#67 – What Size FDC Break Cap or Plug Do I Need?

Why and how you need to keep those fire department connections covered

When firefighters need to supply water to a building’s fire sprinkler or standpipe system, they turn to fire department connections (FDCs). Clean, well-maintained swivels on an FDC allow firefighters to quickly connect fire hoses to supplement or supply a building’s standpipe and/or sprinkler system.

But debris, garbage, or dirt—and even vandalism—can render a fire department connection inoperable. With a cap or plug of the right size, building owners can keep an FDC intact and their building’s fire protection system fully operational.

In this article, we’ll talk about the easy way to properly size an FDC breakable cap, fire department connection plug, and Storz cap.

If you already know what you need, click here to view our selection of FDC accessories, including FDC caps and plugs.

The inner waterway of an FDC, often at 2 1/2″ in size, determines the size of the breakable cap or plug needed

If you have a standard FDC like the one pictured and you measure the outside of the inlet, you’ll come up with a measurement of approximately 3 ½”. What size break cap or plug do you think you need?

FDC cap measurements
An internal measurement (left) places this FDC at about 2 1/2″ wide. The outside of the swivel (right) measures approximately 3 1/2″.

The correct answer: 2 1/2″. The only measurement that counts when selecting an FDC breakable cap or fire department connection plug is the inside waterway of the inlet.

This is the most common size of a commercial FDC. About 98% of our customers have a 2 1/2″ FDC. Only select locations like New York and San Francisco commonly use a larger 3″ size. And family homes are the only consistent users of 1 ½”.

Pictured below are the standard inside measurements for a 2 1/2″ breakable FDC cap and plug. While the fire department connection plug is close at roughly 2 1/2″, the breakable cap is wide enough to cover the full inlet—nowhere near 2 1/2″.

Fire department connection cap and plug measurements
A 2 1/2″ break cap (left) measures about 3 1/2″ at its widest point, while the inside diameter of a 2 1/2″ plug (right) is about 2 1/2″.

In the end, though, both the fire department connection plug and the breakable cap fit perfectly. They completely cover the entryway to the swivel and thread nicely.

A fire department connection with FDC breakable cap and plug
These measurements facilitate a snug fit for both the 2 1/2″ breakable cap and the fire department connection plug.

Some jurisdictions require 3″ FDC plugs and breakable caps

As stated above, caps and plugs used with some fire department connections, notably San Francisco and New York, typically measure 3″. While the outside of the swivel or cap measures more than 4″ in diameter, the actual cap size needed is significantly smaller.


Measuring a 3 inch FDC swivel for caps or plugs
The inside of this swivel (left) looks to be about a 1/4″ larger than we’d expect, but the outside measurement (right) is a 4 1/4″ diameter.

Again, this is where the measurement of the internal waterway counts. The inner ring of the inlet is around the 3″ mark.

A breakable 3 inch FDC cap
The innermost portion of the fire department connection plug measures 3″ (left). The back of the FDC breakable cap measures around 4″ (right).

Once again, looks can be deceiving. The inner portion of the fire department connection plug measures 3″, matching the size of the swivel closely. But the measurement taken of the break cap is around 4″—just like the outer measurement of the FDC inlet.

3 inch fire department connection plug
The 3″ measurement provides a perfect fit between this plug and the fire department connection inlet.

Despite the outer measurements of the FDC inlet reaching more than 4”, it’s the 3″ plug that fits snugly into these FDC swivels.

Residential fire department connection plugs and caps often measure 1 1/2″ in diameter

Residential fire department connection swivel measurement
The inside measurement is at roughly 1 1/2″ (left), while the outside measurement is about an inch larger (right).

Like the other swivels shown so far, swivels on residential fire department connections should also be measured from the inside to get a cap with a proper fit. This 1 1/2″ swivel requires a 1 1/2″ cap.

Residential fire department connection plug
The 1 1/2″ measurement taken from the inside of this plug provides a perfect fit on a 1 1/2″ residential FDC.

What about Storz FDCs?

Some municipalities require Storz connections on FDCs (and hydrants), a non-threaded hook-up that allows firefighters to connect quicker. To learn more about Storz connections and FDCs, read this blog.

Common sizes for Storz connections are 5″ and 4″, and the measurements work the same way as traditional inlets. To get an accurate number, you need to measure the interior waterway, not the outer diameter.

Storz connection interior measurement
Measuring the interior waterway of this 5″ (x 6″ FNPT) Storz FDC adapter gives you the right number for the connection and its cap: 5″.

FDC breakable caps, fire department connection plugs, and Storz caps are in stock at QRFS

At QRFS, we offer a full selection of 2 1/2″ and 3″ plugs made with brass or lightweight aluminum. We also stock a selection of color-coded iron plugs to comply with New York City building codes, including:

  • Red for use with standpipe systems
  • Green for fire sprinkler systems
  • Yellow for combination standpipe and fire sprinkler systems
FDC caps and plugs
Click here to view our full selection of FDC accessories, including 2 1/2″ and 3″ caps, along with color-coded iron plugs.

If you’re purchasing a red, green, or yellow plug it’s important to note that they’re only available with New York Fire Department (NYFD) Threads. We carry unpainted chrome and cast-brass plugs in 2 1/2″ sizes, and 3″ brass plugs with both NYFD and NST threads.

Now that you know your options, check out the table below to find the best choice for your FDC inlet, or click here to browse our full selection of caps, plugs, and more.

  • Our most popular option!
  • Easy installation
  • Breaks easily with the butt of an ax
  • Low cost—sold in sets of 2
Aluminum Break Cap
  • Durable
  • Easy installation
  • Breaks easily with the butt of an ax
  • Low cost—sold in sets of 2
3 Inch Break Cap
  • Durable
  • Easy installation
  • Breaks easily with the butt of an ax
  • Low cost
  • Suitable for New York and San Francisco FDC inlets
Brass Plug and Chain Brass FDC Plug and Chain – 1 1/2″, 2 1/2″, or 3″

  • Heavy-duty brass exterior
  • NST in stock, other threads easily sourced
  • Includes chain to avoid loss in an emergency
Aluminum Plug
  • Lightweight, yet sturdy
  • Comes in a gold/brass finish to match your FDC
  • Includes chain to avoid loss during an emergency
Polished Chrome Plug
  • Polished chrome finish over brass
  • Threaded to meet your local requirements
  • Includes chain to avoid loss in an emergency
Green FDC Plug
  • Available in red, yellow, green, or unpainted
  • NYFD-threaded in stock
  • Lightweight, yet sturdy
  • Includes chain to avoid loss during an emergency
Polished Chrome Plug
  • Protects Storz adapters on fire hydrants and FDCs
  • Polished, aircraft-grade aluminum
  • Includes 18″ chain to avoid loss in an emergency

All options are available online 24/7 – and in-stock in multiple locations within the United States. Order today and you’ll receive an excellent product delivered extremely fast, backed by QRFS’s customer service.

Still have questions or concerns? You’re always welcome to call us at +1 (888) 361-6662 or email support@qrfs.com.

This blog was originally posted by Jason Hugo and Anna Hartenbach at blog.qrfs.com on June 15, 2017, and updated on February 7, 2022. If this article helped you find what you needed, check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.

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