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.

                  = (I_fc x√t )/0.08

Where, I_fc = 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 -

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