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Active Contributor

Join Date: Nov 2006
Posts: 15

# bus bar rating

07/28/2007 5:34 AM

dea sir,

iam very cinfused that for designing the bus bar size for both copper and aluminium what is the current density per mm sequre.

i have transformer of 1500KVA and for the power distribution board which have the load of 900kw what is the size of copper bus bar. my incomer rating for this load is 2500A aircircuit breaker .

can i take 1A= 1MM sequre

1A= 1.2MMsequre

1A= 1.6 MM sequre

is there any standerd for this rating , can i use the factor like bus bar joint ,no of holes please explain in breif,because due to this the system prices goes high.

thanks

bhupernder

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Anonymous Poster
#1

### Re: bus bar rating

07/29/2007 12:27 AM

Hi,

Please refer IEC-890 amendment 1. You can find out the required size of busbar and also it depend ambient temperature.

Iqbal

Guru

Join Date: Jun 2007
Location: chennai,India
Posts: 593
#2

### Re: bus bar rating

07/29/2007 2:38 AM

Since the transformer feeding the switch board is 1500 kVA, the switch board Incomer ACB and Bus bars are to designed for full load current of transformer.

In this case the FLC is 2100 Amps.It is therefore suggested for 2500 A ACB on the incomer.The load may be 900kw but the board has to be designed for trafo rating.

Sizing of Bus bars depends on ambient temperature, enclosure sizing,location of the switch board and also material of bus bar like Aluminum or copper.Short circuit with stand capacity of Bus bar,disposition of bus bar and number of bus bars.

You may go through a book on Aluminum Bus bar by Thomas and RATA

this book gives detailed calculations and selection of bus bars

What you have mentioned is only thumb rule.

Thumb rule for copper is 1.25 Amps per sq.mm and for Aluminum 0.8 amps for sq.mm

I am Switch board manufacturer and freelance consultant, for any other details you can post messages.

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Ramesh,Freelance Electrical/automation Consultant
Anonymous Poster
#12

### Re: bus bar rating

12/18/2010 1:42 AM

Sir i want to learn about the detailed calculation of fabrication of LT Panels,SDB,PDB etc Regards

Guru

Join Date: Oct 2006
Posts: 1571
#3

### Re: bus bar rating

07/29/2007 2:57 AM

"Rule of thumb": If the copper bar is tin plated, not insulated and has free air circulation you can run 1000Amps per square inch.

If there is forced ventilation (and you check the thermal rise) you may be able to double the figure.

Tin plating prevents the copper oxidizing.

Don't use silver, it is too reactive and forms whiskers between the bars that eventually short circuit.

Guru

Join Date: Dec 2006
Location: Long.92E,Lat.26N
Posts: 1336
#4

### Re: bus bar rating

07/29/2007 6:22 AM

1 Amp/mm2 is fine for OFHC Copper

0.6Amp/mm2 is fine for Alu.

Avoid holes whenever/wherever.

Clamp downcomers .

Some makers used to offer Spring-loaded wiping contacts -fine for Copper.

Clean/Scrape/Vaseline and tightly clamp for aluminium.

Guru

Join Date: Feb 2006
Posts: 1758
#6

### Re: bus bar rating

08/01/2007 7:58 PM

>>> Clean/Scrape/Vaseline ... <<<
For Aluminum Bus-Bars there is a "special-Protective-grease" or Vaseline.
Not a normal one.
Will you please name it. Any Brand. I forgot its make.

Power-User

Join Date: Oct 2006
Location: Indonesia
Posts: 102
#5

### Re: bus bar rating

07/30/2007 12:36 AM

Many things to be considered especially maximum permissible temperature rise (heat energy) as current increasing. You can refer to BS-159, ANSI C37.20, BS-5486, IEC 439. Normally maximum temp. on the bus bar are set 110oC for bar's cross sectional 3b 100x5 3400A with ambient temp.near bar is 50oC in still air. This is the suitable size for your bus bar, I guess.

But must be considered to bus bar deformation and rupture of insulation support due to short circuit fault. Duration of SC usually 1 to 4 seconds permitted for bus bar. At normal ambient temp, busbar specific heat is 385 J/kg K, and at 300oC the specific heat is around 410 J/kg K.

SC heating characteristics are difficult to state because the present of AC and DC components. But for approximately approach, follow the formula below :

t = 0.051 (A/I)2 [(√1+0.0076θ) - 1] , t=max.SC time(s), A=cross section(mm2), I=current(kA), θ=temp.rise(K).

Then temp.rise formula with regard to current ;

θ = 5.2 (I/A)2 x 103 , I/A should be less than 0.25 for reasonable accuracy.

This is subjected to copper busbar.

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Active Contributor

Join Date: Aug 2007
Location: Orange County, California.
Posts: 13
#7

### Re: bus bar rating

08/16/2007 4:46 AM

current ratings of laminated bars

Number and size of bars

Total Section

Current for stated temperature rise above 20°C ambient

mm

mm2

20°C

30°C

40°C

50°C

2 - 20 x 5

200

430

540

640

720

2 - 25 x 5

250

500

640

750

855

2 - 30 x 5

300

590

750

880

1000

3 - 30 x 5

450

800

1020

1200

1360

4 - 30 x 5

600

1030

1300

1530

1740

2 - 40 x 5

400

750

950

1120

1270

3 - 40 x 5

600

1030

1300

1530

1740

4 - 40 x 5

800

1260

1600

1890

2140

2 - 50 x 5

500

880

1120

1320

1500

3 - 50 x 5

750

1200

1520

1790

2030

4 - 50 x 5

1000

1500

1900

2240

2540

2 - 60 x 5

600

1030

1300

1530

1740

3 - 60 x 5

900

1380

1750

2060

2340

4 - 60 x 5

1200

1700

2150

2540

2880

2 - 80 x 5

800

1260

1600

1890

2140

3 - 80 x 5

1200

1700

2150

2540

2880

4 - 80 x 5

1600

2080

2630

3100

3520

2 - 100 x 5

1000

1460

1850

2180

2470

3 - 100 x 5

1500

1990

2520

2970

3370

4 - 100 x 5

2000

2420

3060

3610

4090

4 - 50 x 10

2000

2330

2950

3480

3950

4 - 60 x 10

2400

2580

3260

3850

4360

4 - 80 x 10

3200

2970

3760

4440

5030

3 - 100 x 10

3000

2880

3650

4300

4880

4 - 100 x 10

4000

3240

4100

4840

5480

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Participant

Join Date: Feb 2010
Posts: 4
#8

### Re: bus bar rating

02/17/2010 1:32 AM

Hello Everybody! Can any 1 give the calculations for breaking capacity of panel ?? or any related books.

Commentator

Join Date: Dec 2009
Location: Bangalore,India
Posts: 80
#9

### Re: bus bar rating

03/05/2010 12:32 AM

Dear Ankur,

The panel doesn't have any breaking capacity. it depends upon the short-circuit fault at the load end. the design of the entire electrical system including the panels varies from case to case basis. So one example will not be sufficient to explain everything. if you have any specific case, please mention the same with figures and following data

Transformer kVA, V,HZ, type of load,

Thanks

Participant

Join Date: Feb 2010
Posts: 4
#13

### Re: bus bar rating

12/19/2010 4:00 AM

Thanks Shanbhag.

If the Transformer is of 1500 KVA, 415V and 50 Hz. What is to be considered to design 50kA busbar short ckt rupturing capacity in a panel??

Regards,

Ankur Jariwala.

D.J.Electro Controls.

Anonymous Poster
#14

### Re: bus bar rating

12/20/2010 8:33 AM

Dear Ankur,

i have got a bit confused regarding your question because of the subsequent blogs after your question and the fact that its already 8 months after you have replied.

as for the answer, you have to refer some of the opinions and answers provided by other forum members above. As far as the design and dimensions of the busbars is concerned, some of the important features are :

1) short circuit withstand level at the outgoing side ( may be at load side or outgoing of busbar side)

2) temperature rise withstood by the busbars considering the panels enclosure dimensions.

3) the short circuit breaking capacity and the fault clearance time of the main breaker.

in order that the busbar withstand a particular short-circuit KA, say for 1 second, the busbar supports should be designed in such a way that the busbars do not deform during the prescribed time. There is no common standard in the world for the above design and it depends upon application to application. the design of the busbars for short circuit withstand is to be done in conjunction with the breakers withstand capacity which is mentioned in the IEC standard and it is called total type tested panels.

For further inputs, i request other forum members to contribute their views.

Anonymous Poster
#10

### Re: bus bar rating

03/05/2010 1:07 AM

Dear Bhupender,

U can consider 1Sqmm = 1.25Amps for Tinned Copper Busbars and 0.8 Sqmm or 1 Sqmm= 1Amps for E.C Grade Aluminium Busbars.

Regards,

Ankur Jariwala.

D.J.Electro Controls,

Mumbai.

Participant

Join Date: Feb 2010
Posts: 4
#11

### Re: bus bar rating

03/05/2010 1:38 AM

Dear Bhupender,

U can consider 1Sqmm = 1.25Amps for Tinned Copper Busbars and 1Sqmm=0.8 or 1Amps for E.C Grade Aluminium Busbars.

Regards,

Ankur Jariwala.

D.J.Electro Controls,

Mumbai.

Guru

Join Date: May 2008
Posts: 1851
#15

### Re: bus bar rating

10/17/2017 12:47 PM

Dear Mr. bhupnder,

For electrical utility, the Standards are defined by International Copper Association for copper and International Aluminium Association for Aluminium,

If you refer copper association, for copper you will find the current density of 1.550003 Amp. for one Sq.MM of copper conductor, absolute value.

If you refer aluminium association, for aluminium you will find the current density of 0.625 Amp. for one Sq.MM of aluminium conductor, absolute value.

Then, this absolute current value per sq.mm should be multiplied by De-Rating Factor and there are 7 or 8 parameters are involved for arriving at the de-rating factor such as cable laid over head, or buried under ground, laid in cable trench, group of cable, and gap between cables when it is bunched, Short Circuit current level, voltage drop permissible, temp. rise in the cable permissible etc., etc..And then your Local Code also will have a say or role.

Considering all the above 0.9 to 1.0 amps per sq.mm for Copper and 0.45 to 0.50 amps per sq.mm of Aluminium will be safe.

Pl. consult local authority or expert and proceed.

DHAYANANDHAN.S

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