Type MI and Type MC Cables are now allowed to be installed under buildings without a raceway.

300.5(C) Cables Under Buildings. Cables installed under a building must be installed in a raceway that extends past the outside walls of the building.

Ex. 2: Type MC Cable listed for direct burial is permitted under a building without installation in a raceway [330.10(A)(5)]. (click here to see Fig. 10)

Analysis: Although certain types of MC cable are listed for direct burial and concrete encasement, this rule has prohibited them from being installed underneath buildings. This change now allows cables to be installed under the floor slab of a building, which has been accepted by many inspectors for some time. Interestingly, other cables that are listed for this application, such as UF cable, aren’t recognized by this change.

This change clarifies the use of single conductor cables installed in parallel.

300.5(I) Conductors Grouped Together. All conductors of the same circuit, including the equipment grounding conductor, must be inside the same raceway or in close proximity to each other. See 300.3(B).

Ex. 1: Conductors can be installed in parallel in raceways, multiconductor cables, or direct-buried single-conductor cables. Each raceway or multiconductor cable must contain all conductors of the same circuit, including the equipment grounding conductor. Each direct-buried single-conductor cable must be located in close proximity in the trench to the other single-conductor cables in the same parallel set of conductors, including equipment grounding conductors.

Ex. 2: Parallel circuit conductors installed in accordance with 310.10(H) of the same phase or neutral can be installed in underground PVC conduits, if inductive heating at raceway terminations is reduced by the use of aluminum locknuts and cutting a slot between the individual holes through which the conductors pass as required by 300.20(B).

Analysis: Conductors of the same circuit are required to be grouped in the same raceway or cable to help reduce the inductive reactance of the conductors. A very literal reading of previous Code editions didn’t address the use of single-conductor cables, such as many USE cables, in parallel installations. The NEC now recognizes this practice while giving guidance on how to install these cables. The issues of inductive reactance are addressed by requiring these single-conductor cables to be installed within close proximity of each other.