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Electrical Cable Sizing

This article will define the minimum requirements for sizing of electrical high voltage (HV) and low voltage (LV) cable.

• Codes and Standards

In general, below list of standards is normally required for cable sizing:

ReferenceTitle
IEC 60183Guide to the selection of high voltage cables
IEC 60228Conductors of insulated cables
IEC 60287Electric cables - Calculation of the current rating - All parts
IEC 60502-1Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Parts 1
IEC 60502-2Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) - Parts 2

• Electrical Cable Sizing Criteria

The sizing of electrical cables is primarily based on the following criteria:

  1. Current carrying capability of cable after applied cable derating factors
  2. Voltage drop calculation in cable within acceptable limit
  3. Cable Short-circuit Rating Capability

• Cable Current Carrying Capability

The current rating of cable should be satisfied:

Cable Current Carrying Capability ≥ Continuous Load Current

Continuous Load Current shall be calculated with note below:

  • First, incoming feeders to switchgears are sized to deliver the equipment site rated output (viz. GTGs, Transformers, EDG, etc).
  • Second, incoming feeders to MCCs are sized based on rating of incoming Circuit Breaker.
  • Third, incoming feeders to LV distribution boards are sized based on equipment rated output (UPS Rating, Small Power and Lighting Transformers) and outgoing feeders are sized based on the full load current.

Cable Current Carrying Capability is calculated as follows:

I = k0 x k1 x k2 x k3 x k4 x I0

where:

I:Cable Current Carrying Capability at actual site condition
I0:Cable Current Carrying Capability at typical condition i.e. ambient temperature 30oC
k1:Cable Derating factor for variation of surrounding temperature, i.e., ambient temperature (for above-ground installation) or surrounding soil temperature (for underground installation)
k2:Cable Derating factor for variation in ground thermal resistivity of soil (for underground installation)
k3:Cable Derating factor for depth of laying (for underground installation)
k4:Cable Derating factor depending on the cable arrangement, i.e., number of installed cables in a layer, number of layers, cable spacing horizontally and vertically

See Also: Cable Derating FactorsAboveground Cable Derating CalculationUnderground Cable Derating Calculation

• Voltage Drop Calculation in cable within acceptable limit

In checking the Voltage Drop in cables, the following criteria should be satisfied:

Calculated Voltage Drop ≤ Permissible Voltage Drop

Cable Voltage Drop Calculation is performed by the following equations:

Three-phase circuit voltage drop formula:

AC-three-phase-voltage-drop-formulas

Single phase circuit voltage drop formula:

AC-single-phase-voltage-drop-formulas

DC circuit voltage drop formula:

DC-voltage-drop-formulas

where:

VD:Voltage drop, in volts
cosΦ:Load power factor
R:Resistance of cable, in ohms per phase per 1000m
X:Reactance of cable, in ohms per phase per 1000m
I:Load current, in amperes
L:Total length of cable, in meter

See Also: Voltage Drop Calculator

Unless otherwise specified by project design basic, permissible percent voltage drops in cables should be:

– 2% for main feeder cables

– 5% for motor cables under normal operation conditions and 15% for motor cables during motor starting.

In addition, power factor of motors will be as per motor catalog and a power factor of 0.3 during motor starting.

Moreover, the starting current for LV motors is assumed to be 7 times the motor full load current for DOL starting.

• Cable Short Circuit Rating Capability

The following condition should be satisfied so that cables can carry the short circuit current without any damage to cable integrity.

Allowable Cable Short-Circuit Rating ≥ Calculated Short Circuit Current

The basis shall be such that the let-through (I2t) energy of the current limiting devices shall be less than cables thermal withstands (I2t), which is dependent on cable type and cross-section. On other hand, the calculated short circuit at the load end of cable shall consider the fault level of switchgear and cable characteristics (cable resistance, cable reactance and cable length).

See Also: Cable Short Circuit Rating Calculation

Allowable cable short-circuit rating shall be higher than the expected short circuit current flowing through the respective cable. The allowable cable short-circuit rating is calculated by the following equation:

Cable Short-circuit Rating Formular

where

Isc:Allowable short-circuit of a given cable size (kA)
k:Coefficient whose value is relevant to type of insulation material and final temperature reached for short circuit
– k = 143 for copper conductor, XLPE or EPR insulation
– k = 95 for aluminum conductor, XLPE or EPR insulation
S:Cable nominal cross-section (sqmm)
t:Short circuit duration (s)

• Main feeder electrical cable sizing sample

electrical cable sizing sample of main feeder

• Motor feeder electrical cable sizing sample

electrical cable sizing sample of motor feeder

• Conclusion

Three major criteria for electrical cable sizing have been discussed. In addition, calculation tool for each criteria also introduced. These calculation tools comply with IEC standards (i.e. IEC 60502).

It is recommended that during the Detailed Engineering Phase, electrical cable sizing report shall be reissued updating Vendor, Manufacturer data (i.e. Cables, protection devices).