PBS - restrict total number of processors per user

Hello everyone!

I am a bit inexperienced with administering queueing programs. I installed Torque (a PBS derivative) on a Linux cluster and it is running well. There is one annoying problem though: users can run massively parallel jobs and serial jobs too. Almost all users do a mix of the two. I would like to restrict the total number of processors per user at any given time, say 40 processors, whether it is 20 jobs x 2 CPUs or 2 jobs x 20 CPUs, etc. Is there a way to do that in PBS/Torque with or without Moab/Maui?

Thanks in advance for any suggestions!