How this calculation works
The two numbers
Secure within. How close to a box, cabinet or fitting the first support has to be. Rigid raceways are generally 3 feet. Flexible methods and cables are generally 12 inches. NM cable gets a relief to 8 inches at a single-gang nonmetallic box with no clamp.
Maximum spacing. How far apart supports can be along the run. 10 feet for the rigid metallic raceways, 4-1/2 feet for flex and most cables, and varies by trade size for PVC.
The exceptions that save you
Bored holes count as support. Cable or raceway run through bored or punched holes in framing members is considered supported and secured at those points. This is why nobody staples romex inside a stud bay every four feet — the holes are doing the work.
Fished lengths need no support. Unbroken lengths fished into concealed spaces in finished walls do not require support, because you cannot get to them to install any. Same for unbroken raceway in an existing or prefinished wall.
Flex gets slack for a reason. FMC and LFMC are permitted up to 3 feet unsupported where flexibility is needed at an equipment connection, and up to 6 feet from the last support in an accessible ceiling. That slack is the entire point of flex — it isolates vibration.
Threaded rigid risers. Vertical runs of rigid or IMC with threaded couplings can go up to 20 feet between supports if the riser is fastened at top and bottom, per Table 344.30(B)(2).
The one that fails inspections
PVC support spacing changes with trade size, which people forget. Half-inch through 1 inch needs support every 3 feet. Anything larger gets more. And PVC needs expansion fittings where the temperature swing exceeds 25°F — a long outdoor run without one will pull itself out of a coupling over a season.
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Frequently asked questions
Does running through a stud count as support?
Yes. Cable and raceway run through bored or punched holes in framing members is considered both supported and secured at those points. That is why romex inside a stud bay does not need staples every four and a half feet — the holes already satisfy the requirement.
How close to the box does the first strap go?
Three feet for rigid metallic raceways, twelve inches for flex and most cables. NM cable gets a relief to eight inches at a single-gang nonmetallic box with no internal clamp, which is the specific case that comes up on old work.
Can flex be left unsupported at a motor?
Yes, up to three feet where flexibility is necessary at the equipment connection. That is what flex is for — the slack isolates vibration from the raceway system. In an accessible ceiling you also get up to six feet from the last support.
Why does PVC need expansion fittings?
PVC expands and contracts far more than metal raceway with temperature. Where the expected temperature change exceeds 25°F, 352.44 requires expansion fittings. A long outdoor run installed on a cool day without one will work itself out of a coupling by mid-summer.
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