Small-diameter high-density MPO trunk cables

What small-diameter MPO trunk means

A small-diameter or high-density MPO trunk cable carries the same fibres as a standard trunk in a thinner cable. The goal is to fit more fibres through a tray, a conduit or a cable manager that is already crowded, and to make the cable easier to route behind a rack. There is no single industry definition of "slim" or "small diameter", and makers use the words for cables that range from slightly thinner than standard to much thinner. For a buyer, the useful definition is a number: the outside diameter of the cable, in millimetres, for the fibre count and jacket you need.

This page explains how a trunk can be made smaller, what that costs in strength and handling, how to work out whether the saving matters for your route, and what to put in a request. It is buyer education, and not a statement that any specific diameter is stocked. Radix Fibrotech builds MTP and MPO assemblies to the customer's drawing in Pune. For small-diameter constructions, send us the drawing and we reply with what we can build. The wider family of cables is on the MTP and MPO cables page, and the general trunk type is described at MPO trunk cables.

How cable makers reduce diameter

A trunk cable is a stack of layers: fibre, coating, buffer or sub-unit, strength members, and an outer jacket. Reducing diameter means thinning one or more of them, and each change has a consequence.

A well-made small cable balances these, so that the cable still meets the mechanical tests in the data sheet. For indoor cables the relevant cable standards are the IEC 60794-2 family, which sets out tests for tensile load, crush, impact, bend and temperature. Ask the supplier which tests the offered cable passes and at what values. Do not compare diameters alone, because two cables with the same outside diameter can differ greatly in strength.

Worked example: how many trunks fit in a conduit

Conduit and duct fill rules limit the share of the cross-section that cables may occupy. A commonly used limit for three or more cables in a conduit is 40 percent of the internal area, as in US electrical and telecommunications practice, but you must check the code or specification that applies to your site.

Assumed values: a conduit with a 35 mm internal diameter, 40 percent fill, and 12-fibre MPO trunk cables of three different outside diameters (the diameters are assumed for the example and are not product figures).

1. Conduit area = π ÷ 4 × 35² = 962 mm².
2. Allowed cable area = 40 percent × 962 = 385 mm².

Fill is proportional to the square of the diameter, so a one-third cut in diameter (4.5 to 3.0) more than doubles the number of cables, and a cut to 2.0 mm gives five times as many. The table is an upper bound, because round cables cannot pack perfectly and a real pull needs clearance. It also shows the limit of the saving: the MPO connector at each end is roughly 12 mm wide, and it has to be pulled through the same conduit or protected by a pulling sleeve. For a full run, the connector, not the cable, may set the smallest bore you can use. Our guide to MPO trunk cables with a pulling eye explains how the connector is protected.

Bend radius and pulling tension on a small cable

A smaller cable has a smaller minimum bend radius in absolute terms, which is the reason it routes well. Many indoor data sheets state a bend radius of about 10 times the cable outside diameter with no load, and about 20 times under pulling load. A 3.0 mm cable would then need 30 mm at rest and 60 mm under load, and a 4.5 mm cable would need 45 mm and 90 mm. Those are examples of a common rule of thumb and the data sheet for the offered cable decides. The bend-insensitive fibre inside may tolerate a much tighter bend than the cable jacket does, but the cable limit is the one that counts. Our guide to bend radius for fibre cable and the page on macrobend and microbend loss explain the loss mechanisms.

The cost of a smaller cable is usually a lower rated pulling tension, because there is less strength member. Hauling a thin trunk through a long conduit with a cable grip on the jacket can overload it. Use the pulling eye or the strength member as designed, pull by hand or with a tension-limited tool, and use lubricant if the duct run is long. The limits and practical checks are set out in our guide to MPO bend radius and pulling tension and the installation checklist.

The connector end: where the size limit really sits

The cable can shrink, but the MPO ferrule and housing cannot. An MTP or MPO connector is a fixed-size part with a boot at the rear that guides the cable out. A smaller cable needs a matching boot so the cable does not bend sharply at the entry, and so the strain relief works. In a dense panel, the stack of connectors across the face and the cable leaving each one can interfere with the neighbours, and a boot with a low profile or a short body helps.

• Confirm that the boot and crimp sleeve are designed for the smaller cable, because a standard boot on a thin cable leaves it unsupported.
• Check the length of the boot in a cassette, panel or transceiver port where space is tight.
• Check that the pulling eye, if one is fitted, is no larger than the conduit allows.
• Ask whether the connector is the MTP brand or generic MPO, and see our MTP versus MPO guide for what the brand covers.

Fire rating and jacket choice in a thin cable

The jacket does two jobs in a thin cable: it protects the fibres and it carries the fire rating. In many data halls the specification requires a low smoke, zero halogen jacket, covered in our guide to fibre cable jacket types. In North America the cable has to carry a rating such as riser or plenum under the national electrical code (Article 770 for optical fibre cables), and in Europe the Construction Products Regulation classes cables by reaction to fire. A thinner jacket must still pass the fire test, so check that the offered construction carries the rating you need, and ask for the rating in writing.

Do not assume a thin cable has the same fire class as a standard one from the same maker. Ask for the data sheet and the rating mark for the exact construction. Our page on low smoke LSZH MPO trunk cables covers the zero halogen option.

Common mistakes

When a small-diameter trunk is the wrong choice

• Harsh routes. Where rodents, crush or abrasion are a risk, a thin unarmoured cable is the wrong tool. See armoured MPO trunk cables.
• Long vertical risers. Cable weight puts load on the strength members, and a thin cable has less. Check the rated tension and support spacing.
• Frequent re-patching. A thin cable that is often handled wears faster. Use a standard cord for patching and a thin trunk for the fixed run only.
• Generous pathways. If the tray has space, spend the money elsewhere.
• Unfamiliar handling crew. A cable that needs careful handling can fail where a rugged one would not.

Request a quote

Radix Fibrotech makes MTP and MPO assemblies to the customer's drawing and bill of materials in Pune, with lengths, fibre counts, polarity, jacket and marking as you state them, and quotes by form, phone or WhatsApp. Radix tests each cord before dispatch. Whether a particular small-diameter construction can be built depends on the cable and the connector, so send us the drawing and we reply with what we can build. Include your target outside diameter and the pathway it must fit. Use the request a quote page, and see the other options under MPO cable assemblies for OEMs and integrators.