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Electrical > Energy Efficiency > Busbars > FAQs

Busbars

  1. How is the current carrying capacity of a busbar determined?
  2. Some sources suggest painting busbars black to increase the current rating.  Is this a good idea?
  3. Why is copper the preferred material for busbars?
  4. How should busbars be jointed?

Q How is the current carrying capacity of a busbar determined?

A The current rating of a busbar is limited by the maximum ambient temperature and the maximum permitted working temperature.  In use, heat is generated in the bar due to the resistance losses and is lost from the bar by convection and radiation.  At the working temperature, the heat generated is equal to the heat lost.  The calculation process is to make an educated guess at the bar size and determine the heat loss at working temperature and then calculate the current that would produce this amount of heat. The bar size is modified in light of the result and the calculation repeated until the result is close enough to the required rating.   The result is the smallest possible bar size.  As energy costs rise, it is worth considering the lifetime cost of a busbar system, including capital cost and the cost of waste energy.

To simplify the calculations, try using our Busbar calculation software Busbar Design Program download software and see also Pub 22 - Copper for Busbars web link

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Q Some sources suggest painting busbars black to increase the current rating.  Is this a good idea?

A No.  Black matt surfaces are better at radiating heat than bright shiny surfaces, and that is why painting is sometimes suggested.  The efficacy of radiation is the emissivity of the surface; for a perfect radiator (matt black), the emissivity is 1 and for a perfect reflector, emissivity is zero.  In practice, bright shiny copper has an emissivity of about 0.3, dull copper is about 0.7 and darkened copper – such as a busbar after a few weeks of service – can reach 0.9.  Since only around 10-20% of heat is lost by radiation, the difference between dull copper and a matt black surface is only 3-6% of the total.  To achieve this apparent improvement, the 90% of the heat that escapes by convection has to pass through the thermal insulation provided by the paint layer, so the temperature of the bar increases rather than decreases.

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Q Why is copper the preferred material for busbars?

A There are several reasons.  Firstly, the bars are more compact because copper has a higher conductivity than aluminium.  Secondly, it is much easier to joint copper. A hard, highly insulating oxide grows very rapidly on the surface of aluminium, making it very difficult to make good reliable joints in the field.  On the other hand, the thin oxide layer that forms, more slowly, on copper is not an insulator so jointing is simple. Finally, copper is much stronger than aluminium, less susceptible to creep and better at withstanding short circuit current forces.

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Q How should busbars be jointed?

A During installation, the preferred method is to bolt through overlapping sections.  For reliability, recommended practice regarding the size of overlap, number of bolts and, especially, torque settings should be followed. For modification of an existing installation, where drilling is undesirable due to the risk of swarf, clamped joints are preferred.

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