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Sunday, 27 October 2013

Busbar size and calculation


Busbar
Bus bar 

A bus bar (also spelled busbar, buss bar or busbar), is a strip or bar of copper, brass or aluminum that conducts electricity within a switchboard, distribution board, substation, battery bank or other electrical apparatus. Its main purpose is to conduct electricity, not to function as a structural member.
Busbars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. A hollow section has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor switch yards.
A busbar may either be supported on insulators, or else insulation may completely surround it. Busbars are protected from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Power Neutral busbars may also be insulated. Earth (safety grounding) busbars are typically bare and bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus.
Busbars may be connected to each other and to electrical apparatus by bolted, clamp, or welded connections. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses, corona around the connections becomes a source of radio-frequency interference and power loss, so connection fittings designed for these voltages are used.
Busbars are typically contained inside switchgear, panel boards, or busway. Distribution boards split the electrical supply into separate circuits at one location. Busways, or bus ducts, are long busbars with a protective cover. Rather than branching the main supply at one location, they allow new circuits to branch off anywhere along the route of the busway.
Advantages
 Following are some advantages of Bus bar trunking system over normal cabling system:-
1.      On-site installation times are reduced compared to hard-wired systems, thus leading to cost savings.
2.      It provides increased flexibility in design and versatility with regard to future modifications.
3.      Greater safety and peace of mind for specifiers, contractors and end-users.
4.      Because of the simplicity of busbar, it is easy to estimate costs from the design/estimating stage through to installation on site.  This is because the technical characteristics and price of each component are always known.
5.      It is short sighted to compare the cost of busbar against that of a length of cable — and not the real cost of a cable installation to include multiple runs of cable, tray and fixing, let alone the protracted time and effort of pulling cables.
6.      Distribution busbar distributes power along its length through tap-off points along the busbar at typically at 0.5 or 1 m centers. Tap-off units are plugged in along the length of the busbar to supply a load; this could be a sub distribution board or, in a factory, to individual machines. Tap-offs can normally be added or removed with busbar live, eliminating production down time.
7.      Installed vertically the same systems can be used for rising-mains applications, with tap-offs feeding individual floors. Certified fire barriers are available at points where the busbar passes through a floor slab. Protection devices such as fuses, switchfuses or circuit breakers are located along the busbar run, reducing the need for large distribution boards and the large quantities of distribution cables running to and from installed equipment.
8.      Very compact so provides space savings.
9.      Where aesthetics have to be considered, busbar trunking can be installed with natural galvanized, aluminium, or painted finish. Special colours to match switchboards or a specific colour scheme are also available on request.
10.  Busbar trunking has several key advantages over conventional forms of power distribution including: -
11.  (a) Reduced, onsite installation times when compared to hard-wired systems thus leading to cost savings.
a.        Increased flexibility in design and versatility with regard to future modifications.
b.       Increased safety features brought about by the use of high quality, manufactured components, which provide greater safety and peace of mind for specifies, contractors and end-users.
12.  Uneven distribution of current takes place where multiple runs of cables are used in parallel.
13.  Busbar trunking has tap-off points at regular intervals along each length to allow power to be taken off and distributed to where it is needed. Because it is fully self-contained it needs only to be mechanically mounted and electrically connected to be operational.
14.  For higher ratings of power distribution we need to have multiple runs of cable. In such conditions unbalanced distribution of current takes place and causing overheating of some cable. This is completely avoided in the BTS systems.
15.  When multiple runs of cables are used it often leads to improper end connections thereby causing overheating of contacts, burning of cables ends, and is a major cause of fire. This is completely avoided in Bus Bar Trunking systems.
Current carrying capacity
The current-carrying capacity of a busbar is usually determined by the maximum temperature at which the bar is permitted to operate, as defined by national and international standards such as British Standard BS 159, American Standard ANSI C37.20, etc. These standards give maximum temperature rises as well as maximum ambient temperatures.
BS 159 stipulates a maximum temperature rise of 50°C above a 24 hour mean ambient temperature of up to 35°C, and a peak ambient temperature of 40°C.
ANSI C37.20 alternatively permits a temperature rise of 65°C above a maximum ambient of 40°C, provided that silver-plated (or acceptable alternative) bolted terminations are used. If not, a temperature rise of 30°C is allowed.
A very approximate method of estimating the current carrying capacity of a copper busbar is to assume a current density of 2 A/mm2 (1250 A/in2) in still air. This method should only be used to estimate a likely size of busbar, the final size being chosen after consideration has been given to the calculation methods. Refer catalogue of manufacturers.
The more popular thumb rule being followed in India is to assume current density of 1.0 Amps / Sq.mm for Aluminium and 1.6 Amps for Copper for any standard rectangular conductor profile.
Standard size of bus bar
Sr.
Application area
Cable
busbar
1
Number of circuits
One circuit per floor. Hence for a 20-floor building, you need 20 circuits.
Just one circuit can cover all floors.
2
Main Switchboard
Need 1 outgoing for each circuit. Hence 20 nos. MCCB outgoings. Higher cost and larger space requirement in electrical room
Need only 1 outgoing for each riser. Lower cost and size of main panel.
3
Shaft Size
Using 4 core cables, and considering 1 cable per feeder, you need 20 cables on the lowest floor. Large space required for cables/ cable tray.
Typical size of 1600A riser is 185mm x 180mm. Leads to big savings on riser shaft size, and hence more usable floor area on every floor.
4
Fire & safety
The high concentration of insulating materials used in cables and conductors involves a very high level of combustive energy.
The volume of insulating materials used in trunking is reduced to a minimum so combustive energy is considerably lower than cables. The insulating materials used do not release corrosive or toxic gases in the event of a fire. Once the source of the fire is removed, these materials are extinguished in a few seconds so that the effect of the fire is minimised
5
Future expansion
load on any floor exceeds initial plan, owner has to run an additional cable from a spare feeder on main board to that floor.
By providing extra tap off slots on each floor at the design stage, owner only has to procure a tap off box and plug it in wherever additional load is required. As the plug in can be done live, there is no shut down required for any of the existing clients / circuits. Future Flexibility.
6
Fault withstand levels
Limited by conductor size of each circuit.
Much higher – typically a 1600 A riser has a fault withstand capability of 60 to 70 kA. Safer in an electrical fault.
7
Installation time
Much longer
Each riser on a 20-floor building can be installed in approximately 2 to 3 days.
8
Voltage drop
High impedance if you choose cable size based on each floor current rating.
Much lower impedance. Hence substantially lower voltage drop.

Busbars Reduce System Costs
A laminated busbar will lower manufacturing costs by decreasing assembly time as well as internal material handling costs. Various conductors are terminated at customer specified locations to eliminate the guesswork usually associated with assembly operating procedures. A reduced parts count will reduce ordering, material handling and inventory costs.
 Bus bars Improve Reliability
 Laminated bus bars can help your organization build quality into processes. The reduction of wiring errors results in fewer reworks, lower service costs and lower quality costs.
Bus bars Increase Capacitance
 Increased capacitance results in decreasing characteristic impedance. This will ultimately lead to greater effective signal suppression and noise elimination. Keeping the dielectrics thin and using dielectrics with a high relative K factor will increase capacitance.
Eliminate Wiring Errors
By replacing a standard cable harnesses with bus bars, the possibility for miss-wirings is eliminated. Wiring harnesses have high failure rates relative to bus bars, which have virtually none. These problems are very costly to repair. Adding bus bars to your systems is effective insurance.
Bus bars Lower Inductance
 Any conductor carrying current will develop an electromagnetic field. The use of thin parallel conductors with a thin dielectric laminated together minimizes the effect of inductance on electrical circuits. Magnetic flux cancellation is maximized when opposing potentials are laminated together. Laminated bus bars have been designed to reduce the proximity effect in many semiconductor applications as well as applications that involve high electromagnetic interference (EMI).
Bus bars Lower Impedance
 Increasing the capacitance and reducing the inductance is a determining factor in eliminating noise. Keeping the dielectric thickness to a minimum will accomplish the highly desired low impedance.
Bus bars Provide Denser Packaging
 The use of wide, thin conductors laminated together led to decreased space requirements. Laminated bus bars have helped decrease total system size and cost.
Bus bars Provide Wider Variety of Interconnection Methods
 The flexibility of bus bars has allowed an unlimited number of interconnection styles to choose from. Bushings, embossments, and fasten tabs are most commonly used.
Bus bars Improve Thermal Characteristics
 The wide, thin conductors are favourable to allowing better airflow in systems. As package sizes decrease, the cost of removing heat from systems has greatly increased. A bus bar cannot only reduce the overall size required, but it can also improve airflow with its sleek design.
Material: The copper will be of ETP grade as per DIN 13601-2002 and with oxygen free copper.
Chemical composition: Purity of copper will be as per DIN EN 13601:2002. Copper + Silver 99.90% min.
Typical example
Rating Current: 3200Amp.
System:415Vac, TPN, 50Hz.
Fault Level: 50KA. For 1 Sec.
Operation Temp:40° C rise over 45 ° C ambient.

CONSIDERATION
Enclosure size: 1400 mm. wide X 400mm. height
Bus bar Size: 2:200x10 per Ph., 1:200x10 for Neutral.
Bus bar material: Electrolytic gr. Al. (IS 63401/AA6101)

 Short Circuit Rating
-upto 400A rated current:                   25KA for 1 sec.
-600 to 1000A rated current:              50KA for 1 sec.
-1250 to 2000A rated current:            65-100KA for 1 sec.
-2500 to 5000A rated current:            100-225KA for 1 sec.

The minimum cross section needed in sqmm for busbar in various common cases can be listed as below-

Material
Fault level (KA)
Withstand time


1 sec.
200 msec.
40 ms.
10 ms.

Aluminium
35
443
198
89
44
50
633
283
127
63
65
823
368
165
82







Copper
35
285
127
57
28
50
407
182
81
41
65
528
236
106
53


Let us select a busbar with an example:
1)  Aluminium busbar for 2000A, 35 kA for 1 sec withstand – From the table the minimum cross-section needed would be 443 mm2. Thus we can select a 100mm x 5mm busbar as the minimum cross-section. Considering a current density of 1A/ mm2 by considering temperature  as well as skin effect, we shall require 4 x 100mm x 5mm busbars for this case.

2)  Copper busbar for 2000A, 35 kA for 1 sec withstand – From the table the minimum cross-section needed would be 285 mm2. Thus we can select a 60mm x 5mm busbar as the minimum cross-section. Considering a current density of 1.6A/ mm2 by considering temperature as well as skin effect, we shall require 4 x 60mm x 5mm busbars for this case.

 Thus, by using the above formula and table, we can easily select busbars for our switchboards.
Size in mm
Area sqmm
Weight/ km
current carrying capacity in amp ( copper ) at 35 deg.C
AC ( no. of bus)
DC ( no. of bus)
I
II
III
II II
I
II
III
II II
12X2
24
0.209
110
200


115
205


15X2
30
0.262
140
200


145
245


15X3
75
0.396
170
300


175
305


20X2
40
0.351
185
315


190
325


20X3
60
0.529
220
380


225
390


20X5
100
0.882
295
500


300
510


25X3
75
0.663
270
460


275
470


25X5
125
1.11
350
600


355
610


30X3
90
0.796
315
540


320
560


30X5
150
1.33
400
700


410
720


40X3
120
1.06
420
710


430
740


40X5
200
1.77
520
900


530
930


40X10
400
3.55
760
1350
1850
2500
770
1400
2000

50X5
250
2.22
630
1100
1650
2100
650
1150
1750

50X10
500
4.44
920
1600
2250
3000
960
1700
2500

60X5
300
2.66
760
1250
1760
2400
780
1300
1900
2500
60X10
600
5.33
1060
1900
2600
3500
1100
2000
2800
3600
80X5
400
3.55
970
1700
2300
3000
1000
1800
2500
3200
80X10
800
7.11
1380
2300
3100
4200
1450
2600
3700
4800
100X5
500
4.44
1200
2050
2850
3500
1250
2250
3150
4050
100X10
1000
8.89
1700
2800
3650
5000
1800
3200
4500
5800
120X10
1200
10.7
2000
3100
4100
5700
2150
3700
5200
6700
160X10
1600
14.2
2500
3900
5300
7300
2800
4800
6900
9000
200X10
2000
17.8
3000
4750
6350
8800
3400
6000
8500
10000

Temperature rise

During the short circuiting, the bus bar should be able to withstand the thermal as well as mechanical stress. When a sort circuiting takes place, the temperature rise is directly proportional to the squire of the rms value of the fault current. The duration of short circuiting is very small i.e. one second till the breakers opens and clears the fault. The heat dissipation through convection and radiation during this short duration is negligible and all the heat is observed by the busbar itself. The temperature rise due to the fault can be calculated by applying the formulae.

T = K (I/A) 2 (1+αθ) 10-2
T=temperature rise per second
A= conductor cross section area
α = temperature coefficient of resistivity at 20 deg.C/deg.C
   = 0 .00393 for copper
   = 0 .00386 for aluminium
K = constant
    =0.52 for copper
    =1.166 for aluminium
θ = temperature of the conductor at the instant at which the temperature rise is being calculated.

Typical calculation

Rated current = 1000A
Fault current = 50KA for 1 sec
Permissible temperature rise= 40 deg.C
Busbar material =aluminium ally E91E
De-rating factor due to material =1
De-rating factor due to temperature rise =0.86
De-rating factor due to enclosure =0.75
Total de-rating factor = 1x0.75x0.86=0.66
Minimum cross section area required to withstand short circuit for 1 sec.
                  = (Ifc xt )/0.08
Where, Ifc = fault level current in KA
                 t= 1 second
Area A = (50x√1 )/0.08 = 625 sqmm
 Considering all de-rating factor, A = 625/0.66 =946.97
Say, cross sectional area per phase = 1000 sqmm
For neutral, cross sectional area per phase = 500 sqmm


for more study -refer a practical guide to cable installation and tool box talk
In India -

Available with book shop and -


Price: Rs. 375/- excluding delivery charges




23 comments:

  1. main switchboards
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    ReplyDelete
  2. I would like to recommended your blog. You can also refer to us 11KV Busduct see this for more information.

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  3. Reader can see more information on
    http://www.sanjaytechnical.in/11kv-bus-duct/11kv-bus-duct.html

    ReplyDelete
    Replies
    1. How to select BUS BAR for chiller control panel when chiller takes 352 AMPS,400V, 50KA

      Delete
    2. 40 X 6 MM COPPER BUSBAR CAN HOLD 50KA FOR 0.25 SEC WITH RUNNING BUSBAR CAPACITY OF 415 AMPS

      Delete
  4. Under Typical Calculation,How do you have taken 0.08 value,please confirm?

    ReplyDelete
  5. How to calculate bus bar current

    ReplyDelete
    Replies
    1. IF BUSBAR IS OF 40X6 MM
      SO 40X6=240
      CONDUCTIVITY OF COPPER=1.6 A/MM2
      CONDUCTIVITY OF ALUMMINIUM=0.8 A/MM2

      SO IF COPPER BUSBAR = 240 X 1.6 =384 AMPERE
      ALUMINIUM BUSBAR= 240 X 0.8=192 AMPERE

      Delete
  6. How to calculate bus bar current

    ReplyDelete
  7. I want to clearly which product make cross sectional area. Please clear me.

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  8. Calculate your BMI by using our Body mass index Calculator for adults & children. Calculated by dividing the weight in kilograms by the square of height in meters. Weight Calculator

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  9. which bus bar is better? Having current density 1.2 or 1.6

    ReplyDelete
    Replies
    1. SURELY 1.6, SO CURRENT FLOWING THROUGH MM2 OF BUSBAR IS MORE

      Delete
  10. Calculate your BMI by using our Body mass index Calculator for adults & children. Calculated by dividing the weight in kilograms by the square of height in meters. Weight Calculator

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  11. My understanding of doing this is that due to that BELLY FAT DOES NOT HAVE ANY VAINS IN IT - which makes it imposible for blood stream to reach the fatty sources there to consume them (Thats why the belly fat resists to be burned out and lasts longer than other body fats), when some certain demage there is made, a body needs to develop a way to reach the damaged areas and clean that out .. when a body at a same time lacks an energy - voila - an energy source is finally found عملية بالون المعدة . After some belly punching sessions بالون المعدة, body may develop a circulation system there and burn the belly fat along your regular training. I dont have any prove for this, it is just my theory, I just begun this prctice few days ago, because I remembered when I was taking some self defense lessons some years back, each lesson was starting with this "unplesant" excercise (in pairs). A trainer was former boxer عملية البالون , so I think this is something a boxers have some experience with. I can say just from this few days experience I started now, Im positive about the effects, I can say my belly is for few days much warmer - which I hope is due to blood circulation, before that it was cold all the time even during ab trainings. Im trying to not hurt myself, always asking a friend in gym, when my abs are relaxed and tensed enough in a same time to punch me all over my abs for few minutes (2-3m) with increasing intensity - until I feel fine with the intensity. عملية بالون المعده في مصر

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  12. As per my requirement, I am searching Bus Bar Cross Section (Thickness & Width) for 3A, 5A, 10A, 30A, 40A, 100A, 250A Current rating.

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  14. if i have a load 500 kva and 800 kva both have feed from (far 50m and 150m from panel resp.)transformer panel, how can calculate the specified bus duct (only one bus duct for both),also can we devide main bus duct into two branches with two different rates for two loads?

    ReplyDelete
  15. = (Ifc x√t )/0.08

    what is 0.08? how did u calculated it ? please explain

    ReplyDelete
  16. what is the current of 25x3mm copper bus bar

    ReplyDelete