# How To Calculate The Bus Bar Size For Panel Design

Bus Bar Size:In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high voltage equipment at electrical switchyards, and low voltage equipment in battery banks. They are generally uninsulated, and have sufficient stiffness to be supported in air by insulated pillars. These features allow sufficient cooling of the conductors, and the ability to tap in at various points without creating a new joint.

The term busbar is derived from the Latin word omnibus which translates into English as “for all”, indicating that a busbar carries all of the currents in a particular system.

The material composition and cross-sectional size of the busbar determine the maximum amount of current that can be safely carried. Busbars can have a cross-sectional area of as little as 10 square millimetres (0.016 sq in), but electrical substations may use metal tubes 50 millimetres (2.0 in) in diameter (20 square millimetres (0.031 sq in)) or more as busbars. An aluminium smelter will have very large busbars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminium from molten salts.

Busbars are produced in a variety of shapes, such as flat strips, solid bars, or rods, and are typically composed of copper, brass, or aluminium as solid or hollow tubes. Some of these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes 50–60 Hz AC busbars more than about 8 millimetres (0.31 in) thickness inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section also has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor electrical switchyards.

A busbar must be sufficiently rigid to support its own weight, and forces imposed by mechanical vibration and possibly earthquakes, as well as accumulated precipitation in outdoor exposures. In addition, thermal expansion from temperature changes induced by ohmic heating and ambient temperature variations, as well as magnetic forces induced by large currents, must be considered. In order to address these concerns, flexible bus bars, typically a sandwich of thin conductor layers, were developed. These require a structural frame or cabinet for their installation.

Following details are required for calculating the size of Busbar

• Bus bar Current Details:
• Rated Voltage = 415V,50Hz ,
• Desire Maximum Current Rating of Bus bar =630Amp.
• Fault Current (Isc)= 50KA ,Fault Duration (t) =1sec.
• Bus bar Temperature details:
• Operating Temperature of Bus bar (θ)=85°C.
• Final Temperature of Bus bar during Fault(θ1)=185°C.
• Temperature rise of Bus Bar Bar during Fault (θt=θ1-θ)=100°C.
• Ambient Temperature (θn) =50°C.
• Maximum Bus Bar Temperature Rise=55°C.
• Enclosure Details:
• Installation of Panel= Indoors (well Ventilated)
• Altitude of Panel Installation on Site= 2000 Meter
• Panel Length= 1200 mm ,Panel width= 600 mm, Panel Height= 2400 mm
• Bus bar Details:
• Bus bar Material= Copper
• Bus bar Strip Arrangements= Vertical
• Current Density of Bus Bar Material=1.6
• Temperature Co efficient of Material Resistance at 20°c(α20)= 0.00403
• Material Constant(K)= 1.166
• Bus bar Material Permissible Strength=1200 kg/cm2
• Bus bar Insulating Material= Bare
• Bus bar Position= Edge-mounted bars
• Bus bar Installation Media= Non-ventilated ducting
• Bus bar Artificial Ventilation Scheme= without artificial ventilation
• Bus bar Size Details:
• Bus bar Width(e)= 75 mm
• Bus bar Thickness(s)= 10 mm
• Number of Bus Bar per Phase(n)= 2 No
• Bus bar Length per Phase(a)= 500 mm
• Distance between Two Bus Strip per Phase(e)= 75 mm
• Bus bar Phase Spacing (p)= 400 mm
• Total No of Circuit= 3 No.
• Bus bar Support Insulator Detail:
• Distance between insulators on Same Phase(l)= 500 mm
• Insulator Height (H)= 100 mm
• Distance from the head of the insulator to the bus bar center of gravity (h)= 5 mm
• Permissible Strength of Insulator (F’)=1000 Kg/cm2

De rating Factors for Bus bar:

• Per Phase Bus Strip De rating Factor (K1)
• Bus bar Width(e) is 75mm and Bus bar Length per Phase(a) is 500mm so e/a is 75/500=0.15
• No of Bus bar per phase is 2 No’s.
• From following table value of de rating factor is 1.83

Per Phase Bus Strip De rating Factor (K1)= Bus bar Width(e) / Bus bar Length per Phase(a)

• Bus bar Insulating Material De rating Factor (K2)
• Bus bar having No insulating material. It is Bare so following Table
• De rating Factor is 1.
• Bus bar Position De rating Factor (K3)
• Bus bar Position is Edge-mounted bars so following Table
• De rating Factor is 1
• Bus bar Installation Media De rating Factor (K4)
• Bus bar Installation Media is Non-ventilated ducting so following Table
• De rating Factor is 0.8
• Bus bar Artificial Ventilation De rating Factor (K5)
• Bus bar Installation Media is Non-ventilated ducting so following Table
• De rating Factor is 0.9
• Enclosure & Ventilation De rating Factor (K6)
• Bus bar Area per Phase = Bus width X Bus Thickness X Length of Bus X No of Bus bar per Phase
• Bus bar Area per Phase = 75x10xX500X2= 750000mm
• Total Bus bar Area for Enclosure= No of Circuit X( No of Phase + Neutral )X Bus bar Area per Phase
• Here we used Size of Neutral Bus is equal to Size of Phase Bus
• Total Bus bar Area for Enclosure=3X(3+1)X750000mm
• Total Bus bar Area for Enclosure=9000000 Sq.mm
• Total Enclosure Area= width X Height X Length
• Total Enclosure Area=1200x600x2400=1728000000 Sq.mm
• Total Bus bar Area for Enclosure / Total Enclosure Area =9000000/1728000000
• Total Bus bar Area for Enclosure / Total Enclosure Area=0.53%
• Bus bar Artificial Ventilation Scheme is without artificial ventilation so following Table
• De rating Factor is 0.95
• Proxy Effect De rating Factor (K7)
• Bus bar Phase Spacing (p) is 400mm.
• Bus bar Width (e) is 75mm and Space between each bus of Phase is 75mm so
• Total Bus length of Phase with spacing  =75+75+75+75+75=225mm
• Bus bar Phase Spacing (p) / Total Bus length of Phase with spacing  = 400 / 225 =2
• From following Table De rating factor is 0.82
• Altitude of Bus Bar installation De rating Factor (K8)
• Altitude of Panel Installation on Site is 2000 meter so following Table
• De rating Factor is 0.88
• Total De rating Factor= K1XK2XK3XK4XK5XK6XK7XK8
• Total De rating Factor =1.83x1x1x0.8×0.9×0.95×0.82×0.88
• Total De rating Factor =0.90

Bus bar Size Calculation:

• Desire Current Rating of Bus bar (I2) =630 Amp
• Current Rating of Bus bar after De rating Factor (I1)= I2 X De rating Factor or I2 / De rating Factor
• Current Rating of Bus bar after De rating Factor (I1)=630×0.9
• Current Rating of Bus bar after De rating Factor (I1)=697Amp
• Bus bar Cross Section Area as per Current= Current Rating of Bus bar / Current Density of Material
• Bus bar Cross Section Area as per Current= 697 / 1.6
• Bus bar Cross Section Area as per Current= 436 Sq.mm
• Bus bar Cross Section Area as per Short Circuit= Isc X√ ((K/( θtx100)x(1+ α20xθ) xt
• Bus bar Cross Section Area as per Short Circuit= 50000X√ ((1.166/( 100×100)x(1+ 0.00403×85) x1
• Bus bar Cross Section Area as per Short Circuit=626 Sq.mm
• Select Higher Size for Bus bar Cross section area between 436 Sq.mm and 626 Sq.mm
• Final Calculated Bus Bar Cross Section Area =626 Sq.mm
• Actual Selected Bus bar size is 75×10=750 Sq.mm
• We have select 2 No’s of Bus bar per Phase hence.
• Actual Bus bar cross section Area per Phase =750×2= 1500 Sq.mm
• Actual Cross Section Area of Bus bar =1500 Sq.mm
• Actual Bus bar Size is Less than calculated Bus bar size.

Forces generated on Bus Bar due to Short Circuit Current

• Peak electro-magnetic forces between phase conductors (F1) = 2X(l/d)X(2.5xIsc)2/100000000
• Total width of Bus bar per Phase(w)=75+75+75=225mm =2.25cm
• Bus bar Phase to Phase Distance (d)=400+225=625mm=6.25cm
• Peak electro-magnetic forces between phase conductors (F1) =2x(50/63)x(2.5×50000)2/100000000
• Peak electro-magnetic forces between phase conductors (F1)=250 Kg /cm2
• Peak electro-magnetic forces between phase conductors (F1)=2.5 Kg /mm2
• Actual Forces at the head of the Supports or Bus Bar (F)=F1X(H+h/H)
• Actual Forces at the head of the Supports or Bus Bar (F)=2.5x(100+5/100)
• Actual Forces at the head of the Supports or Bus Bar (F)= 3 Kg /mm2
• Permissible Strength of Insulator (F’) is 10 Kg/mm2
• Actual Forces at the head of the Supports or Bus Bar is less than Permissible Strength
• Forces on Insulation is in within Limits

Mechanical strength of the bus bars

• Mechanical strength of the bus bars=(F1X i /12)x(1/ Modulus of inertia of a bus bar )

Value of Modulus of inertia of a bus bar (i/v) = No of Bus Strip per Phase

• From above table Value of Modulus of inertia of a bus bar=14.45
• Mechanical strength of the bus bars=(250×50/12)X(1/14.45)
• Mechanical strength of the bus bars= 72 Kg/cm2
• Mechanical strength of the bus bars= 0.72 Kg/mm2
• Permissible Bus bar Strength is 12 Kg/mm2
• Actual Mechanical Strength is less than Permissible Strength
• Mechanical strength of Bus bar is in within Limit

Temperature Rise Calculation

• Specified Maximum Temperature Rise (T1) is 35°c
• Calculated Maximum Temperature Rise (T2)=T/(log(I1/I2)1.64)
• Calculated Maximum Temperature Rise (T2)=35/(Log(697/630)1.64)
• Calculated Maximum Temperature Rise (T2)= 30°c
• Calculated Bus bar Temperature rise is less than Specified Max Temperature rise
• Temperature Rise is in within Limit

## Results:

• Size of Bus bar = 2No’s 75x10mm per Phase.
• Total No of Feeder =3 No’s
• Total No’s of Bus bar = 6 No’s 75x10mm for Phase and 1No’s 75x10mm for Neutral.
• Forces at the head of the Supports or Bus Bar (F)= 3kg/mm2
• Mechanical strength of the bus bars= 0.7 Kg/mm2
• Maximum Temperature Rise=30°c