Cable Support Spacing Calculator
Look up the maximum National Electrical Code (NEC) support interval for Type NM (Romex), AC, MC, and SE cable, and the distance within which each run must be secured to a box. Enter a straight-run length to get the number of support points the run needs.
How to use this calculator
- Pick the cable type: NM (Romex), AC (armored), MC (metal-clad), or SE (service-entrance).
- For MC, leave the box checked for a standard 2- or 3-conductor whip (4 or fewer conductors, 10 AWG or smaller), or uncheck it for larger MC.
- Optionally enter the straight-run length in feet to get the number of support points, counting a fastener at each end.
- Read the maximum interval between supports and the distance within which the cable must be secured to each box or termination.
NEC reference
This calculator follows the National Electrical Code (NEC) 2020 cable securing and supporting rules: Type NM/NMC cable uses 334.30; Type AC armored cable uses 320.30; Type MC metal-clad cable uses 330.30 for the 6 ft support interval and the small-MC 12 in. securing rule; and Type SE service-entrance cable used as interior wiring uses 338.10(B)(4)(a), which routes to the Article 334 rules for NM. The numeric values are unchanged in the 2017, 2020, and 2023 editions; the Securing and Supporting subsections within each cable article are cited here at the article level because that lettering can differ by edition.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
How cable support spacing works
Every Chapter 3 cable article in the NEC carries a securing and supporting rule with two parts: a maximum interval between fasteners along the run, and a maximum distance from each box or termination within which the cable must be secured. Unlike raceway support, which steps up with trade size, the cable interval is a fixed number that depends only on the cable type. The tool reads these maxima straight from the code; you can always support a cable more closely than the maximum.
Cable support and securing by type
| Cable type | Max between supports | Box-securing distance |
|---|---|---|
| NM / NMC (Romex) - 334.30 | 4.5 ft (1.4 m) | 12 in. (300 mm) |
| AC armored - 320.30 | 4.5 ft (1.4 m) | 12 in. (300 mm) |
| MC, small whip - 330.30 | 6 ft (1.8 m) | 12 in. (300 mm) |
| MC, larger - 330.30 | 6 ft (1.8 m) | none |
| SE interior - 338.10(B)(4)(a) | 4.5 ft (1.4 m) | 12 in. (300 mm) |
"Small" MC is Type MC containing four or fewer conductors sized no larger than 10 AWG, the common 2- and 3-conductor MC whip. Only that size class carries the 12 in. securing rule under 330.30; larger MC has only the 6 ft support interval.
Worked example: 30 ft of NM cable
- NM cable under 334.30 is supported at least every 4.5 ft.
- Number of gaps for a 30 ft run: 30 divided by 4.5 is 6.67, which rounds up to 7 gaps.
- Support points, with a fastener at each end: 7 + 1 = 8.
- Each end fastener also satisfies the requirement to secure the cable within 12 in. of the box it terminates in.
The support-point count is a planning estimate for a straight run fastened at both ends; it is not itself an NEC value, and closer support is always allowed.
Exceptions that allow longer unsupported lengths
The tool returns the base support rule. The NEC also allows longer unsupported lengths in specific cases: NM, AC, and MC cable fished through concealed spaces in finished buildings where support is impracticable; a cable run no more than 4.5 ft (NM) or 6 ft (AC and MC) from the last support to a luminaire or other equipment within an accessible ceiling; and the short AC and MC whips at terminations where flexibility is needed. Cable run horizontally through bored or punched holes in framing members is considered supported where the framing spacing stays within the interval. Flat two-conductor NM cable must not be stapled on edge. Vertical risers and other wiring methods have their own rules.
Frequently asked questions
How often do you have to secure NM (Romex) cable?
NEC 334.30 requires NM cable to be supported at least every 4.5 ft and secured within 12 in. of every box, cabinet, or fitting. Fasteners can be closer, but not farther apart. A single-gang nonmetallic box without a clamp instead needs the cable secured within 8 in. under 314.17(C).
Why does MC allow 6 ft when AC allows only 4.5 ft?
The two cable types have similar interlocked-armor construction, but the NEC sets the Type MC support interval at 6 ft (330.30) and the Type AC interval at 4.5 ft (320.30). Select the cable you actually installed; the tool uses the interval the code assigns to that type.
Does the 12 in. securing rule apply to all MC?
No. Under 330.30 the 12 in. box-securing rule applies to MC containing four or fewer conductors sized no larger than 10 AWG. Larger MC is supported at the 6 ft interval only and is not subject to the 12 in. securing distance. Uncheck the MC box in the tool for that larger cable.
Is this the same as conduit support spacing?
No. This tool covers cable methods (NM, AC, MC, SE) under the Chapter 3 cable articles. Raceways such as EMT, RMC, and PVC have their own support tables; use the Conduit Support Spacing Calculator for those.
Related calculators
- Conduit Support Spacing Calculator: maximum support spacing for PVC, RMC, EMT, FMC, and LFMC raceways by trade size.
- Box Fill Calculator: required outlet or junction box volume per NEC 314.16(B) for your conductors.
- Wire Size Calculator: minimum conductor size from ampacity and voltage drop, with the OCPD.
- Cable Tray Fill Calculator: multiconductor cable fill in ladder and ventilated trays per NEC 392.22(A).
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