Bus Bar Current Ratings 3

[scaifie]

I am looking for a general technique or formula to find the current rating for bus bars in both copper and 1350 aluminium.

I have heard of general A/mm^2 but am not sure of numbers and it seems a rough approximation not to include temperatures, looking through all of my text books from back at uni days doesnt actually give any numbers for bus bars, just tables of expected ratings for cables.

Does anyone have a formula to use.

Thanks

 

[dpc]

http://www.cda.org.uk/megab2/elecapps/pub22/index.htm

 

[alehman]

The U.S. standard for copper bus in low voltage switchboards is 1000A/sq inch, but there are a number of exceptions such as allowance for higher densities where thermal testing shows it is acceptable.

 

[scaifie]

Thanks for your help dpc and alehman,

It was exactly what I was after and gives me both a starting point and a final rating and can use the formula from cda.org.uk for 1350 aluminium knowing its properties.

I was surprised I couldnt find something like this from google or the text books I have.

Thanks again.

 

[scaifie]

Correction, I jumped the gun a little,

I can use some of the formulas,

but it is still a great help.

 

[tulum]

as alehman mentioned the 1000A/sq inch rule of thumb is just that a rule of thumb. If your a consultant taking a peak at some switchgear this rule is good..... if you are designing switchgear.... throw this rule out.

Regards,
TULUM

 

[alehman]

Well I wouldn't necessarily say throw it out. Some switchboard manufacturers have type tested their designs for temperature rise and can use less conductor. Other (typically smaller) shops that don't have tested designs must stick to the 1000A/sq in rule (these rules are stipulated by U.L.). In some cases even lower densities are required for connections at large circuit breakers and switches. Every mfg I've worked with can provide 1000A/sq in boards if required.

Thermal designs can result in considerable reduction in conductor size.

 

[scaifie]

Thanks Tulum and Alehman,

Obviously there are more factors to be taken into account to change this value but it provides a starting point for me which is what I asked for, looking at the standards from NEC, 1000A/sq inch is the maximum for copper and 700A/sq inch is the maximum for the 1350 aluminium.

I was surprised to see that there was only a 30% difference between the two considering the resistivities are 40% apart. I am currently running the number to make sure it will work out to be equivalent to the existing copper design.

I still need to investigate other factors like the dangers using aluminium instead of copper such as short cct currents, arcing, joint methods, electromechanical stresses, configuration of bars etc.

I cant seem to find any reason why not to use aluminium except for the size, but weight is more important to me than size.

Thanks

 

[alehman]

Aluminum is considered an inferior material by some engineers. The main concerns are thermal expansion, creep and corrosion resistance. All of these can affect the integrity of connections. Bolted aluminum connections must use spring tension washers due to the difference in thermal expansion of aluminum and steel and potential for permanent deformation of the aluminum (creep).

If it is manufactured correctly and not used in a corrosive environment, aluminum can be as reliable as copper.

 

[LionelHutz]

UL508A specifies that bus bar should be copper and at most carry 1000A/square inch. Also, insulators/supports must be 90C rated.

Now, you can build a minimum buss to support whatever current you want and use it if you do temperature rise testing. This is not easy to calculate because you need to figure out how much heat can dissipate through the buss duct or enclosure to the outside air. As an example, we have used 1/4" x 2" copper buss to support 800A which is 1600A/sq in. The temperature rise test passed so it is good.

I looked at a lot of buss different companies provided and could generalize and say the general pattern looked something like this;
800A = 1400A/sq in.
1200A = 1200A/sq in.
2000A = 1000A/sq in.
3000A = 800A/sq in.

Of course, these are not set in stone. You would still have to heat run test the bus bar.

As another note, when I talked to one engineer he told me how their tin plated buss just failed so they run the buss through the enclosure powdercoating line and paint it grey because that passes.

 

[SteveSmith]

Try this link:

http://www.copper.org/applications/busbar/ampacity/bus_table3.html

 

[waross]

There are also some configurations where the allowable current is less than 1000 A per sq in.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Freeportpower]

Scaifie,

Sizing LV busbars is quite straightforward when they are in free air however when installed in an enclosure this is quite a different matter.

I have seen 2 x 150 x 6.3 busbars rated at 3000A in free air when enclosed burn down a switchboard at full load.

The CDA publication is quite useful but you are much better off with empirical data such as that provided in the Termate Guide.

For example in the guide 2 x 150 x 6.3 bars are rated at 3287A in free air but 2165A in still air. (I am assuming you are designing in accordance with BS EN 60947)

Another pitfall that is often overlooked is the heat contribution of the LV switchgear. A 3000A ACB at full chat will generate about 1kW of waste heat most of which ends up in the coppers.

Hope the above helped,

FPC