Electrical Cable Sizing Calculation & Introduction to Cable Sizing Tool

Table of Content

1. Cable Sizing Calculation
2. Cable Construction
3. Standards for Sizing of Conductors
4. Cable Manufacturers in KSA
5. Criterion of Cable Sizing 
6. Use of Cable Sizing Tool for optimal sizing

Cable Sizing Calculation

Electrical cable sizing is a critical aspect of electrical system design to ensure that the cables can safely carry the expected electrical load without overheating or causing voltage drop issues. To perform a cable sizing calculation, you need to consider several factors, including the maximum current the cable will carry, the voltage drop allowed, the type of cable insulation, and the ambient temperature. Here’s a step-by-step guide on how to perform electrical cable sizing calculations:

• Determine the Load Current (I)
• Select the Cable Type
• Ambient Temperature (Tambient)
• Voltage Drop (Vd)
• Cable Resistance (R)
• Calculate Minimum Cable Size
• Derating Factors
• Check Cable Ampacity
• Check Short-Circuit and Overload Protection

Cable Construction

Construction Format:

Conductor / Insulation / Armor / Outer Sheath

1. Conductor (e.g. Cu / Al)
2. Insulation (PVC / XLPE)
3. Armour (SWA / STA /Unarmored)
4. Outer Sheath (PVC)      

Examples:

 Cu / XLPE / SWA / PVC

• PVC / CU / SWA / PVC

• XLPE / CU / Unarmored / PVC

• CU / PVC / Unarmored

Standards for Cable Sizing:

Local Standards

• SEC TES-P-104-03-R0
• NEC Article 310
• SBC Chapter 52
• IEC 60364-5-52
• MAADEN MD-405-10P0-EG-EL-SPC-00003

Cable manufacturers in KSA

• Saudi Cables
• Jeddah Cables
• Riyadh Cables

Cable sizing Criterion:

There are THREE primary checks for sizing of conductors:

• Voltage Drop 
• Ampacity
• SC Withstand capability

Ampacity Check:

• Ampacity (Current carrying capacity)

Ampacity:

Ampacity refers to the current carrying capacity of the conductors under standard conditions.

Cables having the same cross-sectional areas may not.     

 Necessarily have the same Ampacity, as Ampacity also depends upon:

•  Conductor type (i.e., Cu / Al), 
•  Insulation type (XLPE/ PVC), 
•  Armour Type (SWA / Unarmored) 
•  Method of Installation 
• Above ground in Cable trays
• Below ground in trenches

Ampacity:

Example:

Suppose we have four cables with CSA=50mm2.  

Cable Spec                                     Install. Method      Ampacity

1-3/C x 50mm2 Cu/XLPE/SWA/PVC Installed in Air 182 Amps

1-3/C x 50mm2 Cu/XLPE/SWA/PVC Installed in Gnd      189 Amps

1-3/C x 50mm2 Cu/PVC/SWA/PVC Installed in Air 128 Amps

1-3/C x 50mm2 Cu/XLPE/UnArm/PVC Installed in Air 175 Amps

Ampacity Condition:

For a conductor to satisfy Ampacity criteria, the following conditions must 

Meet after selecting a cable:

Ib: 

Continuous Load Current.

Ir:  

Current Setting of Protective Device

Iz: 

Corrected Permissible Conductor Current

(Derated Ampacity based on Site conditions & installation method)

Ampacity:

Iz (IDerated) = K x Iz’(IBase) 

Iz’: Permissible Conductor Current

        (from Manufacturers cable catalogue- Base Ampacity)

Where K= K1 x K2 x K3 x K4

K1= Correction factor for Temperature

K2= Correction Factor for Group of cables

K3= Correction factor for Soil thermal resistivity

K4= Correction factor for depth of burial

Ampacity:

Correction Factors (K):

K2= Correction Factor for Group of cables 

(IEC-60364-5-52 ,Table A.52-17 & 18)

K1= Correction factor for Temperature

K2= Correction Factor for Group of cables

K3= Correction factor for Soil thermal resistivity

K4= Correction factor for depth of burial

Example:

For 50mm2

Ampacity Condition (Summary):

Ib < Ir < Iz

Where Iz (IDerated) = K x Iz’(IBase) 

K= K1 x K2 x K3 x K4

Voltage Drop Check:

Voltage Drop

Every electrical load is characterized by Rated Voltage

that must be within the tolerance as specified by the manufacturer.”

Voltage Drop Formulae:

1-Phase: 

               Vd = 2 x I x L x (R.Cosø + X.Sinø)

3-Phase:

                   Vd = Sqrt(3) x I x L x (R.Cosø + X.Sinø)

Where, 

                   Vd = Voltage Drop (Volts)

                     I = load Current (Amps) ~ Ib

                     L = Length of Cable (km)

                     R = Resistance (Ohms/km)

                     X = Reactance (Ohms/km)

               Cosø = Load Power Factor

               Sinø = Sin (Cos-1 ø)

Voltage Drop Formulae:

1-Phase: 

               Vd = I x L x mV/A/m

3-Phase:

                   Vd = Sqrt(3) x I x L x mV/A/m

Where, 

                   Vd = Voltage Drop (Volts)

                     I = load Current (Amps) ~ Ib

                     L = Length of Cable (km)  

       mV/A/m = Cable Voltage Drop factor 

% Voltage Drop:

Usually, Voltage Drop is expressed in the percentage of the source voltage. 

Vd % = (Vd x 100)/Vs

Where Vd = Voltage Drop (Volts)

                    Vs. = Power Supply Source Voltage

Voltage Drop Calculations for Motors:

1. For motor loads, we need to check:

Steady State Voltage Drop

(Calculations will be same as that of non-motor loads)

1. Starting Voltage Drop

               Vd = Sqrt (3) x Istarting x L x (R.Cosøs + X.Sinøs)

Where, 

                   Vd = Voltage Drop (Volts)

                  I starting = Motor Starting Current (Amps) ~ 4 – 6 x IFL

                     L = Length of Cable (km)

                     R = Resistance (Ohms / km)

                     X = Reactance (Ohms / km)

               Cosøs = Starting Power Factor

               Sinøs = Sin (Cos-1 øs)

Allowable Voltage Drop for Motors:

Allowable Voltage Drop during motor starting depends on the Torque speed characteristic curve. 

For example, the starting voltage drop for NEMA Design B motors should not be less than 20%. 

As per IEC, if the steady state voltage is more than 8%, then It would not let the motor accelerate because of high voltage drop during starting (i.e., approximately 40%).

Cable SC Withstand Capability:

Cable SC Withstand Capability

For a given cable size “S” (in mm2), the cable

Short Circuit Withstand capability for a particular

this equation calculates duration of time

Where,

                “K” is the Temperature Coefficient of Conductor Resistance

                “t” is the duration of fault current (typically taken as 1 sec)

Cable SC Withstand Capability:

For a cable to satisfy the Criteria of SC Withstand capability.

The SC Withstand Capability shall be greater than the available fault current (3-Ph Sym. SC current).

Example#1: 

Can a cable with specifications 1-3/C x 50mm2, CU/ XLPE/SWA/PVC be used as an outgoing feeder from a unit substation with an Available fault current of 21kA rms?

Solution: By putting values in the given equation,

S=50 

K= 143 (for Cu/XLPE type cables) 

t = 1 sec

I= 7.15kA < 21kA 

Hence, cable needs to be more suitable regarding the SC Capability of cable.

Cable SC Withstand Capability:

Example#2:

Can a cable with specifications 1-3/C x 150mm2, CU/ XLPE/SWA/PVC be used as an outgoing feeder from a unit substation with an Available fault current of 21kA rms?

Solution: 

By putting values in the given equation,

S=150 

K= 143 (for Cu/XLPE type cables) 

t = 1 sec

I= 21.45kA > 21kA 

Hence, cable is suitable with regards to the SC Capability of cable.

Cable sizing Criterion:

There are THREE primary checks for sizing of conductors:

• Voltage Drop 
• Ampacity
• SC Withstand capability

Remember that cable sizing is a complex task, and it’s essential to follow local electrical codes and standards, as they may provide specific guidelines and requirements for cable sizing in your region. Additionally, consulting with a qualified electrical engineer or professional is advisable for critical applications or if you have limited experience in cable sizing Calculation.