Amperage capcity of copper bus bars 2

[drax]

I'm trying to find out any information at all in calculating the capacity in amperes of cu. bus. I have been told that a rule of thumb is 1000 amps per square inch. of material. I know through some past engineering courses that current flows near the surface (skin effect). How can I calculate an accurate rating of bus bars or assemblies (with air gap between bars, etc.) Any help or ref. would be appreciated.<br>
<br>
Thanks<br>

 

[jbartos]

3/3/00<br>
Here is your choice:<br>
1. Go to<br>
<A HREF="

http://www.thomasregister.com"

TARGET="_new">

http://www.thomasregister.com</A><br>

find companies which manufacture buses, e.g. Siemens, Cutler Hammer, Westinghouse, Busway Systems, etc. and ask them for engineering/design literature which is usually helpful for applications.<br>
2. Try libraries for literature, e.g.<br>
2a. Standard Handbook for Electrical Engineers<br>
2b. IEEE Standards 141, 241, 242, 605, C37.20.1, C37.20.2, C37.20.3, etc.<br>
2c. NEMA Standard BU1<br>
2d. ANSI/UL Standard 857

 

[kswitzer]

The references are good sources.<br>
In general, 1000A per sq. in. is a rough starting point but only rough.<br>
The bar size will change that number - the thicker, the lower.<br>
Also, the bar position can change it - on edge higher that flat.<br>
Further, multiple bars will lower the ampacity.<br>
Finally, air circulation, or lack thereof, will affect the number.

 

[drax]

Thanks to both of you for your help. The references will help me, however I am surprised that the thicker the bar is, the lower the ampacity?

 

[bfuchs]

When one says 'the thicker the bar the lower the ampacity', what KSwitzer actually meant is 'the lower the ampacity per sq inch'.<br>
In other words, the amount of current the bar can carry is not linear, the rule 1000A per sq inch is rough and valid only in a given range. Beyond that range, the rule isn't true anymore.<br>
<br>
IEC60364-523 appendix B gives a formula that can be used to calculate the current carrying capacity of a given cable based on its installation method :<br>
<br>
I = A x S(m)- B x S<br>
<br>
where (m) means 'to the power m'<br>
Usually A between 10 - 15, and m approx 0.6<br>
In many cases B=0, so lets ignore it here.<br>
<br>
You can see, I is not linear : if you double S, you don't double I, you multiply it by approx 2(.6) = 1.5<br>
<br>
I am sure the formulas for the busbar ampacity are similar (even accoring to NEC). And if you have a value of B different to 0, it gets even more complex !<br>
That's why you should work from tables, and forget about the formula.<br>
<br>
<br>
Good luck !<br>

 

[drax]

Thanks for the info...good stuff!!

 

[corona]

Bus bars:<br>General applications:<br><br>Switchboards, panelboards, Bus duct....<br><br>In these type of appications they are rated for amps and voltages.<br><br>For estimating purposes 1000 amps/sq-in is ok.<br><br>Source frequency drives the skin effects, the higher the frequency the higher the Z losses are....<br><br><br>

 

[motorman]

try this neat site!<A HREF="

http://busbar.copper.org/ampacity/busbar.htm"

TARGET="_new">

http://busbar.copper.org/ampacity/busbar.htm</A><br>

 

[drax]

This is excellant! and exactly what I have been looking for.<br>Thanks very much......

 

[Marke]

The current rating of a bar or cable is directly related to the temperature rise of that bar or cable and the standards determine a maximum operating temperature depending on material etc.
When you pass current through the conductor, there is power dissipated by the conductor which is dependant on the effective resistance of that conductor.
The temperature rise is a function of the power dissipated and the thermal resistance of the conductor. If you increase the surface area of the conductor, you will increase the ability of the condcutor to get rid of heat, so the current carrying capacity will increase, especially if the bar is running horisontally with the largest face vertical. (i.e. on it's edge)
Many other factors can come into the equation also, such as the proximity of other objects limiting the airflow over the surface.
Have a play with the Electrical Calculations software from

http://www.lmphotonics.com/busbar32.zip

and you will see what happens as you alter the profile of the bar without changing the cross sectional area.
Best regards,
Mark Empson

http://www.lmphotonics.com