RG 58 Coax
I've seen guys run a few hundred watts through it. However, it wouldn't be something I'd consider doing. The amp finals are the weakest part of the TX equipment typically, and with mini 8 available, why would anyone run RG58U? If you plan on running RG58U, I personally would try and keep the wattage down under a few hundred (or less). Few manufacturers have quality RG58U that I've ever seen (i.e. shielding).