The B744 has two ground power sockets. While one is adequate for normal use on the ground, part of the start procedue is to turn on the hyd pumps. These use a lot of AC power and it is a good idea to reduce the load on the ground power before turning on the hyds. The B777 is even worse. If possible it is a good idea to start one engine, and get the generator on line before switching on the hyd pumps. It can be embarrassing to turn on the AC hyd pump, and the power trips off!