NEC 344.30 · 352.30 · 352.44

Conduit support spacing and PVC expansion fittings

How RMC and PVC support rules work, when PVC needs an expansion fitting, and how to calculate the movement and set the piston — with our own worked example and a free link to the official tables.

Need the actual tables? Look up 344.30, 352.30, and 352.44 in the free NFPA viewer →

Which support rule applies

Raceway / situation	Rule	What it controls
RMC — securing near terminations	344.30(A)	Fasten within a set distance of every box, cabinet, conduit body, or termination
RMC — general support	344.30(B)(1)	A basic maximum support interval for any RMC run
RMC — straight runs with threaded couplings	344.30(B)(2) and its table	Longer spans allowed, growing with trade size, when couplings are threaded and the run is straight
RMC — exposed vertical risers to fixed equipment	344.30(B)(3)	Longer riser spacing under listed conditions
IMC	342.30	Same structure as RMC — check its own section
PVC — securing and support	352.30(A) and (B), and its table	Fasten near terminations; support spacing is much shorter than steel and grows with trade size
PVC — thermal movement	352.44	Expansion fittings required when calculated length change reaches the code threshold
Any raceway — thermal movement generally	300.7(B)	Expansion, expansion-deflection, or deflection fittings where needed

Our own summary, not the code text (2023 NEC). Look up 344.30, 352.30, and 352.44 in the free NFPA viewer (nfpa.org/freeaccess), follow the expansion-fitting manufacturer’s instructions, and verify against the edition adopted by your AHJ.

How to lay out supports, step by step

Step	What you do	Watch out for
1	Identify the raceway type and trade size	Each wiring-method article has its own .30 section — don’t carry RMC spacing over to PVC
2	Place the first strap within the termination distance of every box, fitting, and cabinet	This applies at both ends of every run and at every conduit body
3	For RMC, check whether the run qualifies for the longer 344.30(B)(2) spans	Only straight runs with threaded couplings qualify — set-screw or compression couplings don’t
4	For PVC, look up the trade-size span in the 352.30 table	PVC sags when hot — in hot spaces, supporting closer than the maximum is good practice
5	For PVC, calculate thermal movement (below) and add expansion fittings where needed	Straps must let the pipe slide — tighten them snug, not crushing
6	Recheck spans at the fittings	Most expansion fittings need support on both sides per the manufacturer

Worked example — support spacing concept

Run	Rule applied	Result
4" RMC, straight horizontal run, threaded couplings	344.30(B)(2) table	Spans up to 20 ft, plus a strap near each termination
2" PVC, exposed on a wall	352.30 table	Supports at no more than 5 ft, plus a strap near each termination

Worked-example values only — not the tables. Look up your trade size in the free NFPA viewer.

Worked example — 150 ft straight rooftop run of PVC, with our own thermal calculation

Input / step	Value	Source
Coefficient of thermal expansion (rigid PVC)	3.38 × 10⁻⁵ in/in/°F	Manufacturer data (Carlon)
Run length	150 ft = 1,800 in	Design
Air temperature range	10 °F to 100 °F = 90 °F	Local climate
Direct-sun allowance	+30 °F → range 10 °F to 130 °F = 120 °F	Manufacturer instructions
Total length change ΔL = L × α × ΔT	1,800 × 0.0000338 × 120 ≈ 7.3 in	Our calculation
Expansion fitting required?	Yes — far above the 352.44 threshold of 1/4 in.	NEC 352.44
Number of fittings	7.3 in ÷ fitting stroke, rounded up	Manufacturer stroke rating
Installed at 70 °F: travel still to expand	1,800 × 0.0000338 × (130 − 70) ≈ 3.65 in	Our calculation
Piston setting	Leave ~50% of the travel open (60 ÷ 120 of the range)	Our calculation + manufacturer chart

Our own physics calculation using manufacturer data — not the NEC table. PVC moves about 0.4 in per 10 ft per 100 °F. Always follow the expansion fitting manufacturer’s setting chart.

Common field mistakes

Mistake	What to do instead
Using the long RMC spans on a run with set-screw couplings	The 344.30(B)(2) spans are only for straight runs with threaded couplings; otherwise use the general interval
Strapping PVC at steel spacing	PVC needs far more supports than rigid — use the 352.30 table
Cranking PVC straps down tight	The pipe has to slide through the straps toward the expansion fitting
Setting the expansion fitting fully closed or fully open	Set the piston for the installation temperature so it can travel both ways
Forgetting sun load on rooftops	Manufacturers call for adding about 30 °F for direct sun exposure
Skipping expansion on vertical PVC risers	Risers move too — and underground-to-above-grade transitions need movement allowance (300.5(J))

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Hold it up, and let it move

Look up 344.30, 352.30, and 352.44 in the free NFPA viewer. NFPA offers free read-only access to current NEC editions. This page explains how the rules work (2023 NEC); it does not reproduce the tables.

Rigid metal conduit (344.30). RMC has to be fastened near every termination and supported at a general maximum interval. The 344.30(B)(2) table lets straight runs made up with threaded couplings span farther — the allowable span grows with trade size, because bigger pipe is stiffer. The allowance is for threaded couplings only; a run made up with set-screw or compression fittings falls back to the general rule.

PVC (352.30). PVC is fastened near terminations too, but its support table is far tighter than steel, and it grows with trade size. PVC also softens and sags in heat, so on hot roofs and in mechanical rooms many of us strap closer than the minimum.

PVC expansion (352.44). Rigid PVC grows and shrinks roughly six times as much as steel. When the calculated length change in a straight run between fixed points reaches 1/4 in., the code requires an expansion fitting. The NEC includes a table of movement per 100 ft, but you can calculate it directly: ΔL = length × coefficient × temperature change, using the manufacturer’s coefficient of about 3.38 × 10⁻⁵ in/in/°F. Carlon’s instructions add 30 °F for runs in direct sun.

Worked example. A 150 ft rooftop run that sees 10 °F to 100 °F air, plus 30 °F for sun, has a 120 °F range: 1,800 in × 0.0000338 × 120 ≈ 7.3 in of total movement. Divide by your fitting’s stroke to get the number of fittings. Installed at 70 °F, there is 60 °F of warming left out of a 120 °F range, so set the piston about half open (about 3.65 in of expansion travel) and let the straps slide.

For underground runs coming up out of the ground, see the burial depth chart (NEC 300.5). Size the raceway with the free conduit fill calculator or conduit fill chart. Field PM keeps install photos, QC checklists, and inspection sign-offs with the job.

FAQ

How far apart do conduit supports need to be?+

It depends on the wiring method and trade size. RMC has a general maximum interval under 344.30(B)(1) and longer spans for straight runs with threaded couplings under 344.30(B)(2). PVC has its own, much shorter spans in the 352.30 table. All of them also require a strap within a set distance of each box or termination. Look up the tables free at nfpa.org/freeaccess.

When does PVC need an expansion fitting?+

Under 352.44, when the expected length change of a straight PVC run between securely mounted items (boxes, cabinets, elbows, terminations) is 1/4 in. or more. With a coefficient of about 3.38 × 10⁻⁵ in/in/°F, almost any long exposed run outdoors or in an attic will reach that.

How do I calculate PVC conduit expansion?+

Multiply length (inches) × coefficient (about 3.38 × 10⁻⁵ in/in/°F for rigid PVC, per manufacturer data) × temperature change (°F). Add about 30 °F to the range for direct sun. Example: 150 ft over a 120 °F range moves about 7.3 in.

How do I set a PVC expansion fitting piston?+

Set it by the installation temperature: the share of travel left open for expansion equals (max temp − install temp) ÷ (max temp − min temp). Installing at mid-range means setting it about half open. Use the manufacturer’s setting chart.

Does RMC need expansion fittings?+

Steel moves roughly a sixth as much as PVC for the same temperature swing, so it rarely needs them for temperature alone — but 300.7(B) still requires expansion or deflection fittings where movement is expected, such as building expansion joints.

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