Chapter A General rules of electrical installation design

G - Sizing and protection of conductors
2 Practical method for determining
the smallest allowable crosssectional
area of circuit conductors
The layout of the trunking system depends on the position of the current consumers,
the location of the power source and the possibilities for fixing the system.
v One single distribution line serves a 4 to 6 meter area
v Protection devices for current consumers are placed in tap-off units, connected
directly to usage points.
v One single feeder supplies all current consumers of different powers.
Once the trunking system layout is established, it is possible to calculate the
absorbed current I n on the distribution line.
I n is equal to the sum of absorbed currents by the current I n consumers: I n = Σ I B.
The current consumers do not all work at the same time and are not permanently on
full load, so we have to use a clustering coefficient k S : I n = Σ (I B . k S).
Application Number of current consumers Ks Coefficient
Lighting, Heating 1
Distribution (engineering
workshop)
2...3
4...5
6...9
10...40
40 and over
Schneider Electric - Electrical installation guide 2008
0.9
0.8
0.7
0.6
0.5
Note : for industrial installations, remember to take account of upgrading of the machine
equipment base. As for a switchboard, a 20 % margin is recommended:
I n ≤ I B x k s x 1.2.
Fig G24 : Clustering coefficient according to the number of current consumers
G19
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