Chapter A General rules of electrical installation design

G48
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G - Sizing and protection of conductors
8 Worked example of cable
calculation
The resistances and the inductive reactances for the six conductors in parallel are,
for a length of 5 metres:
for a length of 5 metres:
22.5 x 5
R = = 0.20 mΩ (cable resistance: 22.5 22.5 m mΩ.mm
95 x 6
Ω
2 /m)
X = 0.08 x 5 = 0.40 mΩ (cable reactance: 0.08 mΩ/m)
Dimensioning circuit C6
Circuit C6 supplies a 400 kVA 3-phase 400/400 V isolating transformer
3
400. 10
Primary current = = 550 A
420. 3
A single-core cable laid laid on on a a cable cable tray tray (without (without any any other other cable) cable) in an in ambient an ambient air air
temperature of 30 °C is proposed. The circuit-breaker is set at 560 A
The method of installation is characterized by the reference letter F, and the “k”
correcting factors are all equal to 1.
A c.s.a. of 240 mm 2 is appropriate.
The resistance and inductive reactance are respectively:
22.5 x 15
R = = 1.4 mΩ
240
X = 0.08 x 15 = 1.2 mΩ
Calculation of short-circuit currents for the selection of circuit-breakers
Q 1 and Q 6 (see Fig. G67)
Circuits components R (mΩ) X (mΩ) Z (mΩ) Ikmax (kA)
parts
500 MVA at 0.04 0.36
the MV source network
1 MVA transformer 2.2 9.8 10.0 23
Cable C1 0.20 0.4
Sub-total for Q1 2.44 10.6 10.9 23
Busbar B2 3.6 7.2
Cable C6 1.4 1.2
Sub-total for Q6 4.0 8.4 9.3 20
Fig. G67 : Example of short-circuit current evaluation
The protective conductor
Thermal requirements: Figures G58 and G59 show that, when using the adiabatic
method the c.s.a. for for the the protective earth earth (PE) (PE) conductor conductor for circuit for C1 circuit will be: C1 will be:
34,800 x 0.2
143
2
= 108 mm
A single 120 mm 2 conductor dimensioned for other reasons mentioned later is
therefore largely sufficient, provided that it also satisfies the requirements for indirectcontact
protection (i.e. that its impedance is sufficiently low).
For the circuit C6, the c.s.a. of its PE conductor should be:
29,300 x 0.2
2
= 92 mm
143
In this case a 95 mm2 conductor may be adequate if the indirect-contact protection
conditions are also satisfied.
Schneider Electric - Electrical installation guide 2008