In a project that is currently under construction, the engineer has decided to route instrumentation circuits (4-20ma) in the same conduit as control circuits (120VAC discrete signals). This is the first time that I've seen this done, and I'm wondering if this is OK. The conduit runs are in the order of 100-200 feet long. The application is for modulating flow control valves (Limitorque). My experience has been that instrumentation and control circuits should be kept separated. This is to prevent noise and interference off the analog signals. To me this is a good enough reason by itself, but its my understanding that the separation requirement may also be from the NEC. Isn't the analog signal considered a Class 1 or Class 2 circuit, therefore requiring separation to protect against fire or shock? All this isn't very clear to me, which is why I'm trying to pick your brains. Are the analog signals Class 1 circuits? Are the analog signals Class 2 circuits? Does it really make any difference if it's Class 1 or Class 2? Ignoring the noise issue, are we allowed by NEC to combine instrumentation circuits with control circuits? What if the cables have 600V rated insulation? Do you any experience combining instrumentation circuits with control circuits in the same raceway? Is this common practice? At this point, is it worthwhile to pay for another conduit run to separate these signals? I believe that some of the conduit has been installed, but none of the cables at this point. If we change the control circuit from 120VAC to 24VDC, do all the issues (interferance and NEC separation requirements) go away? This probably would be less expensive than installing separate conduit. Thank you very much for your input.