Type NM cable is being bent where it turns a corner in a wall cavity. Measured at the inner edge of the bend, what is the smallest radius the Code permits?
- Five times the diameter of the cable
- BThree times the diameter of the cable
Three times the diameter is tighter than the article allows and will visibly flatten the jacket on common cable sizes.
- CTen times the diameter of the cable
Ten times the diameter is a conservative practice figure used for some large cables and for fiber, but it is not what the nonmetallic-sheathed cable article requires.
- DTwice the diameter of the cable
Twice the diameter is a sharp bend that compresses the inner edge of the jacket severely and is well outside anything the article permits.
Why A is correct
The cable article states one bending limit: the inner edge radius must not be less than five times the cable diameter. That is a small radius for common two-conductor cable, but it is easy to violate when cable is forced around a corner post or into a shallow box. The intent is to preserve the jacket and the conductor insulation from mechanical damage that would never be seen again.
What this question is testing
The item tests recall of a simple, frequently cited numeric limit and whether the candidate knows the radius is measured at the inner edge of the bend rather than the centerline.
On the job
The sharp bend usually appears where cable enters a shallow old-work box or turns down a corner post in a remodel. The jacket splits at the inside of the bend, and by the time the drywall is up nobody will see it. Rough-in inspectors look at cable bends near boxes for exactly this reason. On larger cables the number matters more: a big feeder cable forced around a corner can crush its conductor insulation and fail years later under load.
Memory technique
Five times the diameter, measured on the inside of the turn.
Exam tip
Several wiring methods state bend radius as a multiple of diameter. Learn the cable figure separately from raceway bend rules.
Where to look it up
Look for the bends section near the installation requirements of the nonmetallic-sheathed cable article.