Conduit as a Wireway in Hazardous Locations

Rigid metal conduit (RMC) is sometimes used as a wireway in hazardous locations.

Where RMC is joined together, a union is used. This union section must be wrench tightened to provide electrical conductivity if the conduit is likely to become energized due to internal wires shorting to the conduit. The reliability of the connection between conduit pipe sections is stated within the IEC 60079-14 clause 9.4:

Where the conduit system is used as the protective earthing conductor, the threaded junction shall be suitable to carry the fault current which would flow when the circuit is appropriately protected by fuses or circuit-breakers

Therefore, a pipe sealing material used in the union must be electrically conductive.

NOTE: Some materials such as “dielectric grease” have insulating properties and therefore should NOT be used. Teflon tape, too, insulates to some degree and should NOT be used.

Fault current

The next question might be: “what is the fault current” (sometimes referred to as the short circuit current rating, SCCR)? This question can be partly answered by reviewing the fuse or circuit breaker ratings. The ratings must be sufficient to interrupt the supply transformer or line circuit supply.

How much conduit is allowed?

To determine how much conduit is allowed for a circuit, review NEMA bulletin no. 97. If ¾ inch conduit is used and the over current rating is 30 amps, then 386 feet of RMC is allowed.