The original generator design supplemented the fan pole-generated air flow with through-rotor air flow. During the factory acceptance tests one generator had been subjected to a heat run but in order to keep the windage and friction losses within the allowable 10 percent over the guaranteed value the manufacturer had blocked off the through-rotor flow which reduced air flow in the generator, with the coolers in the closed circuit, to 19.5 m3/s, which was about 90 percent of the design flow. This modification was applied to all of the generators. The limited time taken to commission the generating units had meant that no heat runs were performed, which would have identified the impact of this modification on the stator winding temperatures. As a result, the Roxburgh generators always operated at higher temperatures than most other hydro generators in New Zealand.