Well, hopefully the wire would be sized to the max load it will see also. Normal rating for 10ga is 30amps.
If the wire insulation is rated high, it might go up to 40 amps. I would not run 40 amps through it though.
Of course an alternator might never see full load under normal circumstances.
If the battery is fully charged, all it needs it to replace the amount of charge used during cranking.
Plus, the operating load of the vehicle.
But, the wire really should be sized for the maximum load it can see.
Here is a link to a chart that shows what can be done. I would not go by this chart unless I went by the 25' rating.
And then I would be skeptical.
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm
This link is a little better. It comes from the problem from the opposite direction.
http://www.offroaders.com/tech/12-volt-wiring-gauge-chart.htm.
There are two considerations with wire size. You can keep loading the amps on until it melts. That would be the actual limit of its capacity.
The practical limit is the insulation. There are different temperature ratings for insulation, so you can find different current ratings for different temp insulations.
For our purposes, we do not want to even approach these limits. When you are specing out a job that has thousands of feet of wire, and you are trying to win a bid, you use the minimum ratings.
They are safe, and are cost effective.
But, when you are working on your own equipment, looking for reliability, you don't try to save 10 cents. You go to the higher rated wire.
The other side of the situation, is voltage drop.
We are not working with hundreds of volts here. Every volt counts.
For instance, the difference between a functioning HEI and a non-functioning one, is only about 3 volts.
From 12 to 9.
The smaller the wire, the more voltage it loses per foot per amp.
Conversely, the bigger the wire, the closer to full voltage you get.
Ever try to jump off a vehicle with small jumper cables? It doesn't work. They lose too much voltage to crank it.
Instead, you have to leave them hooked up for a while with the other car running until it charges the dead battery enough for it to help.
I have a set of jumper cables that are about 3/0. Its pretty much the same as actually replacing the dead battery with the good one.
So, when in doubt, go up one size.
The best way, is to have heavy enough wire so that your fuse protects the equipment.
Then you can be sure the fuse also protects the wire.
For instance, making up numbers, for a 30 amp alt, use wire rated to 40 amps and a 30 amp circuit breaker or fuse.
But, when in doubt, protect the wire. Its what will catch the truck on fire.
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