Electrical Cable Sizing and Selection in Pakistan: A Complete Engineering Guide

Incorrect cable sizing is one of the most common and costly electrical engineering errors on Pakistani industrial and commercial projects. Undersized cables overheat, degrade insulation, and cause fire hazards. Oversized cables waste capital and create unnecessary installation cost. Both failures are avoidable through systematic application of the cable sizing methodology defined in IEC 60364-5-52 — but this methodology, with its derating factors, voltage drop calculations, and short-circuit checks, is rarely applied rigorously on Pakistani projects below the scale of major infrastructure work.

This guide sets out the complete cable sizing process in the sequence that practicing electrical engineers and MEP contractors use on real Pakistani projects: current rating selection with derating, voltage drop verification, short-circuit withstand check, and final cable type selection. It is written by the engineering team at Bilal Switchgear Engineering, which delivers MEP project services including complete electrical design and cable scheduling for industrial and commercial facilities across Pakistan since 1978.

Step 1: Determine the Design Current

The starting point for every cable sizing calculation is the design current — the maximum continuous current the cable must carry under normal operating conditions. For a motor feeder, this is the motor’s full load current from its nameplate. For a distribution feeder supplying multiple circuits, it is the sum of the individual circuit currents multiplied by an appropriate diversity factor that reflects the probability of all circuits being simultaneously at full load.

For three-phase motor circuits in Pakistan, the full load current in amperes is calculated from the motor power rating using: I = P / (sqrt(3) x V x PF x efficiency), where P is motor power in watts, V is line-to-line voltage (415V), PF is the motor power factor (typically 0.85 for standard induction motors), and efficiency is the motor efficiency (typically 0.88 to 0.95 for modern motors). For a 37 kW motor at 0.87 PF and 0.92 efficiency: I = 37,000 / (1.732 x 415 x 0.87 x 0.92) = 64 A.

Step 2: Apply Derating Factors for Pakistani Conditions

IEC 60364-5-52 publishes current carrying capacity tables for cables in various installation methods, based on a reference ambient temperature of 30 degrees Celsius. Pakistani summer conditions, with ambient temperatures regularly reaching 45 to 50 degrees Celsius in industrial plants, require derating that significantly reduces the permissible current below the tabulated values. Four derating factors apply:

Ambient Temperature Derating

The standard XLPE cable tables are referenced at 30 degrees Celsius ambient. At 50 degrees Celsius — a realistic summer temperature inside a Pakistani factory or outdoor cable route in sunlight — the derating factor for 90 degree XLPE cables is approximately 0.82. This means a 95 mm2 XLPE cable rated 250A at 30 degrees ambient is derated to 250 x 0.82 = 205A at 50 degrees Celsius. Applying standard European cable tables without temperature derating in Pakistani conditions is a significant safety error that produces cables running above their rated temperature in summer.

Installation Method Derating

Cables installed in conduit, trunking, or buried ducts cannot dissipate heat as effectively as cables in free air. IEC 60364-5-52 defines installation method reference numbers (A1, A2, B1, B2, C, D, E, F, G) each with different current carrying capacities. Cables on open cable trays in free air (Method E or F) have higher current ratings than the same cables in conduit (Method A or B). For the 37 kW motor example, cables in steel conduit (Method B2) are derated by approximately 0.77 compared to free air, requiring a larger cable than the open-air calculation would indicate.

Grouping Derating

When multiple cables are installed together in a cable tray, conduit, or duct, mutual heating reduces each cable’s current carrying capacity. IEC 60364-5-52 Table B.52.20 provides grouping factors: a single circuit on a tray has a factor of 1.0; 6 circuits on a single tray layer have a factor of 0.70; 12 circuits have a factor of 0.60. In Pakistani industrial cable tray installations where dozens of cables run together in crowded trays, grouping derating can be the most significant factor in the cable sizing calculation and is frequently overlooked in practice.

Combined Derating

All applicable derating factors multiply together. A cable on a crowded tray (grouping factor 0.70) in a hot factory (temperature factor 0.82) requires a tabulated current rating of at least: Design Current / (0.70 x 0.82) = Design Current / 0.574. For the 64A motor circuit, the required tabulated current rating is 64 / 0.574 = 111A minimum, which selects a 35 mm2 copper XLPE cable (rated approximately 120A at 30 degrees in free air). Without derating, a 16 mm2 cable rated 80A might appear sufficient — and would run dangerously hot in Pakistani summer conditions.

Step 3: Check Voltage Drop

A cable that passes the current rating check must also be verified for voltage drop — the reduction in voltage between the supply end and the load end of the cable caused by the cable’s resistance and reactance. Excessive voltage drop causes motors to run at reduced torque, lighting to dim, and equipment to malfunction or fail to start.

The permissible voltage drop under IEC 60364 is 4 percent for lighting circuits and 5 percent for power circuits, measured from the supply transformer secondary to the load terminals. Pakistani industrial practice typically uses tighter limits of 2.5 to 3 percent on main feeder cables to leave headroom for voltage drop in sub-distribution cables and final circuit wiring.

For a three-phase cable, voltage drop in volts is calculated as: Vd = sqrt(3) x I x L x (r x cos(phi) + x x sin(phi)) / 1000, where I is the load current in amperes, L is cable length in metres, r is the conductor resistance in milliohms per metre per phase, x is the conductor reactance in milliohms per metre per phase, and cos(phi) is the load power factor. The percentage voltage drop is then: %Vd = (Vd / V_nominal) x 100.

Worked example: 37 kW motor, 64A full load current, 0.87 PF (sin phi = 0.493), cable run 80 metres, 35 mm2 copper XLPE (r = 0.668 mohm/m, x = 0.0875 mohm/m). Vd = 1.732 x 64 x 80 x (0.668 x 0.87 + 0.0875 x 0.493) / 1000 = 1.732 x 64 x 80 x (0.581 + 0.043) / 1000 = 1.732 x 64 x 80 x 0.624 / 1000 = 5.53V. Percentage Vd = 5.53 / 415 x 100 = 1.33 percent. This is within the 5 percent limit. If the cable run were 300 metres, Vd = 20.7V = 5.0 percent — exactly at the limit and marginal in practice, requiring an upsize to 50 mm2 or 70 mm2 to provide adequate margin.

Step 4: Verify Short-Circuit Withstand Rating

A cable must withstand the maximum prospective fault current at its supply end for the maximum time before the protective device clears the fault. If the cable cannot withstand this fault energy, the conductor and insulation can be permanently damaged even by a fault that the protective device clears within its rated time.

The minimum conductor cross-section to withstand a short-circuit fault is calculated from: S = (I_sc x sqrt(t)) / k, where I_sc is the prospective short-circuit current in amperes, t is the fault clearance time in seconds, k is a constant depending on conductor material and insulation type (k = 143 for copper PVC, k = 176 for copper XLPE), and S is the minimum conductor cross-section in mm2.

For a cable supplied from a 500 kVA transformer with 5 percent impedance on a 415V system, the prospective fault current at the transformer secondary is approximately 13,900A. With a 63A MCCB clearing in 0.1 seconds: S = (13,900 x sqrt(0.1)) / 176 = (13,900 x 0.316) / 176 = 24.9 mm2. A 35 mm2 cable satisfies this requirement with margin. This check confirms the cable selected for current rating and voltage drop also survives a fault event without damage.

Cable Types Used in Pakistan: Selection Guide

PVC Insulated Cables (70 degrees Celsius)

PVC insulated cables are the lowest cost option and are widely used for internal wiring in Pakistani buildings and industrial panels. Their 70 degree Celsius maximum conductor temperature limits current carrying capacity compared to XLPE, making them a poor choice for high-load circuits or hot ambient conditions. PVC also releases toxic fumes in fire conditions, which has led to restrictions on PVC cables in occupied buildings and escape routes in international specifications.

XLPE Insulated Cables (90 degrees Celsius)

Cross-linked polyethylene (XLPE) insulation allows a maximum conductor temperature of 90 degrees Celsius under normal conditions and 250 degrees Celsius under short-circuit conditions, giving XLPE cables approximately 30 percent higher current carrying capacity than equivalent PVC cables. XLPE cables are the standard specification for power distribution cables in Pakistani industrial projects at 415V and 11kV. XLPE also provides better resistance to moisture, chemicals, and UV exposure than PVC for outdoor or direct-buried applications.

Armoured Cables for Pakistani Projects

Armoured cables add a metallic armour layer between the insulation and the outer sheath, providing mechanical protection against physical damage during installation and in service. Two armour types are common in Pakistan: Steel Wire Armour (SWA) provides robust mechanical protection for cables installed in cable trays, ducts, and direct buried routes in industrial areas, and is the standard specification for main distribution cables in Pakistani factories and infrastructure projects. Steel Tape Armour (STA) provides lighter mechanical protection suited to cables installed in less aggressive environments. Both types also provide a continuous earth path through the armour, which can serve as the circuit protective conductor where specifically designed to do so.

Low Smoke and Fume (LSF/LSZH) Cables

Low smoke zero halogen (LSZH) cables use insulation and sheath compounds that release minimal smoke and no toxic halogen gases in fire conditions, significantly improving survival conditions in occupied buildings during a fire event. LSZH cables are specified in Pakistani hospitals, hotels, shopping malls, and high-rise buildings where fire life safety standards are applied. They carry a cost premium of approximately 20 to 40 percent over standard PVC cables but are mandatory for routes in escape routes, plenum spaces, and areas with high occupant density.

Common Cable Sizing Errors on Pakistani Projects

• Omitting ambient temperature derating: applying European cable tables at 30 degrees ambient without derating for Pakistan’s 45 to 50 degree summer temperatures produces undersized cables that run hot and degrade insulation prematurely
• Ignoring grouping derating: specifying individual cable sizes correctly but installing them in crowded trays without applying the grouping factor — common on Pakistani projects where cable routing is decided after the cable schedule is finalised
• Sizing for rated current only, not for starting current: motor feeder cables that are adequate for running current may not withstand repeated motor starts without accelerated insulation ageing if the starting current duration is not considered in the thermal rating
• Not checking voltage drop on long runs: acceptable at 30 to 50 metre cable lengths but critical on 100 to 300 metre main feeder runs common in large Pakistani industrial facilities
• Specifying PVC cables for outdoor or hot ambient locations: PVC insulation softens and degrades at sustained temperatures above 60 to 65 degrees Celsius, which is routinely exceeded in direct sunlight on Pakistani summer days
• Using aluminium cables without applying the conversion factor: aluminium conductors require approximately 1.6 times the cross-section of copper for the same current rating — using copper tables without adjustment for aluminium conductors produces significantly undersized aluminium cables

Cable Sizing for Switchgear and MCC Installations

Every cable connected to a motor control centre or distribution switchboard must be individually sized through the three-step process above. A complete cable schedule documenting the design current, derating factors, selected cable size, voltage drop result, and short-circuit withstand verification for every circuit is a standard deliverable on properly engineered Pakistani electrical projects.

The cable schedule also feeds directly into the earthing system design — protective conductor (earth cable) sizes are derived from the phase conductor sizes using the IEC 60364 tables, ensuring the earth fault loop impedance is low enough for protective devices to operate within their required clearance times.

Bilal Switchgear Engineering’s Power Division prepares complete cable schedules as part of switchgear and MCC project documentation. Our MEP Division performs cable sizing calculations and produces cable schedules for complete building electrical installations as part of turnkey MEP project delivery.

Getting Cable Sizing Right for Your Project

Cable sizing is a fundamental electrical engineering calculation that protects personnel, prevents equipment damage, and ensures reliable system performance over the 25-plus year service life of an electrical installation. The four-step process — design current, derating, voltage drop, short-circuit check — must be applied to every circuit, not just the largest feeders. In Pakistani conditions with high ambient temperatures, crowded cable trays, and long distribution runs, the derating factors involved are substantial and cannot be ignored without creating installations that underperform and degrade prematurely.

Contact Bilal Switchgear Engineering’s engineering team to discuss cable sizing and selection for your project, request a cable schedule review, or obtain engineering support for your electrical installation design.

Frequently Asked Questions

What standard governs cable sizing in Pakistan?

The primary standard for cable selection and installation in Pakistan is IEC 60364-5-52 (Wiring Systems), which defines current carrying capacities, installation methods, derating factors, and voltage drop limits for low voltage electrical installations. IEC 60228 governs conductor construction and cross-sections. XLPE insulated cables at 11kV and above follow IEC 60502. Where British Standard BS 7671 is specified on older Pakistani projects or by some international clients, the methodology is similar but the tables and factors differ in detail.

How do I derate cables for Pakistan’s hot climate?

Apply a temperature correction factor from IEC 60364-5-52 Table B.52.14 based on the maximum expected ambient temperature at the cable installation location. For 90 degree XLPE cables at 50 degrees Celsius ambient (a realistic summer temperature inside Pakistani factories or in outdoor conduit in direct sunlight), the correction factor is approximately 0.82. Multiply the standard table current rating by this factor to get the derated current capacity. For 70 degree PVC cables, the factor at 50 degrees ambient is approximately 0.71 — a much larger reduction that often makes PVC cables uneconomical for Pakistani outdoor or hot-ambient applications.

What is voltage drop and how much is allowed in Pakistan?

Voltage drop is the reduction in voltage along a cable caused by the cable’s resistance and reactance carrying load current. IEC 60364 permits a maximum of 4 percent voltage drop for lighting circuits and 5 percent for power circuits, measured from the supply transformer to the load terminals. Pakistani industrial practice typically designs for 2.5 to 3 percent on main feeder cables to leave headroom for sub-distribution cables. Voltage drop is most critical on long cable runs and for motor starting, where sustained high current combined with long cable length can produce unacceptable voltage at the motor terminals.

Should I use copper or aluminium cables for a factory in Pakistan?

Copper cables are the standard specification for motor feeders, control cables, and all cables below 35 mm2 in Pakistani industrial projects. Copper provides higher conductivity (lower resistance per mm2), better mechanical properties for termination, and lower contact resistance at joints and terminations. Aluminium cables are used for main distribution feeders above 70 mm2 where the weight and cost savings are significant and where proper aluminium termination hardware is specified and installed. Aluminium requires approximately 1.6 times the cross-section of copper for the same current rating, and aluminium terminations require antioxidant compound and regular retorquing due to aluminium’s higher thermal expansion.

What cable type should I specify for direct underground burial in Pakistan?

Directly buried cables in Pakistan should be armoured XLPE cables — typically designated as XLPE/SWA/PVC or XLPE/STA/PVC in cable schedules. The XLPE insulation provides thermal performance for Pakistani conditions, the armour provides mechanical protection against excavation damage and soil movement, and the PVC outer sheath provides moisture and chemical resistance. Cables buried directly in soil should be laid at a minimum depth of 750 mm to 900 mm in pedestrian areas and 1000 mm minimum under roads and vehicle access areas. Route markers should be installed above the cable to warn against future excavation damage.