VOLTAGE DROP AND CABLE SIZING
Table 2.1Sidayu Permissible Voltage Drop (Based on IEC-60092-201)
Permissible
No. From To Voltage Criteria
Vd
1 DC system power cable 2% DC
2 Main Supply Farthest Lighting 5% LV Lighting (2% in feeder,
Fixtures 3% Sub Distribution)
3 Main Supply Farthest Socket 5% 400/230VAC Load (2% in
Outlets feeder,3% Sub Distribution)
3. CALCULATION METHODOLOGY
3.1 Calculation Criteria
Calculation of the cable size is based on the most stringent of the following criteria, wherever
applicable:
a) Permissible current carrying capacity under steady state conditions
b) Permissible voltage drop under steady state conditions
c) Short-circuit thermal withstand capacity
3.2 Derating Factors
The given cable ampacity has to be derated based on the actual installation method and local
environment. The derating factors considered in the calculation are as follows:
3.2.1 Temperature Derating Factor
Derating factor K1 considered for offshore air installation is of 0.91 in accordance IEC 60364-5-52
D Table A.52-14 for 40°C design ambient temperature.
3.2.2 Cable Laying Group Derating Factor
The group derating factor K2 for multi-core cables are considered is 0.73, based on IEC 60092-352
Table A.7 for 9 cables and more, laid on cable ladder and touching each other in single layer.
3.2.3 Overall correction factor
3.3 Full Load Current
The full load current calculation is given by the following equations:
For AC Three Phase System:
√ √
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VOLTAGE DROP AND CABLE SIZING
For DC Systems:
In which,
S = Equipment Rating (kVA)
P = Equipment Rating (kW)
IFL = Full Load Current (Ampere)
Vn = Voltage Nominal (Volt)
cos Ø = Power Factor (per unit)
Eff = Efficiency (%)
3.4 Continuous Current Carrying Capacity
The cable shall be selected so that the design load does not exceed the continuous current carrying
capacity of the cable:
In which,
IZ = Cable continuous current carrying capacity rating (A)
= I N× n × K
IN = Cable ampacity based on ambient temperature rating 30oC (A)
n = Number of parallel cable
K = Total derating factor
IFL = Full load current (A)
3.5 Voltage Drop
3.5.1 Voltage Drop under Steady State Condition
Voltage drops under steady state conditions are given by the following formulae as per IEEE 141-
1993 Section 3.11:
For AC Three Phase Systems:
For DC Systems:
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For all voltage drop equations, the percentage voltage drop is given by:
In which,
Vn = Nominal Voltage (V)
Vd = Voltage Drop across the cable (V)
Vdn = Percentage of Voltage Drop (%)
IFL = Full load current (A)
= Power Factor (per unit)
R = Cable Resistance (ohm/1km)
X = Cable Reactance (ohm/1 km)
L = Estimated Cable Length (m)
n = Number of cable in parallel per phase
3.6 Minimum Cable Size due to Short Circuit Current
3.6.1 Short-Circuit Thermal Withstand Capacity
The maximum temperature for XLPE cables as stated above is 250 ºC so the thermal stress due to
overcurrent and its duration should be so that the core insulation is under 250 ºC.
The cable shall be sized so that the given fault current will not raise the cable temperature over the
permissible limiting temperature that will be damage the cable. The following equation to find the
minimum suitable cable size derived from the IEC 60364-5-54:
In which,
S = Minimum cross-sectional area of conductor in mm2
I = Value of fault current in amperes
t = Trip time of protection 0.01 second for fuse protection.
k = Factor dependent on the material of the conductor, the insulation and other parts and the
initial and final temperatures, For XLPE insulated cable, initial, and final temperatures
equal of, respectively, 90 °C and 250°C, k = 143 as per IEC 60364-5-54 Table A.54.4.
3.6.2 Cables Protected by Fuse/MCCB
Short circuit protection of LV motors (shall be provided by fuses or circuit breaker. The power feeder
service cables shall be protected by the type circuit breakers).
The minimum cable protected by HRC fuses and/or MCCB shall be selected to ensure that the limited
let through energy (I2t) of fuse or MCCB is less than the I2t energy withstand of the cable.
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VOLTAGE DROP AND CABLE SIZING
Based on equation in section 3.6.1;
The above equation characterizes the time in seconds (t) during which a conductor of c.s.a (S) mm2
can carry current (I) without thermal damage to the insulation. For purpose of these calculations,
conductor is assumed to be copper (k = 143) with XLPE insulation.
4. SUMMARY OF CALCULATION RESULTS
The calculation is carried-out based on the method described previously. The results of the
calculations are presented in under Attachment-3 on the following pages.
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VOLTAGE DROP AND CABLE SIZING
IEC REFERENCE
Temperature Correction Factor
(Excerpts from IEC 60364-5-52, Table A.52-14)
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Group Correction Factor
(Excerpts from IEC 60092-352 Ed.3, Table A.7)
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VOLTAGE DROP AND CABLE SIZING
Group Correction Factor
(Excerpts from IEC 60092-352 Ed.3, Table A.8)
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VOLTAGE DROP AND CABLE SIZING
Group Correction Factor
(Excerpts from IEC 60364-5-54 Table A.54.4)
APPENDIX 2 – Minimum Cable Size Chart Based On MCCB Selection
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