Cable Size for a Motor or Panel: Current Rating, Voltage Drop and Safety Margin

The short answer

• Three checks set the size – current rating after derating, voltage drop, and protection by the breaker upstream. Take the smallest standard size that passes all three.
• The rule – design current (IB) ≤ protective device rating (In) ≤ cable capacity after derating (Iz).
• Starting current is brief and handled by the breaker curve or starter, not by a bigger cable.
• Safety margin – between two sizes take the larger, and allow for future load.

Step 1: Load current and protection

For a three-phase motor, take the full-load current from the nameplate, or a typical value from the kW-to-amps chart in our contactor sizing guide. The calculation is I = P ÷ (√3 × V × efficiency × power factor), with P the shaft output in watts and V the 400 V line voltage. For single-phase, leave out the √3 and use 230 V; it draws far more current per kW.

For a panel, add up the full-load currents of everything that runs together (apply diversity only if the designer or code allows it) and size the incomer cable to the incomer breaker, which is what protects it.

A direct-on-line start typically draws 5 to 8 times the running current for a few seconds. The breaker curve, motor protection breaker or starter is chosen to let that through; the cable is not oversized for it (see our MCB vs MCCB vs RCCB guide). Some codes also add a margin for motor cables.

IB, In and Iz explained

The current the circuit really carries (IB) must not exceed the rating or setting of the overload protective device (In), which must not exceed what the installed cable can carry (Iz, its tabulated rating after the Step 2 derating).

In a motor circuit the overload relay (or motor protection breaker), set to the nameplate current, is the overload device. The short-circuit device may be larger so the motor can start, but the cable must survive a fault until it trips. On a long run, check that it still trips quickly on a fault at the far end (fault-loop rules in your wiring code).

Step 2: Derate for heat, grouping and method

Rating tables, such as those in IEC 60364-5-52 (wiring systems and current-carrying capacities), assume one circuit at 30 °C air, or 20 °C ground if buried. What counts is the air around the cable, which in a closed steel panel or a roof duct can be far higher in a Pakistani summer.

The standard’s table gives no PVC factor above 60 °C: ask the cable maker or move the cable into cooler air.

Cables bunched in one conduit or trunking, or clipped together, heat each other: typical factors are about 0.8 for two circuits, 0.7 for three, 0.57 for six and 0.5 for nine. Spare conduit or tray space does not replace them.

Installation method matters: a cable in conduit in a wall or insulation carries less than one clipped to a wall or on a perforated cable tray in free air, and buried cables depend on soil temperature and dryness. Use the maker’s table for your method, and the lower figure if unsure.

To pick a size, work backwards: required tabulated rating = In ÷ (temperature factor × grouping factor). For an 11 kW motor with a 22 A overload setting, PVC cable, 40 °C and three bunched circuits: 22 ÷ (0.87 × 0.7) = 36.1 A.

Orientation only, not design data: bunched, buried or hot cables carry less, and XLPE and aluminium differ. Confirm on the maker’s datasheet and in your wiring code. Here, 36.1 A points to 10 mm², whose low end of 38 A covers it.

Step 3: Check the voltage drop

Voltage drop is the voltage lost in the cable’s resistance; it grows with current and length. Limits are commonly 3 to 5 percent in total, depending on the code and circuit type; check the applicable code.

• Single-phase (230 V): ΔV = 2 × I × R × L
• Three-phase (400 V): ΔV = √3 × I × R × L

ΔV is the drop in volts, I the current in amps, L the one-way length in km, and R the resistance in Ω/km at working temperature: about 1.2 times the 20 °C datasheet value for PVC, 1.28 for XLPE. For large cables use the maker’s mV/A/m figures, which include reactance.

Worked example: an 11 kW, 400 V pump motor (22 A) sits 200 m (0.2 km) from the panel. Assume a 3 percent limit, which is 12 V.

1. 10 mm² copper (from the current check): 1.83 Ω/km at 20 °C, so R ≈ 1.83 × 1.2 = 2.2 Ω/km. ΔV = 1.732 × 22 × 2.2 × 0.2 = 16.8 V, or 4.2 percent of 400 V. Fails.
2. 16 mm²: 1.15 Ω/km at 20 °C, so R ≈ 1.38 Ω/km. ΔV = 1.732 × 22 × 1.38 × 0.2 = 10.5 V, or 2.6 percent. Passes.
3. Current alone allowed 10 mm²; the long run needs 16 mm². Check the start too: the drop is several times higher.

For a motor fed by a variable-speed drive, use the drive maker’s motor cable type and maximum cable length instead.

Insulation, conductor and construction

• PVC or XLPE – XLPE is rated for 90 °C at the conductor, PVC for 70 °C, so XLPE carries more current at the same size and loses less of its rating in a hot ambient, provided the terminals at both ends are rated for it.
• Copper or aluminium – aluminium conducts about 60 percent as well as copper, so it typically needs one or two sizes larger. Use lugs and terminals marked for aluminium, and check your code allows it.
• Stranded, flexible or solid – stranded (class 2) for fixed wiring; flexible (class 5) where the cable vibrates or moves; solid only for small fixed wiring.
• Armoured or unarmoured – armour resists damage and suits burial, plant areas and outdoor runs; it needs an armour gland at each end and serves as earth only where the code allows.
• mm² or AWG – they do not match: 14 AWG is about 2.1 mm², less than 2.5 mm². Convert from the maker’s table and take the next larger metric size.

Lugs, ferrules, earth and joints

• Lugs and ferrules – crimp lugs sized to the conductor and terminal stud, with the lug maker’s tool and die; ferrules on flexible conductors. Check each device’s terminal size range, temperature rating and torque.
• Earth – the protective earth conductor is sized separately, from the table or calculation in your wiring code, which depends on the fault current and the protective device’s clearing time.
• No splice-and-hope – never twist and tape lengths together or insert a smaller section: the breaker must protect the smallest section, and every joint can overheat. Use one continuous length, a terminal box or a joint kit.

Cable buying checklist

1. Size in mm² – the largest of the derated-current, voltage-drop and protection results for the real route.
2. Cores and colours – three live cores plus earth for a direct-on-line three-phase motor (six live cores plus earth from a star-delta starter to its motor; more if a neutral is needed), two plus earth for single-phase; match your site’s colour code, with green/yellow for earth only.
3. Voltage grade and standard – 450/750 V for general wiring, 0.6/1 kV for power distribution; the datasheet should name the standard (for example IEC 60502-1 for 0.6/1 kV power cable).
4. Conductor class – class 2 stranded for fixed wiring, class 5 flexible where the cable moves (IEC 60228).
5. Certification – ask for the datasheet or test certificate; look for a recognised certification mark and sheath markings.
6. Length – measure the real route plus slack, order one continuous length, and rerun the voltage-drop check.

Browse cables, conduit & flexible pipes and cable trays & accessories, plus circuit breakers and contactors & motor starters from Schneider Electric and ABB. Send us the nameplate photo and route length on WhatsApp and our team will help you choose.