Physics modelling and engineering analysis have been carried out to determine the operating limits of the upgraded JET neutral beam injector from duct re-ionisation and beam shine-through. The JET neutral beam duct is only 23cm wide and 90cm tall at its throat and yet it presently has to transmit more than 11MW of Do beam particles, resulting in power densities in excess of 200MW/m2. Even at this power level, the copper duct liner can be the limiting component with respect to the pulse length of the Octant 4 injector, depending on plasma current and power. The upgrade to the Octant 8 injector in 2002 will increase the power to ~15MW of Do at 130kV, so it is necessary to determine the new limits. It is shown that at full power, the duct will become the major limiting component with respect to pulse length for this injector. The shine-through power density and integrated energy for various in-vessel components have also been evaluated for the upgraded injector. Thermo-mechanical finite element stress calculations on elements of the ICRH antenna show that the injector can be operated at full power without further restrictions being imposed on the plasma characteristics, e.g. density and shape. For the CFC inner wall guard limiter tiles and their internal reinforcement, however, there is a bulk temperature limit and an enhancement to the existing real-time protection system is proposed.