Type MC cable containing three 12 AWG circuit conductors and an equipment grounding conductor is run through a commercial ceiling space. At what maximum interval must this cable be supported and secured along the run?
- A4-1/2 feet
4-1/2 feet is the interval for armored cable and nonmetallic sheathed cable. MC cable is allowed the longer 6-foot interval.
- B10 feet
Ten feet is the support spacing for EMT and other rigid metal raceways, not for a cable assembly.
- 6 feet
- D3 feet
Three feet is the securing distance from a box for rigid metal raceways and the support interval for small PVC, not the MC cable interval.
Why C is correct
Interlocked armor gives MC enough stiffness to bridge a longer span without deforming, so NEC 330.30(B) allows 6 feet where the softer cable assemblies are held to 4-1/2. The interval is still short enough that the armor never carries a pulling load into the connector. Support must also come from the building structure itself, which is why draping the cable over grid wires or ductwork does not satisfy the rule.
What this question is testing
This item tests whether you can distinguish the support interval for MC cable from the intervals used by other cable assemblies and by rigid raceways.
On the job
In commercial ceilings MC is often laid across the grid wires or draped over ductwork, which is not support. It must be independently supported from the building structure, and it may not be attached to the ceiling grid support wires unless those wires are additional wires identified for the purpose. Inspectors pop ceiling tiles at random and look for cable resting on tile. Bridle rings or J-hooks on 6-foot centers solve it.
Memory technique
MC has muscle: it stretches to 6 feet while AC and NM stop at 4-1/2.
Exam tip
MC is the odd one out among cables at 6 feet; AC and NM are both 4-1/2 feet.
Where to look it up
Go to the .30 section of the metal-clad cable article and read the securing subsection separately from the supporting subsection.