The optical connections (lightpaths) offered to the client by optical networks have large capacities. On the other hand, the client traffic flows require smaller and heterogeneous bandwidths. Due to this bandwidth gap, client traffic flows are aggregated onto lightpaths to improve network utilization and reduce cost, which is called traffic grooming. In this paper, we develop an analytical model using the single-service reduced load approximation to compute traffic loss probabilities in grooming of dynamic traffic. The model can address arbitrary alternate routing, and arbitrary wavelength conversion in the optical network. We have compared this model with a previous work that uses the multi-service reduced load approximation, with regard to computation time and calculated blocking probabilities. The results obtained by the single-service model are very close to those by the multi-service model, and the computation time can be significantly improved. On the other hand, the results calculated by the (both single and multi-service) analyses match those obtained by the simulation.