Power and Signal Transmission

Is there any body to tell me about below concern?

there is Ultrasonic custody transfer flow meter which for transmission of data to local equipment room with 300 meter distance used external power 24VDC from local equipment room and RS-485 for sending signal from field to flow computer in local equipment room.
vendor considered 2x2x1.5 individual and overal screen cable one pare for RS-485 and one pair for 24VDC in one digital JB.

i have a questione is it according to Standard to use one JB for external power 24VDC and RS-485 signal.

and is it according to standard which use one 2x2x1.5 cable for above transmissin?

for more detail please let me know the referable standard or practice.

B.R

For Power sypply : if u have 24 VDC power and based on accptable voltage drop of 20% (Means you will get @19 V at transmitter with 1 amp load and is acceptable for working of instrument), minimum 2.5 mm2 cable is required if your load is @1 amp and distance is @ 300 meters

visit
[url]http://www.nooutage.com/vdrop.htm[/url]

For RS 485 it is ok and for specific application u can have supply and signal in same JB as votage level are low but it should not be apply to complete project.