Chapter A General rules of electrical installation design

G - Sizing and protection of conductors
8 Worked example of cable
calculation
Protection against indirect-contact hazards
For circuit C6 of Figure G65, Figures F45 and F61, or the formula given page F27
may be used for a 3-phase 3-wire circuit.
The maximum permitted length of the circuit is given by :
0.8 x 240 x 230 3 x 1,000
Lmax
2 x 22.5 1+ 240
=
= 70 m
⎛ ⎞
x 630 x 11
⎝ 95 ⎠
(The value in the denominator 630 x 11 I=
Im i.e. the current level at which the
instantaneous short-circuit magnetic trip of the 630 A circuit-breaker operates).
The length of 15 metres is therefore fully protected by “instantaneous” overcurrent
devices.
Voltage drop
From Figure G28 it can be seen that:
b For the cable C1 (6 x 95mm2 per phase)
-1 -1
∆U
0.42 (V A km ) x 1,374 (A) x 0.008 = = 1.54 V
3
∆U%
100 = 1.54 = 0.38%
400 x
b For the circuit C6
∆U
0.21 (V A km ) x 433 (A) x 0.015 -1 -1
= = 1.36 V
3
∆U%
100 = 1.36 = 0.34%
400 x
At the circuit terminals of the LV/LV transformer the percentage volt-drop
ΔU% = 0.72%
Schneider Electric - Electrical installation guide 2008
G49
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