For the nuoc cham, stir the sugar, fish sauce and lime juice in a bowl to dissolve the sugar, then stir in chilli and garlic. Set aside.
Prepare the rice noodles according to packet instructions, drain well, then place in a large bowl.
Add the carrot, cucumber, chilli, prawns, bean sprouts and herbs to the bowl of noodles. Add the dressing, toss well to combine, then serve.
