Measure the route the cable will take, not the straight line, including every vertical rise and drop, then add slack at each end, a service loop and a small contingency, and round up to the next orderable length. Specify a plus-only tolerance and say where the length is measured to. A trunk that is short cannot be corrected on site.
Why length matters more for a trunk than for a patch cord
An MPO trunk is a factory-terminated assembly. The connectors are fitted, polished, tested and capped at the factory, and a trunk cannot be shortened on site without cutting off a connector and fitting a new one, which needs fusion splicing or a field termination method that a pre-terminated trunk was bought to avoid. A trunk that is too short has no repair other than an extension through an extra connection. A trunk that is too long can be coiled, but it takes space and has costs of its own.
For that reason the order length is a design output and not a catalogue pick. The same logic applies to the MPO trunk cables range as a whole and to specific builds, such as the MTP-12 OM4 trunk. This guide gives a method for getting from a floor plan to a number on the order. It covers the route, slack, service loops, tolerance and the ordering mistakes that cause most re-orders.
What the length means: agree the reference points
Before measuring, settle what the number on the order refers to. Two suppliers can quote the same length for different cable.
| Reference | Measured between | Effect |
|---|---|---|
| Tip to tip | The end faces of the two connectors | The longest figure. It includes the plug body at each end. |
| Boot to boot | The ends of the strain relief boots | Shorter than tip to tip by the length of each plug body, a few centimetres per end. |
| Cable to cable | The length of cable between the transitions | Excludes tails or breakout legs. Used when legs are specified separately. |
| Excluding pulling grip | Any of the above, without the pulling arrangement | A grip or sleeve adds length that is not part of the working cable. |
A few centimetres do not matter on a 40 m trunk and they matter on a 2 m jumper. Write the reference on the order line, such as "length 44 m, connector tip to connector tip, excluding pulling grip". If your quote does not state it, ask.
The seven parts of an order length
Build the length from parts and write each on the drawing. A single total hides the arithmetic and makes checking impossible.
| Part | What it covers | How to get it |
|---|---|---|
| 1. Horizontal run | Along tray, duct or trunking | Measure along the pathway on site or from a drawing of the pathway, not the room |
| 2. Vertical rise and drop | Cabinet to tray, tray to cabinet | Measure heights of panel, cabinet top and tray |
| 3. Offsets and corners | Around columns, junctions, steps | Add the extra path at each offset |
| 4. Inside the cabinet | Panel position to the cabinet exit, via cable manager | Measure at the rack, from the actual panel position |
| 5. End slack | Dressing and strain relief at each end | Allow about 0.5 to 1.0 m per end, more on tall cabinets |
| 6. Service loop | Extra cable so a panel can slide out or be re-seated | Allow a loop sized to the bend radius |
| 7. Contingency | Route changes and measurement error | A small percentage, for example 3 percent |
Parts 5 to 7 are judgement, and the figures above are working values, not rules. Choose them once for a site, record them, and apply them to every trunk, which keeps the lengths consistent.
Worked example: one trunk between two cabinets
A trunk runs from cabinet A, the main distribution cabinet, to cabinet B in the same hall, in an overhead tray at 2.8 m. The panel in A is 1.4 m above the floor, the panel in B is 1.2 m above the floor, and both cabinets have a top cable exit at 2.2 m.
| Segment | Working | Length (m) |
|---|---|---|
| Panel A up to the cabinet top, plus 0.5 m across to the cable manager | 0.8 + 0.5 | 1.3 |
| Cabinet top up to the tray | 2.8 minus 2.2 | 0.6 |
| Tray run along the hall | Measured on site | 31.0 |
| Offset around a column and a tray junction | Measured on site | 4.0 |
| Tray down to the cabinet B top | 2.8 minus 2.2 | 0.6 |
| Cabinet B top down to the panel, plus 0.5 m across | 1.0 + 0.5 | 1.5 |
| Measured route | 39.0 | |
| End slack, 1.0 m at each end | 2 x 1.0 | 2.0 |
| Service loop, 0.5 m at each end | 2 x 0.5 | 1.0 |
| Contingency, 3 percent of the route | 39.0 x 0.03 | 1.17 |
| Total | 43.17 |
The order length is the next whole metre up, 44 m, with a plus tolerance. Check it against the optics. If the links are 100GBASE-SR4 on OM4, the channel limit is 100 m, and 44 m of trunk plus two 3 m patch cords is 50 m, well inside it. On OM3 the limit is 70 m, so the same channel passes there too. The fibre contributes very little loss at this length: 50 m at the 3.5 dB/km limit of ISO/IEC 11801 for multimode at 850 nm is under 0.2 dB. The OM3 trunk page and the loss budget guide explain how the limits are used.
Service loops and slack: how much is enough
A service loop is a length of cable stored in a loop, so a panel can be pulled forward or a cable can be re-routed without being stretched. The loop must stay within the bend radius of the cable. A commonly used rule from ANSI/TIA-568 is 10 times the cable outside diameter when the cable is not under load, and 20 times while it is under pulling tension. Use the figure on the datasheet if it differs.
| Loop diameter | Cable stored in one full turn | Fits a cable of outside diameter up to |
|---|---|---|
| 0.20 m | 0.63 m | 10 mm (radius 100 mm at 10 x) |
| 0.30 m | 0.94 m | 15 mm (radius 150 mm at 10 x) |
| 0.40 m | 1.26 m | 20 mm (radius 200 mm at 10 x) |
| 0.50 m | 1.57 m | 25 mm (radius 250 mm at 10 x) |
The middle column is the circumference of the loop, which is the length the turn uses. A trunk with a thin cable can have a small loop, and a heavy armoured trunk needs a larger one. Armoured cable is thicker and stiffer, so allow more length for the same slack. The armoured MPO trunk page has the radii for armour.
- Where to put the loop. In an accessible place, such as the tray near the cabinet or a cable management arm, and not behind a panel where it cannot be reached.
- How to tie it. Loosely, with hook-and-loop straps, never with cable ties pulled tight, which squeeze the cable.
- Airflow. Do not park loops in the cold aisle or in front of vents. A bundle of slack at the rack front blocks cooling air.
- Moves and changes. Slack is also what lets the trunk survive a rack being moved by half a metre. Without it, any move means a new cable.
Tolerance: what to ask for
No factory builds to the exact centimetre. Every trunk comes with a tolerance on its length, and the tolerance is a choice you can make.
- Plus only. A tolerance of plus a few percent and minus zero means the trunk is never shorter than the order. It is the safest form for a trunk, because a short trunk cannot be fixed and a long one can be coiled.
- Symmetric. A tolerance of plus and minus the same amount means the trunk can arrive shorter than ordered. Add the minus part to the order length, or do not accept it.
- Absolute or relative. A figure in centimetres for a short jumper and a percentage for a long trunk. State which you mean.
For example, on a 44 m trunk, a plus-only tolerance of 1 m gives a cable between 44 and 45 m, while a symmetric tolerance of 1 m gives 43 to 45 m. The second can fall below the 43.17 m the example needed. Radix quotes lengths and tolerances to the customer's drawing, so ask the quote to state the tolerance it offers for the construction you chose, and compare that with what your route can absorb.
A second example: four rows, one standard
A hall has four rows of cabinets, with one trunk from the main distribution area to each row. The measured routes, after the method above, are in the table. The extras are 2.0 m of end slack and 3 percent contingency, and the totals are rounded up to the next whole metre.
| Row | Measured route (m) | Plus 2.0 m slack and 3 percent | Exact need (m) | Order length (m) |
|---|---|---|---|---|
| 1 | 22.4 | 22.4 + 2.0 + 0.67 | 25.07 | 26 |
| 2 | 27.9 | 27.9 + 2.0 + 0.84 | 30.74 | 31 |
| 3 | 33.6 | 33.6 + 2.0 + 1.01 | 36.61 | 37 |
| 4 | 39.2 | 39.2 + 2.0 + 1.18 | 42.38 | 43 |
That is four different lengths. A site that wants fewer part numbers can standardise:
- Option 1: four lengths. Every trunk is close to what the row needs, and each carries about 0.3 to 0.9 m of surplus.
- Option 2: three lengths (26, 31, 43). Rows 3 and 4 share the 43 m trunk, and row 3 stores 6 m of extra cable, about four loops of 0.5 m diameter.
- Option 3: one length (43 m). Every row carries surplus, nearly 18 m on row 1, which uses a lot of tray space and is hard to dress.
Option 2 is a fair middle for many halls. The right choice depends on the cost of an error and the space to store a surplus. In every option, the length is chosen to be long enough for the longest row it serves, and not for the average.
The ordering mistakes that cause re-orders
| Mistake | What goes wrong | Fix |
|---|---|---|
| Measuring the straight line on the floor plan | The cable follows tray and walls, not the diagonal. The trunk comes up short. | Measure along the pathway |
| Leaving out vertical sections | Rises and drops of 2 to 3 m each end are missed. | Record heights of panel, cabinet top and tray |
| Using a drawing scale without checking it | A printed plan at the wrong scale shifts every length. | Verify one known distance on site |
| Ordering exactly the measured length | No slack, no loop, no tolerance. The cable is under tension. | Add slack, loop and contingency |
| Over-ordering by a large margin | Metres of surplus fill the tray, block airflow and break bend radius. | Use the method and round up one step |
| Mixing feet and metres | A US or mixed team orders 100 ft thinking 100 m, or the reverse. 100 ft is 30.48 m. | Write the unit with every number |
| One length for all rows | The far rows come up short. | Size by the longest, or list each row |
| Forgetting patch cords or breakout legs | The trunk is right and the channel is too long for the optics. | Add all cable in the channel |
| Not stating the reference point | Tip to tip and boot to boot differ by several centimetres at each end. | State it on the order |
| Measuring before the route is final | A tray change after the order makes the trunk short or long. | Freeze the route and record the date |
The unit mistake is common on projects with a design team in one country and an installer in another. A 100 ft trunk is 30.48 m, and a trunk ordered at 100 m is 328 ft. Write both on the first drawing and keep to one unit afterwards.
Pulling, grips and lengths on long runs
On a run longer than one rack, the pulling method affects the length. A pulling grip or pulling eye is attached to the cable end, usually with a protective sleeve over the connectors, and it adds length that is not usable. Ask whether the order length includes or excludes the grip, and whether the grip is removable on site. Decide the pulling direction before you order, since the trunk is pulled from the end with the grip. For the cable, state the number of bends, the vertical sections and the maximum duct size, and ask the quote for the maximum pulling tension. The pulling method is also in the section on installation in the main trunk page.
After the pull, the connectors are cleaned and inspected before they are mated. The inspection criteria most buyers cite are in IEC 61300-3-35, and the cleaning method is set out in the MPO cleaning guide.
Breakouts, harnesses and the lengths beyond the trunk
When a trunk ends in a breakout or a harness, the legs have their own lengths, measured from the furcation. Those are separate from the trunk length and need their own reference point. The leg method is described on the OM4 MPO to LC breakout page, and the single-mode version on the OS2 MPO to LC breakout page. Do not blend the two into one total, because a longer trunk does not give a longer leg.
If the trunk arrives short or long
- Short. The options are to re-order, or to add an extension trunk through an MPO adapter. The extension adds a mated pair, which adds loss and one more place for dust. Check the channel with the fibre loss budget calculator before choosing, and use a trunk of the same type, gender and polarity method or the channel will not work.
- Long. Store the surplus in a loop that respects the bend radius, in an accessible place, and not in the airflow path. Never cut the trunk.
- Different by a few centimetres. Check the reference point on the order against the measurement method on site. Tip to tip and boot to boot differ by that amount.
Checklist before you send the lengths
- Route measured along the pathway on site, by two people, and checked against the drawing.
- Heights recorded for panel, cabinet top and tray at both ends.
- Slack, service loop and contingency written as separate lines on the drawing.
- Total rounded up to the next orderable length, with the unit written.
- Reference point stated: tip to tip, boot to boot or cable only.
- Tolerance stated: plus only, with the figure.
- Pulling grip: yes or no, included in the length or not.
- Gender, polarity and fibre count on the same line, as set out in the pinned and unpinned MPO guide and the polarity guide.
- Channel length checked against the optic reach, including patch cords.
Send the table and the drawing with your enquiry through the request a quote page. Radix builds MPO trunks to the lengths and drawings supplied, and you can ask the quote to state the tolerance and the measurement reference that apply.
Questions buyers ask
How much slack should I add to an MPO trunk?
A common working figure is 0.5 to 1.0 m at each end for dressing and strain relief, plus a service loop and a small contingency such as 3 percent of the route. Treat these as site standards you set once and apply to every trunk, and increase them for tall cabinets and heavy armoured cable.
Can I shorten an MPO trunk on site?
No. A pre-terminated trunk has factory-fitted connectors, and cutting it removes one. The cable can be coiled if it is too long, but it cannot be shortened without refitting a connector. That is why the order length has to be right.
Should I measure the straight line between cabinets?
No. Measure along the route the cable will actually follow, including each vertical rise and drop and the path around columns and tray junctions. The straight line on a plan is always shorter than the installed cable.
What tolerance should I ask for on trunk length?
Ask for a plus-only tolerance, so the trunk is never shorter than the order. A symmetric tolerance can deliver a trunk below the length you need. State the figure, and ask the quote what tolerance applies to the construction you chose.
Does a longer trunk add much loss?
Very little. Multimode fibre at 850 nm has a maximum attenuation of 3.5 dB/km under ISO/IEC 11801, so an extra metre adds about 0.0035 dB. The reach limit of the optic and the number of connectors matter far more than a few metres of cable.
Is the length measured to the connector tip or the boot?
It depends on the supplier, which is why you should state it. Tip to tip is the longest figure, and boot to boot is shorter by the plug body at each end. A few centimetres do not matter on a long trunk, and they matter on a short jumper.
Is it better to order one standard length for every row?
Only if the longest row sets the length, and the surplus on shorter rows can be stored. One length reduces part numbers and errors but puts more surplus in the tray. Many sites use a few standard lengths, each sized for the longest row it serves.
What if the trunk arrives short?
Re-order the correct length, or extend it with a second trunk through an MPO adapter. The extension adds a mated pair and extra loss, so check the channel against the loss budget first, and make sure the extension has the same type, gender and polarity method.