Conductors and Conduit Size

Another consideration is the number of conductors that can be installed in the various sizes of conduits. Annex C of the
National Electrical Code
shows the maximum number of conductors allowed in each conduit based on the capability of the wire to dissipate heat. However, with conduit runs of any substantial length, it is usually not practical to install the maximum number of conductors allowed in the conduit. A good rule of thumb is to limit the number of conductors to approximately one-half of the number shown in Annex C.
For underground lighting conduit, TxDOT's standard practice is to use 2 inch PVC. Although this is usually larger than required by the NEC for typical TxDOT lighting circuits, the larger size conduit is easier to clean and to pull conductors through. Since the largest cost of trenched conduit is the trench, 2-inch trenched conduit has a similar cost to smaller sizes of trenched conduit.
For underground lighting conductors, TxDOT's standard practice has previously been to use a minimum size of 8 AWG copper. However, it is now recommended to use the smallest size conductor that the NEC and voltage drop will allow, with a minimum size of 12 AWG. Using the minimum size conductor gives three advantages:
  • Lower cost of installation
  • Reduced consequences of copper theft. Smaller conductors reduce the scrap value of the copper and can help to reduce the incentive to steal it. If the conductors are stolen, the smaller conductors are less expensive to replace.
  • Increased safety due to the possibility of reduced arc flash. Smaller conductors have higher resistance, which can reduce arcing if they are accidentally shorted or grounded during maintenance.