Suggestion: Check Reference:
1. Fink D. G., Carroll J. M. "Standard Handbook for Electrical Engineers," 10th Edition, McGraw-Hill, Inc., 1968
pages 10-116 and 10-117, Section 429 "Flat Bars,"
Figure 10-165 (Reflects Amperes versus number of laminations per bus). It appears that your copper bars will fit the bus-bar arrangement L for 3000A
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Can you clarify the arrangment intended to be used:

1.- Is the bus bar for use in a switchgear or are you in need to expand the generator breaker housing for easiness of power cable connection.

2.- Be sure to calculate your temperatures and short circuits forces to correctly establish support locations.

If you are doing something outdoors then you might need to perform some added calcs.

Thanks you both for your advice and replys. Looks like I need to invest in an Handbook For EE's.

The intent is to mount a side car to the pothead and place the breaker in the side car. We need to extend the generator's bars over to the breaker. The distance should not be longer than 20-25 inches. This will be an indoor unit.

Suggestion: Be careful when you invest in EE Handbooks since they tend to remove certain useful stuff and replace it with something what is less practical and widespread. Not every edition may have it. Many libraries have these handbooks or they findout which libraries have them.
The above mentioned figure is relevant to bus bars for 60Hz/sec and the ampacity is for alternating current carrying capacity of bus bars for 35°C rise in still unconfined air. If there are deviations, appropriate revisions shall be applied. Please, notice that buses are traditionally installed with long dimensions in the vertical direction. This is to clarify squares in my posting.

Yes that is how I intended to mount the bars. As far as the temp., I will need to look up the site location but will certainly be aware of it. And for the handbook it's too late. I already purchased the Standard Handbook for Electrical Engineers but thanks for the warning. I think it will do fine along with the NEC handbook I have.

this should help
http://busbar.copper.org/ampacity/busbar.htm

You can use a rule of thumb of 1000A per square inch bus bar and you should be safe ampacity wise.

For motorman
Thanks for the infor on the busbar site-good data

Suggestion: The posted web site reflecting bus bar ampacity is a good start. Then, one may need to configure the bus bars such a way that heat dissipation is adequate and electromagnetic forces are harmless to the bus bar aggregate. It is a good idea not to underestimate electrical engineering aspects.