Originally Posted by gstory
Just for my curiosity can you tell me why there is a problem with running jacketed wire in conduit. Seams to me that would just be doubly safe.
The problem is the heat build up in the conduit. The number of current carrying conductors in the conduit governs what percentage of the conductors original ampacity is safe to use in that conductor under those conditions but those figures are based on individual wires in the conduit rather than cable assemblies. The heat may build up in the conduit beyond what the insulation on the conductors can withstand. Also the total fill in the conduit may not exceed forty percent of it's internal cross sectional area. To allow for the twisting of the cable assemblies that occurs during installation you are required to use the largest cross sectional measurement as the diameter of a circle to calculate the cross sectional area of the cable and when you add up all of the calculated cross sectional areas they may not exceed forty percent of the internal cross sectional area of the conduit. So there is a heavy fill penalty that comes into play when you pull cable assemblies into conduit. With six cables in the conduit you have at least twelve current carrying conductors in the one conduit. That reduces the ampacity of the conductors to half of their originally rated capacity. You now have to run #12 in place of #14 American Wire Gauge (AWG) copper and #10 in place of #12 AWG copper. Your probably over your conductor ampacity and your percentage of wire fill in the conduit.
If you run multi wire branch circuits you can knock the number of current carrying conductors back to six. You would still have ten of the original eighteen conductors but multiwire branch circuits would keep the number that are carrying current down to six. That would allow you to use the conductors at their nominal ampacity under the code which is fifteen amperes for #14 AWG and twenty amperes for #12 AWG. The only problem with that is that if you are required to use AFCI breakers for these circuits you cannot use multiwire branch circuits because those breakers are not available for shared neutral circuits.
Your best remedy may well be to run these circuits in metal clad cable that is run through the holes that the building manufacturer left in the steel joists. Spread the cables out so that there are not more than nine current carrying conductors in any one group of cables. That will allow you to run the cables directly to each box you are trying to serve and the metal clad jacket will resist rodent attack. Most metal clad cable has conductors that are listed for wet locations. You wire it exactly as you do Romex except that you must use metallic boxes or plastic boxes that are fitted with an internal bonding means to bond all of the cable jackets together. Do not use type AC or armored cable as it is not listed for damp locations.