A present challenge in industrial acoustics is the accurate determination of the sound power of large machines in ordinary workrooms. Such a method is being worked out as the revised ISO 3747 standard. It is a comparison method with sound pressure measurements at a limited number of microphone positions in the reverberant field. The accuracy of the method was studied using a high-performance ray-tracing computer model for the propagation of sound in industrial halls. All parameters influencing the accuracy of the method have been considered: room size and wall absorption, dimensions and shape of the machine, source distribution (one single point source or a distribution of point sources over the machine volume), omnidirectional and directional machines, position of the reference sound source with respect to the machine (on top or to the side), and microphone positions. Results analysis shows that sound power can be determined with grade 2 of accuracy even for large directional machines operating in moderately reverberant workrooms provided a specific methodology is used for locating the reference sound source and the microphones based on a zoning of the room. A criterion for validating a microphone position based on the excess of sound pressure level is proposed.