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How to select busbar for electrical systems
Busbar Voltage Drop
The Busbar voltage drop is the expected resistive voltage
drop on a busbar circuit, based on the length
and cross sectional area of the bar. There may be an additional voltage drop
due to the inductance of the bar. This can become particularly important at
high frequencies and high currents. Where there are a number of bars in
parallel, assume the bar width is the actual width multiplied by the number of
bars in parallel. i.e. 5 bars of 50 x 6 mm in parallel would give the same resistive
voltage drop as a single bar of 50 x 30mm.
To calculate the resistive voltage drop of a length of
busbar, enter in the width, length and thickness of the bar. Select the units
as either metric or imperial. and the current passing through the bar. The
circuit configuration also needs to be specified. "Single bar" refers
to the voltage drop along a single length of bar, while "Single
Phase" refers to the voltage drop of two equal lengths of bar, one in the
active circuit and one in the neutral circuit. "Three Phase"
calculates the voltage drop between the supply and a three phase load where
three equal bars are used for the three phase circuits. Enter the ambient
temperature around the bar as Celsius or Fahrenheit and the program will check
the suitability of the bar for that current. The program displays the resistive
voltage drop for both an aluminium bar of these dimensions and a copper bar of
these dimensions
Busbar Power Dissipation
The total Power
Dissipated in the busbar is dependent on the resistance of the bar, it's length
and the square of the RMS current flowing through it The power dissipated in
the busbar is proportional to the square of the current, so if the busbar has a
cyclic load, the current should be the RMS current rather than the average. If
the maximum current flows for a considerable period of time, this must be used
as the current to determine the maximum busbar temperature, but the power
dissipation is based on the square root of the maximum current squared times
the period for which it flows plus the lower current squared times the period
it flows all divided by the square root of the total time. For example, a
busbar carries a current of 600 Amps for thirty seconds, then a current of 100
amps for 3000 seconds, then zero current for 3000 seconds. The power
dissipation is based on an RMS current of sqrt(600x600x30 + 100x100x3000 + 0 x
3000)/sqrt(30 + 3000 + 3000) = 82.25 Amps.
To
calculate the Power Dissipation of a busbar, enter in the width, length and
thickness of the bar, and the RMS Current passing through it. Select the units as
either metric or imperial. The program displays the Power Dissipated in both an
aluminium bar of these dimensions and a copper bar of these dimensions. Enter
the ambient temperaturearound the bar in either Celsius or Fahrenheit
and the program will check the suitability of the bar for this application.Busbar Ratings
Busbar ratings are based on the expected surface
temperature rise of the busbar. This is a function of the thermal resistance of
the busbar and the power it dissipates. The thermal resistance of the busbar is
a function of the surface area of the busbar, the orientation of the busbar,
the material from which it is made, and the movement of air around it. The
power dissipated by the bus bar is dependent on the square of the current
passing through it, its length, and the material from which it is made. Optimal
ratings are achieved when the bar runs horizontally with the face of the bar in
the vertical plane. i.e. the bar is on its edge. There must be free air
circulation around all of the bar in order to afford the maximum cooling to its
surface. Restricted airflow around the bar will increase the surface
temperature of the bar. If the bar is installed on its side, (largest area to
the top) it will run at an elevated temperature and may need considerable
derating. The actual derating required depends on the shape of the bar. Busbars
with a high ratio between the width and the thickness, are more sensitive to
their orientation than busbars that have an almost square cross section. Vertical
busbars will run much hotter at the top of the bar than at the bottom, and
should be
derated in order to reduce the maximum temperature within
allowable limits. Maximum Busbar ratings are not the temperature at which the
busbar is expected to fail, rather it is the maximum temperature at which it is
considered safe to operate the busbar due to other factors such as the
temperature rating of insulation materials which may be in contact with, or close
to, the busbar. Busbars which are sleeved in an insulation material such as a
heatshrink material, may need to be derated because of the potential aging and
premature failure of the insulation material.
The Maximum Current rating of Aluminium Busbars is based
on a maximum surface temperature of 90 degrees C (or a 60 degree C temperature
rise at an ambient temperature of 30 degrees C). If a lower maximum temperature
rating is desired, increase the ambient temperature used for the calculations.
i.e. If the actual ambient temperature is 40 degrees C and the desired maximum
bar temperature is 80 degrees C, then set the ambient temperature in the
calculations to 40 + (90-80) = 50 degrees C. The Maximum Current rating of Copper Busbars is
based on a maximum surface temperature of 105 degrees C (or a 75 degree C
temperature rise at an ambient temperature of 30 degrees C).
The Busbar Width
is the distance across the widest side of the busbar, edge to edge.
The Busbar Thickness is the thickness of the material
from which the Busbar is fabricated. If the busbar is manufactured from a
laminated material, then this is the overall thickness of the bar rather than
the thickness of the individual elements. The
Busbar Length is the total length of busbar used.
The Busbar Current
is the maximum continuous current flowing through the busbar. The power dissipated
in the busbar is proportional to the square of the current, so if the busbar
has a cyclic load, the current should be the RMS current rather
than the average. If the maximum current flows for a considerable period of
time, this must be used as the current to determine the maximum busbar
temperature, but the power dissipation is based on the square root of the
maximum current squared times the period for which it flows plus the lower
current squared times the period it flows all divided by the square root of the
total time. For example, a busbar carries a current of 600 Amps for thirty
seconds, then a current of 100 amps for 3000 seconds, then zero current for
3000 seconds. The power dissipation is based on an RMS current of
sqrt(600x600x30 + 100x100x3000 + 0 x 3000)/ssqrt30 + 3000 + 3000) = 82.25 Amps.
The Ambient
Temperature is the temperature of the air in contact with the busbar. If the
air is in an enclosed space, then the power dissipated by the busbar will cause
an increase in the ambient temperature within the enclosure.
To calculate the rating of a busbar, enter in the width
and thickness of the bar, and the ambient temperature around the bar. Select
the units as either metric or imperial, and the temperature as Celsius or
Fahrenheit. The program displays both the current rating of an aluminium bar of
these dimensions and a copper bar of these dimensions.
