CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A contractor is wiring equipment inside a grain elevator classified as a Class II, Division 1 location due to the presence of combustible dust. Which wiring method is acceptable for this installation?
- AThreaded rigid metal conduit (RMC) with dust-tight fittings
- BElectrical metallic tubing (EMT) with set-screw fittings
- CMetal-clad cable (MC) with standard connectors
- DElectrical nonmetallic tubing (ENT)
Show answer & explanationAnswer & explanation
Correct answer: A. Threaded rigid metal conduit (RMC) with dust-tight fittings
NEC 502.10(A) requires Class II, Division 1 locations to use threaded rigid metal conduit or threaded steel intermediate metal conduit with dust-tight joints and fittings to prevent ignition of combustible dust.
Why the other options are wrong
- B. EMT with set-screw fittings is not dust-tight or rated for Class II, Division 1 locations.
- C. Standard MC cable connectors are not dust-tight and are unsuitable for this classification.
- D. ENT is a non-metallic raceway not permitted in Class II, Division 1 locations.
Class II Division 1 Wiring
Locations with combustible dust in the air under normal conditions require threaded rigid metal conduit or IMC with dust-tight fittings per NEC 502.10.
- Applies to grain elevators, flour mills, and similar dust-producing facilities
- Threaded joints must be dust-tight, not just tight
- Flexible connections require dust-tight flexible fittings listed for the location
Memory trick: Dust demands threads — tight joints keep sparks in their beds.