The electrical power system consists of many incoming & outgoing feeder connections, for which busbars are necessary.
A busbar is just a node (conductor or collection of conductors) that gathers power from the incoming feeder and distributes it to the departing feeders.
A busbar size is determined by its material & current carrying capacity.
Formula
The Busbar Current is calculated using the following formula:
Ibb = w x t x MF
Where,
Ibb – Busbar Current (A)
w – Width (in millimeters)
t – Thickness (in millimeters)
MF – Material Carry Capacity Factor (amps/mm2)
- 1.2 for Copper
- 0.8 for Aluminum
- 0.6 for Iron
- 1.6 for Silver
To find the busbar current, multiply the width & thickness together, then multiply by the material carry capacity factor.
How to Calculate Busbar Current?
- Initially measure the width (mm).
- Following that, measure the thickness (mm).
- After that, calculate the material carrying capacity factor (amps/mm2).
- Finally, use the following formula to determine the busbar current.
Solved Example
Calculate the current carrying capability of a 150 (width) x 25 (thickness) (in mm) busbar in the copper material.
Ibb = w x t x MF
Ibb = 150 x 25 x 1.2
Ibb = 4500A
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Standards & References
IEC 60865-1: Short-circuit currents.
IEC 61439: Low-voltage switchgear and controlgear assemblies
IS 5082: IS 5082 specifies wrought aluminum and aluminum alloy bars, rods & sections for electrical applications.
IEEE Standard 80: Electrical safety in substations.
Why accurate Busbar Sizing is required?
When selecting a busbar, one must consider the application, present carrying capacity and costs since undersized busbars may result in heating and damage to the busbar while oversized busbars can increase the cost of the work.
How to calculate the Busbar Size?
Busbar size is calculated by calculating the desired cross-sectional area based on maximum load current, material and temperature limitations.
- Copper typically has a density of 1.2-1.6 A/mm² and
- Aluminum has 0.8-0.9 A/mm².
The formula
Area = Current/Density
sets the size using a 125% safety factor for safe design.