condutores de cabos isolados
In the full version, you can
- Check three-phase, single-phase and DC circuits from power or current.
- Compare copper and aluminium, insulations and parallel cables.
- See the drop for each commercial section and check Ib ≤ In ≤ Iz with the Iz from the standard.
- Export an unbranded PDF report and keep the work on your device, with a backup file.
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Log in to my accountFrequently asked questions
Is the length out and back?
No. Enter only the one-way length. The factor k already accounts for the return: √3 in three-phase and 2 in single-phase and DC.
Why can the cable still be wrong if the drop passed?
Because voltage drop and current-carrying capacity are different checks. Short cables with high current are usually decided by Iz; long cables, by the drop. Both must be met.
Which drop limit should I use?
It depends on the circuit and the origin of the installation. IEC 60364-5-52 and NBR 5410 give limits for final circuits and from the supply point; use the value for your case.