ads74 said:
... apparently this cable size is wrong because the cables were melting/burned.
ads74 -
Yes they are sized wrong - but unless the gen is running fully loaded, 24/7, I suspect this is not the cause of the cable burning/melting.
The copper is good for 1800A (frank2004) and the gen output is 2100A. If the gen is loaded to less than 90%, the copper might run hot, but should be okay. And, it's rare for gens to be loaded to 90% or more.
As I understand, the cables are burned/melted, so you are changing them out no matter what - good choice. The NEC specifies 115% ampacity, or 100% if the gen can not be overloaded (Art 445). Depending on the rating (Prime, Continuous, Standby) could be either way. Check the gen specs. Interestingly, you can overheat the gen and the copper feeder and not be overloading the engine. A low power factor will do it.
So size copper for 2100A or 2400A. Pay really close attention to the terminations. With the information you have given us, that is the first place I would look for a problem. Also check the gen leads where they terminate. If the leads are hot coming out on the generator, that can cook the feeder.
If you are still running on the existing feeder, use the opportunity to look for other issues. Are the cables burned on one end or both ends? If you have access to a thermographic scanner, look for hot spots. Look for termination issues. Even if you are changing the cables, I would do this. Mostly cause if the same person installs the cables that did the first set, you may end up with the same problems.
Here is the professional advice part. Changing the copper will fix the symptom, but maybe not the problem. You are focused on the size of the copper - and while that may be the problem, or one of the problems, there may well be other root causes. The best advice I could give is to broaden your scope, look for other contributing issues. It might not be just the size of the copper.
Then again, I could be all wet. If you are running the gen flat-out, and the cables are burned in the middle, not the ends, then hummmm.... more copper, good clean installation is about it. In this case, fixing the symptom will fix the problem.
Let us know how it comes out.