Current load capacity of copper and aluminium busbars

Introduction

The entry concerns selected issues related to low-voltage switchgear defined on the basis of IEC 61439 standard, as a set containing low-voltage switching devices equipped with a control, measurement, signalling, protection system together with electrical and mechanical connections and structural elements. In this article, we will focus on the selection of busbars.
The following pictures show a Modulor EPE-type low-voltage power distribution switchgear, manufactured by Radiolex. The busbar set is made of electrotechnical copper using three 80mm x 10mm flat bars per single phase – providing a current load capacity of 2800A.

Current load capacity

Current load capacity is the maximum value of the current flowing through the conductor in an unlimited period of time in certain conditions – it will heat up the conductor to the limit temperature value – without damaging it in the process. Current load capacity depends on multiple factors:

• material of the wires,
• cross-section of the wires,
• routing and number of the wires,
• type of insulation.

Also external conditions should be mentioned, namely:

• ambient temperature,
• routing of the conductors.

Current load capacity of copper busbars

Table 1 compares the current load capacity of copper for busbars: bare and tin-plated for the listed cross-sections. However, this data relates to a certain temperature of the busbars and air around them. A diagram of these temperatures allowing to determine the correction factor for copper busbars is given in DIN 43671 standard. It enables us to determine the cross-section of a copper busbar for the ambient temperature of 0°C to 60°C and the operating temperature up to 125°C – figure 1.

Tab. 1 Continuous currents for rectangular copper busbars in indoor equipment at the air temp. of 35°C and the busbar temp. of 65°C according to DIN 43670.

Fig. 1 The coefficient to determine a copper busbar cross-section depending on the ambient temperature ϑ_u and the busbar temperature ϑ_s.

Current load capacity of aluminium busbars

Table 2 compares the current load capacity of aluminium for busbars: bare and tin-plated for the listed cross-sections.

Tab. 2 Continuous currents for rectangular aluminium busbars in indoor equipment at the air temp. of 35°C and the busbar temp. of 65°C according to DIN 43670.