Well it seems I misspoke earlier, makes no difference if it is service cable or individual wires. The book says #2 aluminum is good for 75, 90, or 100 amps depending on the ambiant temperature (140, 167, or 194 degrees farenheit).
#3 copper is rated for 85, 100, or 110 amps with the same temperature restrictions.
There is a seperate table for service feeders for dwelling units which states you can use #4 copper or #2 aluminum for a 100 amp service. I was thinking this part of the code only applied to service cable which is a factory made cable with 3 or 4 wires under one jacket.
The tricky part of using the first set of numbers which would alow you to run 100 amps on #2 aluminum is that you have to derate. You have to find all the derating factors, ambient temp, number of conductors in conduit, percent fill of conduit, bundling, etc. The real killer to this, the way I was tought, is that you have to start that derating at the lowest rated temperature of any part of the system. Wire rated for 194, check; conduit rated for 194, check; wire nuts (if used) rated above 194, check; that breaker you are using ---rated for 140 or even less -- trouble. Now your #2 aluminum is rated for 75 amps and then you derate.
Or, just use the second table and put 100 amps on it and don't worry.
Familiarity breeds contempt. I have contempt for the book. The number of replies and questions in this post should show that not just any layperson can easily grasp what the intent was.
For anybody thinking of doing electrical work, if you are not familiar with what you are working on, don't do it. Call a Licensed Electrical Contractor, not just an Electrician. There is a differance. If you are prepared to tackle the job yourself, pull a permit if/as required, talk to your inspector, he is there to help you, follow his advice and you should have no problems.
As I said before, this is just my opinion, sorry for any ruffled feathers.
