Selection and Sizing of Electrical Parameters

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  • Breaker Size :
Example: Design Current for the R-TPN Load of Max 10 KW value is 20.89 A

We need to select the Next Higher size of Breaker available in the Market for Calculated Design Current (=20.89 A)
Here In our Case 30/32 A breaker can be selected. It can be MCCB or MCB or RCBO depending upon requirements.
Ib = 21 A
In = 30 A
Iz = 52.48 (From Cable Data)

Note: The operating characteristics of a device protecting a conductor against overload shall satisfy the following two conditions:

Ib <= In <= Iz

I2 <= 1.45 Iz

  • Ib = the current for which the circuit is designed, e.g. maximum demand
  • In = the nominal current of the protective device
  • Iz = the continuous current-carrying capacity of the conductor (see the AS/NZS 3008.1)
  • I2 = the current ensuring the effective operation of the protective device and may be taken as equal to either—
  • (a) the operating current in conventional time for circuit breakers (1.45 In); or
  • (b) the fusing current in conventional time for fuses (1.6 In for fuses by the IEC 60269 series)
Criteria for type gG fuses - Comply with the IEC
Standard requirement I2 ≤ 1.45 Iz:
As per the IEC standard, condition I2 ≤ 1.45 Iz must be taken into account, where l2 is the fusing (melting level) current, equal to k2 x In (k2 ranges from 1.6 to 1.9) depending on the particular fuse concerned.
In such a case an extra factor
k3 = 1.31 for fuses with In < 16A, or k3 = 1.1 for fuses with In ≥ 16A is considered, 
so that 12 ≤ 1.45 Iz will be valid if In s Iz/k3.

"In Simple Words, We should consider these 2 conditions so that the conductor will not be overheated or burned before the breaker trips."
  • Cable Size :
We can Select Cable Size based on Breaker selection directly because it keeps the account of conductor Ampacity in the worst-case scenario. Or we can select Cable from the Manufacturer Data table as well.

If MCCB = 10 Then CABLE = "1 x 4 mm2"
If MCCB = 20 Then CABLE = "1 x 6 mm2"
If MCCB = 30 Then CABLE = "1 x 10 mm2"
If MCCB = 40 Then CABLE = "1 x 16 mm2"
If MCCB = 60 Then CABLE = "1 x 25 mm2"
If MCCB = 80 Then CABLE = "1 x 35 mm2"
If MCCB = 100 Then CABLE = "1 x 50 mm2"
If MCCB = 125 Then CABLE = "1 x 50 mm2"
If MCCB = 150 Then CABLE = "1 x 70 mm2"
If MCCB = 175 Then CABLE = "1 x 70 mm2"
If MCCB = 200 Then CABLE = "1 x 95 mm2"
If MCCB = 225 Then CABLE = "1 x 95 mm2"
If MCCB = 250 Then CABLE = "1 x 120 mm2"
If MCCB = 275 Then CABLE = "1 x 120 mm2"
If MCCB = 300 Then CABLE = "1 x 150 mm2"
If MCCB = 325 Then CABLE = "1 x 150 mm2"
If MCCB = 350 Then CABLE = "1 x 185 mm2"
If MCCB = 350 Then CABLE = "1 x 240 mm2"
If MCCB = 375 Then CABLE = "1 x 240 mm2"
If MCCB = 400 Then CABLE = "1 x 300 mm2"
If MCCB = 450 Then CABLE = "1 x 300 mm2"
If MCCB = 500 Then CABLE = "1 x 400 mm2"
If MCCB = 500 Then CABLE = "2 x 120 mm2"
If MCCB = 600 Then CABLE = "2 x 150 mm2"
If MCCB = 700 Then CABLE = "2 x 185 mm2"
If MCCB = 750 Then CABLE = "2 x 240 mm2"
If MCCB = 800 Then CABLE = "2 x 300 mm2"
If MCCB = 900 Then CABLE = "2 x 300 mm2"
If MCCB = 1000 Then CABLE = "2 x 400 mm2"

In Our Example, We can select a "1 x 10 mm2" Cable. that will fulfil all the conditions required.
  • Voltage Drop :

Several major factors can affect voltage drop in an electrical system. These include:

  1. Cable length: Voltage drop is directly proportional to the length of the cable. The longer the cable, the greater the voltage drop.

  2. Cable size: The size of the cable's cross-sectional area affects its resistance. Larger cables have less resistance and therefore less voltage drop.

  3. Current: Voltage drop is directly proportional to the current flowing through the cable. Higher currents result in higher voltage drops.

  4. Cable material: The type of material used in the cable affects its resistance. Copper has lower resistance than aluminium, for example, resulting in less voltage drop in a copper cable.

  5. Temperature: The temperature of the cable affects its resistance, and therefore its voltage drop. Higher temperatures increase resistance, resulting in higher voltage drop.

Voltage Drop = (vdFactor / No. of Runs of cable) * Current* (Length Of cable / 1000))) / 415)

"Voltage Drop for 600/1000 V XLPE Insulated Cables in V/A/KM for 3 and 4 Core Cables"

If SizeC = "1.5" Then vdFactor = 26.7
If SizeC = "2.5" Then vdFactor = 16.4
If SizeC = "4" Then vdFactor = 10.2
If SizeC = "6" Then vdFactor = 6.8
If SizeC = "10" Then vdFactor = 4#
If SizeC = "16" Then vdFactor = 2.5
If SizeC = "25" Then vdFactor = 1.65
If SizeC = "35" Then vdFactor = 1.15
If SizeC = "50" Then vdFactor = 0.87
If SizeC = "70" Then vdFactor = 0.6
If SizeC = "95" Then vdFactor = 0.45
If SizeC = "120" Then vdFactor = 0.37
If SizeC = "150" Then vdFactor = 0.3
If SizeC = "185" Then vdFactor = 0.26
If SizeC = "240" Then vdFactor = 0.21
If SizeC = "300" Then vdFactor = 0.19
If SizeC = "400" Then vdFactor = 0.17
If SizeC = "500" Then vdFactor = 0.16

So for a "1 x 10 mm2" 1-Run Cable of 100-meter length with VD factor of 4 V/A/KM, the Voltage Drop is 1.68 % = 6.972 Volts."
If the Supply Voltage is 415 volts, the terminal voltage is at (415-6.972) volts = 408 volts.

MEW Kuwait - The maximum permissible drop in voltage from the consumer’s terminal to any point to his installation shall not exceed 2.5% of the nominal voltage when the conductors are carrying full load current (6 volts for 1 phase and 10 volts for 3 phase systems).
  • Short Circuit:

Isc = V / [(Zc x √3) x N]

where:

  • Isc = the short circuit current in amperes
  • V = the voltage of the power system in volts
  • Zc = the total impedance of the cable run in ohms, calculated as Zc = √(R^2 + X^2), where R is the resistance in ohms per unit length of the cable, X is the reactance in ohms per unit length of the cable and √ is the square root function
  • N = the number of parallel cable runs
  • √3 = the square root of 3, which is a constant factor in three-phase power systems
If we have the Resistance and Reactance values of the cable from the Vendor catalogue, we can use that directly. Or
Generalise Value :
Rmcb = (LENGTH * (1 / 56) * 1000) / (Cable Size * NUMBER OF CABLE)
Xmcb = 0.08 * LENGTH

Rtotal = 0.054 + 1.141 + 0.149 + Rmcb + 7.143
Xtotal = 0.539 + 6.357 + 1.6 + Xmcb + 0.8
Ztotal = ((Rtotal ^ 2) + (Xtotal ^ 2)) ^ 0.5

So, Short Circuit Value will Be 1.28 kA for the above-mentioned values.

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