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.