Every Number on a Motor Nameplate Explained — And Why They All Affect Your Cable Size

The kW rating is just the starting point. Getting the cable wrong costs you a motor — and sometimes the cable too.

I have watched engineers size a motor cable by taking the kW rating, calculating the current at unity power factor, picking the next cable up from a table, and calling it done. That approach ignores at least four nameplate parameters that directly affect the cable current, and at least two more that affect the protection settings. The motor nameplate is a dense data sheet stamped into metal, and every number on it is there for a reason.

This article walks through each nameplate parameter, explains what it means physically, and shows how it connects to the cable and protection decisions that follow.

The headline numbers: rated power (kW or HP), rated voltage (e.g., 415 V, 400 V, 380 V, 460 V), and full load current (FLC or FLA). These seem straightforward but contain subtleties.

The rated power is the mechanical output at the shaft, not the electrical input. The electrical input power is higher by the reciprocal of the efficiency. A 75 kW motor with 94% efficiency draws approximately 79.8 kW of electrical power. At 415 V three-phase with a power factor of 0.86, that is:

I = 79,800 / (√3 × 415 × 0.86) = 129 A

Not 75,000 / (√3 × 415) = 104 A, which is the number you get if you ignore efficiency and power factor. That 25 A difference is the difference between a correctly sized cable and one that runs hot.

The nameplate full load current is the most reliable figure — use it directly rather than back-calculating from kW. But understand that the FLC assumes rated voltage, rated frequency, and continuous duty at rated load. Change any of those conditions and the actual current changes.

The service factor (SF) is common on NEMA-rated motors (North American) but less common on IEC motors. It indicates the continuous overload capability of the motor beyond its rated power.

A motor with SF 1.15 can continuously deliver 115% of its rated power without exceeding its thermal limits, provided the voltage and frequency are at rated values. A 75 kW motor with SF 1.15 can continuously deliver 86.25 kW.

The cable sizing implications are direct: if the motor is expected to operate at service factor, the cable must be sized for the service factor current, not the nameplate FLC. The service factor current is approximately SF × FLC.

IEC motors typically have SF 1.0 — the rated power is the maximum continuous power. There is no built-in overload margin. Any continuous overload beyond rated power exceeds the thermal design. This is a fundamental difference between NEMA and IEC motor philosophy, and it matters when an NEMA motor is replaced with an IEC motor (or vice versa) on an existing installation.

The nameplate shows both the full-load power factor and the efficiency. Both affect the current drawn from the supply.

Power factor at full load is typically 0.82–0.90 for standard induction motors. At light load, the power factor drops significantly — a motor running at 25% load might have a power factor of 0.50–0.60. The nameplate value is the full-load figure. If the motor routinely operates at partial load, the actual power factor is lower, and the actual current is higher than the nameplate FLC (because the magnetising current remains roughly constant regardless of load).

Efficiency class follows the IEC 60034-30-1 classification:

• IE1 — Standard efficiency (being phased out in most markets)
• IE2 — High efficiency (minimum legal requirement in many countries)
• IE3 — Premium efficiency (mandatory for new installations in the EU for motors 0.75–375 kW since July 2023)
• IE4 — Super premium efficiency (increasingly available, mandatory in the EU for 75–200 kW since July 2023)

Higher efficiency means less current for the same mechanical output. Replacing an IE1 motor with an IE3 motor of the same kW rating reduces the full load current by 2–5%, depending on size. This is rarely enough to change the cable size, but it does affect the thermal load on the cable over the motor's lifetime.

The duty cycle class, marked as S1 through S9 per IEC 60034-1, describes the load pattern the motor is designed for. This directly affects the thermal design and, by extension, the cable sizing approach.

• S1 — Continuous duty: The motor runs at constant load long enough to reach thermal equilibrium. This is the default assumption for most cable sizing calculations.
• S2 — Short-time duty: The motor runs at constant load for a specified time (e.g., S2 30 min), then is switched off long enough to cool to ambient. The motor can be rated for a higher current than its continuous rating, because it never reaches thermal equilibrium.
• S3 — Intermittent duty: Cyclic operation with identical ON and OFF periods. Specified as a cyclic duration factor, e.g., S3 25% means 25% of the cycle is loaded. Crane motors are typically S3 or S4.
• S4 through S9: Progressively more complex duty cycles involving varying loads, braking periods, and speed changes. These are common in lift motors, rolling mill drives, and other applications with complex motion profiles.

For cable sizing, the duty cycle determines the effective RMS current over the thermal time constant of the cable. A motor rated S3 25% at 100 A does not require a cable rated for 100 A continuous — the cable's thermal time constant is much longer than the duty cycle, so it "sees" a lower effective current. However, calculating the equivalent continuous current correctly requires understanding both the duty cycle and the cable's thermal behaviour.

The insulation class (B, F, or H per IEC 60034-1) defines the maximum winding temperature the motor can sustain:

• Class B: 130°C maximum winding temperature (80°C rise above 40°C ambient + 10°C hot spot allowance)
• Class F: 155°C maximum (105°C rise + 10°C)
• Class H: 180°C maximum (125°C rise + 10°C)

Modern motors are almost universally built with Class F insulation but rated for Class B temperature rise. This means the motor runs at Class B temperatures under normal conditions, with the Class F insulation providing a substantial safety margin. The nameplate will show "Insul. Class F, Temp. Rise B" or similar.

For cable sizing, the motor's terminal box temperature matters. A motor running at Class B temperature rise will have a terminal box temperature of approximately 80–90°C above ambient. The cable termination at the motor must be rated for this temperature, and the cable's current-carrying capacity must be derated for the elevated ambient temperature in the motor terminal box. This derating is often forgotten and can be significant — a cable in a 70°C terminal box has substantially less capacity than the same cable at 40°C ambient.

Here is the complete chain from motor nameplate to cable selection, showing which nameplate parameter drives each step:

1. Design current: Use nameplate FLC directly. If service factor operation is expected, multiply by SF. If the motor operates at partial load consistently, the actual current may differ.
2. Protective device rating: Select based on FLC (or SF × FLC). Motor protection settings (thermal overload) are set to FLC, with the starting current (typically 6–8 × FLC for DOL) determining the trip class.
3. Cable current rating: Must exceed the protective device setting divided by all applicable derating factors. The duty cycle class may allow a reduction in the effective current for intermittent duty motors.
4. Voltage drop: Calculate at FLC (or starting current for DOL starts if the voltage drop during starting is a concern). Motor starting voltage drop should not exceed 10–15% at the motor terminals.
5. Terminal box derating: Apply ambient temperature derating for the cable section inside or near the motor terminal box, where temperatures are elevated above general ambient.

Every number on the nameplate feeds into at least one of these steps. Reading the nameplate correctly is not academic — it is the foundation of a correct cable and protection design.