Re: Exothermic Vs Mechanical
I concur with Dereck's post. There is no Code basis to reject exothermic welds. In fact, originally, exothermic was the only NEC recognized method. I was one of the original proposers to add "permanent, irreversible" mechanical connectors as an alternate to exothermic welds.
My concerns had nothing to do with exothermic welds damaging the conductors. My experience at the time was that, for #6 and larger conductors, exothermic welds often developed "pits" in the weld interior so the cross-sectional area, and therefore the current rating, was indeterminate. The larger the conductor, the greater probability of "pitting." Several manufactures said we were not making the welds correctly, so we invited them to demonstrate the proper procedure. They had identical results.
I would use either method today. I still prefer "permanent, irreversible" mechanical connections personally because I feel they are simpler to install. But if a contractor requested Engineer of Record permission to use exothermic I would probably approve it. I've never had such a request.