Chapter G

Sizing and protection of conductors

Sizing of busbar trunking systems (busways)

From Electrical Installation Guide

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 making the selection of busways much more straightforward than the sizing of a traditional distribution with cables.

The cross section area of any given model has been determined by the manufacturer based on:

  • The rated current,
  • An ambient air temperature equal to 35 °C,
  • 3 loaded conductors.

Rated current

The rated current can be calculated taking into account:

  • The layout,
  • 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 is provided by the busway manufacturer. As an example, for Schneider Electric medium and high power range (up to 4000 A) the correction factor is given in Figure G24.

Fig. G24 – Correction factor for air temperature higher than 35 °C
°C 35 40 45 50 55
Correction factor 1 0.97 0.93 0.90 0.86

Neutral current

Where 3rd 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 G25 represents the maximum admissible phase and neutral currents (per unit) in a high power busbar trunking system as functions of 3rd harmonic level. For more information, see Harmonic currents in the selection of busbar trunking systems (busways).

Fig. G25 – Maximum admissible currents (p.u.) in a busbar trunking system as functions of the 3rd harmonic level

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.

  • One single distribution line serves a 4 to 6 meter area
  • Protection devices for current consumers are placed in tap-off units, connected directly to usage points.
  • 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 In on the distribution line.

In is equal to the sum of absorbed currents by the current In consumers: In = Σ IB.

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 kS : In = Σ (IB . kS).

Fig. G26 – Rated diversity factor according to the number of current consumers
Application Number of current consumers Ks Coefficient
Lighting, Heating 1
Distribution (engineering workshop) 2...3 0.9
4...5 0.8
6...9 0.7
10...40 0.6
40 and over 0.9 0.5
  • Note: for industrial installations, remember to take into account the upgrading of the machine equipment base. As for a switchboard, a 20 % margin is recommended: In ≤ IB x ks x 1.2