Permissive Temperature Rise for Busbars

Look it up on one of the hundreds of web sites that have the information available.

BS 7671 or NEC or Indian electrical spec.

"Heat rise in a conductor would be a good search topic.

I have searched the net and I have found some answers either but actually I want a table or graph wich consider all relative parameters such as number of conductor,cross section of conductor,ambient temperature, ...

"Conductor sizing". Google that.

Have you tried a search on the standards websites for the busbar standard applicable to your country and equipment? Additional information can be found in there also.

Going back a step why don't you ask one of your companies Electrical Engineers or alternatively the Electrical Engineering consultancy your company uses?

Mechanical engineers should not be designing something as dangerous as LV or HV switchgear (do you even know how to size the busbar and supports to handle the short circuit current forces?), which leads me to believe you may be trying to do and sell something substandard, uncertified and possibly even dangerous.

A poorly designed piece of switchgear is just as deadly as a bomb.

Care to elaborate and provide more information on why you are doing what you are doing?

Well, it wouldn't be in the form of a 2-D graph, then. It's more likely a multi-dimensional hypersurface.

As far as I know for bare busbar shall make to withstand 50 to 70 KA the permissive temperature is 180°C for 2 sec. and 250°C for more than 3 sec.

For insulated busbar up to 120°C for 2 sec. and 140°C for more than 2 sec. all this values is related to the short circuit current calculated.

Bye

In India the permissible operating temperature of the busbar inside the switch board is 85 deg C . Accordingly busbar sizes are designed.

I have found some answers according to IEC ,BS 159,ANSI C37.20 ,but they just give an spesific number such as 105k,50c or 65c. Actually I need a table or a graph or formula that consider number of conductor,cross section of conductor,ambient temperature,.... . for example if current is 240A, cross section of conductor is 5x25,ambient temperature is 40c , what would be temp rise in an enclosed busbars?

If I am not mistaken, you would like to determine the steady state temperature under load conditions. The final steady state temperature depends on various factors like conductivity of the conductors, profile & dimensions of the conductor, electrical resistivity factor, type of ventilation, ratio of volume of conductor to its enclosure, material of enclosure, orientation and spacing of & between bus bars, altitude of installation etc. Any standard reference book for bus bar design will first guide you to determine the required current density considering the above factors including the final targeted steady state temperature for stated current carrying requirement. Typically (In India), the permissible temperature rise will be 40-45°C above an ambient of 40°C for non insulated and non segregated bus bars. However lot of bus duct manufacturers (Insulated Sandwich bus ducts) define the current carrying capacity of the bus ducts based on higher permissible temperature rise of about 60-65°C above ambient by using higher class of insulation between the conductors. In the pretext of value engineering, they however assume a higher current carrying capacity for bus bars than is conventionally followed. (Personally I do not advocate this philosophy and de-rate the claim of manufacturer by at least one level when choosing the bus ducts ).

Secondly, consideration of temperature rise during short circuit condition should also be kept in mind and bus bars should also be determined to withstand the rated short circuit KA for required time duration of "t" seconds, predefined in design parameters. The higher cross section value as derived from set of calculations above, should be the chosen conductor size.

IEC 60439-1 Table 2 - Temperature-rise limits

Limited by:- mechanical strength of conducting material ⁷⁾;

For: Busbars and conductors, plug-in contacts of removable or withdrawable parts which connect to busbars=70 dgr.K[=C]. [Ambient air=35 dgr.C].

⁷⁾ Assuming all other criteria listed are met, a maximum temperature rise of 105 K for bare copper busbars and conductors shall not be exceeded. The 105 K relates to the temperature above which annealing of copper is likely to occur .

See: ABB Switchgear Manual ed.11 Table 13-4 "Current-carrying capacity of copper conductors [DIN 43671]".

For 65 dgr.C temperature rise and 35 ambient

For 1 conductor per phase 20*5 [bare]=274 A

For 1 conductor per phase 30*5 [bare]=379 A

For 1 conductor per phase 25*5 [bare]=(274+379)/2=326.5 [extrapolation]

For 65 dgr.C conductor temperature rise and 40 ambient the compartment inside temperature could be 5 dgr. K[C] more . Then inside ambient temperature will be 45.

From the above ABB Switchgear Manual[according DIN 43671]

Fig 13-4 for 65 dgr.C conductor temperature and 45 ambient temperature multiply factor[k2]=0.75 .Then the final permissible current for closed compartment 25*5 mm copper bar 40 dgr.C ambient temperature will be 326.5*0.75=244 A.

wild guess, your company builds more than one to fit multiple applications with different specifications