Chapter A General rules of electrical installation design

G18
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G - Sizing and protection of conductors
2 Practical method for determining
the smallest allowable crosssectional
area of circuit conductors
2.4 Busbar trunking systems
The selection of busbar trunking systems is very straightforward, using the data
provided by the manufacturer. Methods of installation, insulation materials, correction
factors for grouping are not relevant parameters for this technology.
The cross section area of any given model has been determined by the manufacturer
based on:
b The rated current,
b An ambient air temperature equal to 35 °C,
b 3 loaded conductors.
Rated current
The rated current can be calculated taking account of:
b The layout,
b The current absorbed by the different loads connected along the trunking system.
Ambient temperature
A correction factor has to be applied for temperature higher than 35 °C. The
correction factor applicable to medium and high power range (up to 4,000 A) is given
in Figure G23a.
°C 35 40 45 50 55
Correction factor 1 0.97 0.93 0.90 0.86
Fig. G23a : Correction factor for air temperature higher than 35 °C
Neutral current
Where 3 rd harmonic currents are circulating, the neutral conductor may be carrying a
significant current and the corresponding additional power losses must be taken into
account.
Figure G23b represents the maximum admissible phase and neutral currents (per
unit) in a high power busbar trunking system as functions of 3 rd harmonic level.
Maximum admissible current (p.u)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 10 20 30 40
Fig. G23b : Maximum admissible currents (p.u.) in a busbar trunking system as functions of the
3 rd harmonic level.
Schneider Electric - Electrical installation guide 2008
50 60 70
3 rd harmonic current level (%)
Neutral conductor
Phase conductor
80 90