Sub Zero Covers
Green insulated backflow cover installed against a granite outcrop under a clear blue sky, with a padlocked chain and orange valve stems rising above the cover.
Padlocked and sealed against a hard freeze

By assembly and size

4" RPDA backflow cover

4" RPDA backflow cover sizing starts with the assembly on the tag. At 4", 3 RPDA assemblies from the brands we pattern for are common: Ames Colt C500, Apollo / Conbraco RPDALF4A, and Deringer (Ames) 50. The price list maps this pipe size to one stock cover, and the published dimensions below say which one to pull.

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What "4" RPDA" covers on a counter

Reduced pressure detector assembly. At 4" the assemblies your customers bring in are Ames Colt C500, Apollo / Conbraco RPDALF4A, and Deringer (Ames) 50. They are not the same size: the Deringer (Ames) 50 is 18.7" end to end and the Apollo / Conbraco RPDALF4A is 38", so "a 4" RPDA" is not a single envelope.

The stock cover for 4"

The April 2026 price list maps 4" to 85 × 40 (DS8540), with the N-shape NS4 for N-pattern installs. That cover was built around this size class; the table below shows how each assembly sits inside it.

Relief valve drainage

Every assembly on this page has a relief valve that discharges downward from under the body. Leave that path open. Close the cover to the pad around the assembly, never under the relief port.## The detector bypass

RPDA assemblies carry a metered bypass along one side of the mainline. It adds width the stock cover usually accommodates at this size, but send a photo showing the bypass side before quoting; a custom pattern ships in 3–5 business days.

When to measure instead

Unions, strainers, OS&Y gates, and wall-tight mounts all move an assembly off its spec sheet. If the tag is gone or the install is odd, measure it or send photos for a custom cover. For the type as a whole see RPDA covers; for another size, start at the fitment hub.

Sub Zero Covers sells through distributors only. Distributor accounts see pricing and availability in the portal; everyone else orders through their supply house, or applies for distributor access.

Assemblies at this size

4" reduced pressure detector assembly assemblies and the cover that fits

Dimensions in inches from each manufacturer's spec sheet with standard shutoffs. Length end to end; heights from the pipe centerline. The cover column is the closest stock fit by published length; confirm against the assembly in front of you.

AssemblyLengthAbove CLBelow CLClosest stock cover
Ames Colt C50033.75"22.75"7"85 × 40 (DS8540) · N-shape NS4
Apollo / Conbraco RPDALF4A38"22.75"10.375"85 × 40 (DS8540) · N-shape NS4
Deringer (Ames) 5018.7"7.9"8.7"85 × 40 (DS8540) · N-shape NS4

Common questions

Which cover fits a 4" RPDA?
The 85 × 40 (DS8540) is the price-list match for 4". N-pattern installs take the N-shape NS4 instead.
Do all 4" RPDA assemblies take the same cover?
Not always. At this size the published lengths run from 18.7" (Deringer (Ames) 50) to 38" (Apollo / Conbraco RPDALF4A). Order by model.
Is this a heated enclosure?
No. Sub Zero Covers are insulated, unheated covers (R-13 standard, R-30 extreme weather). They are not ASSE 1060 listed enclosures. Where a spec calls for a Class I heated enclosure, ask sales before quoting.

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