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D54 and D56 are two telecom duct sizes, and the lower number is the bigger pipe. D54 has a 96.5 mm outside diameter and a bore of about 90 mm. D56 has a 56.5 mm outside diameter and a bore of about 50 mm. Both have a 3.25 mm wall, but length, socket and fittings differ.
The mix-up is easy to make. A BOQ line reading D56 looks like the size above D54, and an order placed from memory can end up with a duct holding about a third of the cross-section the design assumed. The numbers are names. The dimensions are what decide the fit.
This guide puts the two sizes side by side, shows what the bigger bore and the longer length change on a real route, and explains why the standard written into a tender needs a second look. 4B Factory manufactures both sizes at its Al-Kharj plant. Full dimension sheets are on the D54 duct pipe specification and D56 duct pipe specification pages.
To-scale cross-sections drawn from the nominal outside diameter and a 3.25 mm wall. The bore is calculated from those two figures.
Outside diameter and wall thickness are published dimensions. Bore, cross-section and joints per kilometre are calculated from them, so check them against the drawing you are building to.
| D54 | D56 | |
|---|---|---|
| Outside diameter | 96.5 mm | 56.5 mm |
| Wall thickness (nominal) | 3.25 mm | 3.25 mm |
| Bore (calculated) | about 90 mm | about 50 mm |
| Bore cross-section (calculated) | about 6,360 mm² | about 1,960 mm² |
| Effective length | 5.8 m | 3.0 m |
| Socket length | 100 mm | 70 mm |
| Pipe lengths per kilometre (calculated) | about 173 | about 334 |
Dimensions follow 4B Factory’s D54 and D56 pages and a Qatar National Broadband Network duct specification, which agree on outside diameter. That specification gives plus or minus 0.2 mm on diameter and wall. 4B Factory’s D54 page shows a wall of 3.25 to 3.65 mm and a length of 5.8 to 6 m, so confirm both on your order. Pipe lengths per kilometre are 1,000 divided by the effective length, rounded up.
For cable, bore matters more than outside diameter. D54 has about 6,360 mm² of cross-section against about 1,960 mm² for D56, roughly 3.2 times as much. A D56 is not half a D54. It is closer to a third.
That gap decides the choice in practice. One D54 can take several subducts or leave spare capacity for later pulls. A D56 suits a single small cable bundle on a short route. Neither number is the fill you can use, because pulling tension, bends and the clearance each cable needs cut the usable share. Take the permitted fill from the operator specification or the cable supplier.
D54 has an effective length of 5.8 m and D56 of 3.0 m. Over one kilometre that is about 173 pipe lengths of D54 or about 334 of D56, almost twice the joints to make and inspect, before any allowance for cuts.
The sockets differ as well. In the reference specification the socket is 100 mm long on D54 and 70 mm on D56. Couplers, bends and fittings are specific to each size, so a mixed-up delivery means a joint that will not seat. Check fittings against the same designation as the pipe.
The usual split follows capacity and route type. The table shows the common cases, and the last row overrides the rest.
| Situation | Usual choice |
|---|---|
| Trunk route, duct bank or road crossing carrying several subducts or cables | D54 |
| Route where spare ways are wanted for future pulls | D54 |
| Access route or lead-in to a building, pole or cabinet with one or two small cables | D56 |
| Street lighting, traffic signal or CCTV cabling along a road | D56 |
| Tender or operator specification names a size | The size it names |
4B Factory’s own product notes list D54 for underground power and telecom routes, utility tenders and solar and substation cable runs, and D56 for last-mile telecom, low-voltage power, building risers, street lighting and CCTV. These are typical uses, not rules.
A line that reads D54, 96.5 mm outside diameter, 5.8 m effective length can only be filled one way. A line that reads D54 on its own depends on both sides holding the same assumption about what the name means.
It depends on the operator and the market, so read the tender. D54 and D56 come from telecom duct specifications. A Qatar National Broadband Network specification for uPVC ducts lists both under BS 3506. One UK catalogue sells the 96.5 mm size as Duct 54, also known as BT Duct 54D, and cites BS EN 61386.
BSI lists BS 3506:1969 as withdrawn on 14 December 2022, yet documents still cite it. That is no reason to reject a pipe. It is a reason to ask the consultant which standard and edition they will accept, and to ask the supplier, 4B Factory included, to name that standard on the quotation and supply the matching test report. Certificates and test reports for 4B Factory pipe are listed on the certificates and test reports page.
Related reading: the conduit vs duct selection guide helps you choose between conduit and duct pipe, PVC pipes for telecom ducting covers where rigid duct fits and where HDPE wins, and the uPVC duct pipes range page covers SAS 254/255 duct from 20 to 63 mm.
Send the designation, outside diameter, quantities, the standard named in your tender and the delivery location. The 4B Factory technical team will confirm the specification and quote factory-direct from Al-Kharj.
Request a Factory-Direct QuoteNot exactly. Its calculated bore is about 90 mm, or roughly 3.5 inches. A 4-inch bore is about 102 mm, which is a larger product. If a drawing says 4-inch, check the metric dimension it intends before ordering D54.
No. The two pipes differ in outside diameter, and in the reference specification the sockets differ too: 100 mm long on D54 and 70 mm on D56. Each size needs its own couplers, bends and fittings.
Physically it fits, because a 56.5 mm pipe sits inside a bore of about 90 mm. Whether the practice is allowed, and how the remaining gap is handled, is the operator’s call, so confirm it in the specification before designing it in.
4B Factory supplies compliance documentation to support tender and project approval, and publishes certificates and test reports on its certificates page. Ask for the standard and edition to be named on the quotation so the test report matches it.
