Slots/holes on bus bars, what's the effect?

[choi1167]

Some copper busbars being used on switchgears are pre-fabricated with slots/holes for ease of installation and interconnection. Example is a 1/4" x 4" square busbar with 3-slots in a row/column. The size of the slots is about 1/2" x 1" (oval shaped). Obviously, these slots reduce the cross section of the bus bar from 1sq.in. to 0.625sq.in. on that particular portion. But, what or how much is the effect on the ampacity of the bus bar as a whole? A more specific example is a 4-ply 1/4" x 4" horizontal copper busbars having slots indicated before, will it still be rated for 4000 amperes? Switchgear manufacturer said yes. But I'm not convinced since no technical information or data was provided. Any thoughts?

 

[electricpete]

One qualitative thing to consider - copper has a tremendously high thermal conductivity. So although there may be increased heat generated in the local slotted area due to higher resistance, that heat is quickly transferred along the bus without causing much local temperature rise.

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[Alex68]

Usually I don't buy a busbar indicating the cross section I like, but I ask a busbar rated for a desired ampacity at a rated voltage. The busbar manufacturer guarantees that its product complies with the requirements.

 

[AusLee]

The hole will be filled by the future cable bolt and lug. As long as the hole is smaller or equal to half the width of the bar, it is still larger than the cable. Therefore it is the role of the protection devcice to protect the eathing cable in case of an earth fault, and the worry is about the cable, not the bar.

 

[choi1167]

The holes will not be filled by the future cable bolt and lug. My suggestion was to put spacers between the bus bars where these slots are located then bolt them together. But manufacturer did not follow and instead issue an email statement which guarantee the rating.
From another switchgear manufacturer that I consulted, the normal practice is after the busbars are fitted, the slots that will not be used will be filled before plating the entire length of the bar.
From a table at copper.org, a 4-ply 1/4" x 4" will have an ampacity of 4328amps on a 30degC rise and 40degC ambient. This is for a whole bus bars w/o the slots being mentioned here.

 

[electricpete]

If you want to "bound" the actual ampacity of the bus.

Look up the rating for the reduced cross section. I believe 4" minus 3x 1/2" slots would be equivalent of 2.5" wide bus bar from a standpoint of cross section at that point only (if I have understood the geometry correctly).

The actual ampacity would be somewhere between the 4" and 2.5" busbar (both quarter inch), and I believe much closer to the 4" due to the high thermal conductivity of copper as mentioned above.

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[asco1]

UL 891 says "Part of the bus material may be removed for slots or holes (wether used or not) if the remaining material at any cross section along the bus bar has at least 70 percent of the required ampacity and the remaining metal in any 6 inch length of bus is at least 93 percent of the metal of a bus having the required ampacity in accordance with 13.7.4 and Table 13.1 or 13.3."
Table 13.2 rates 4000A as (4)1/4 x 5 cu bars or (5) 1/4 x 4 cu bars.

These are UL's guidlines. Other bar sizes and the amount of material removed may be different if the heat rise tests are performed on the specific construction.

 

[alehman]

Under U.L., bus bars may be sized based on current density or temperature rise. To use busbar sized for temperature rise, the switchboard must be type tested by the manufacturer. If you can't test temperature rise per U.L., you must size busbar based on current density (1000 amps per sq. inch for copper), with the allowances stated by asco.

 

[Sargardani]

AC (50~60 Hz) current distribution may not be uniform across the cross section of the conductor..... skin effect.... this can be verified by Finite Element Analysis of the current distribution. In the above case its quite possible that the holes may be located such that it doesn't greatly effect the current density and hence the heat dissipated.

Sarg