Quick Answer: Choose LC connectors on 1.25 mm ferrules for anything in a rack, and SC on 2.5 mm ferrules for single links in industrial enclosures. A mated LC pair typically costs 0.10-0.30 dB, an SC pair 0.10-0.25 dB, and the connector budget for a channel is 0.75 dB per pair under normal practice. Polish matters more than the shell: a UPC ferrule on multimode is the default, and an APC end face should never be mated to a PC end face. Inspect and clean every end before mating, because one 5 micron particle can add 3 dB.
Most multimode links that fail a test fail at the connector, not in the glass. A 10GBASE-SR channel over 300 metres of OM3 has a 2.6 dB budget; the fibre takes about 0.9 dB of that at 850 nm, leaving roughly 1.7 dB for connectors and splices. Two careless end faces can consume the whole allowance before the cable is even routed.
Three shells, one fibre
LC uses a 1.25 mm ferrule and a latch, giving twice the port density of SC on a patch panel. SC uses a 2.5 mm ferrule and a push-pull body that suits gloved hands and thick drop cable. ST uses the same 2.5 mm ferrule with a bayonet lock, and remains common on installed industrial and legacy camera cabling.
| Connector | Ferrule | Loss per mated pair | Best use |
|---|
| LC | 1.25 mm | 0.10-0.30 dB | rack and patch panel |
| SC | 2.5 mm | 0.10-0.25 dB | single link, enclosure |
| ST | 2.5 mm bayonet | 0.20-0.50 dB | legacy plant |
| LC uniboot | 1.25 mm | 0.15 dB typical | one jacket, two fibres |
| MPO-12 | 12 fibres | 0.35 dB per lane | 40G and 100G trunk |
Polishing and end face geometry
PC, UPC and APC
A physical contact polish curves the ferrule so the cores touch under spring load; UPC is the tighter, lower back reflection version and is the normal choice on multimode. APC grinds the end face at 8 degrees to push reflection below -60 dB, and because of that angle it must only meet another APC.
Why APC is rare on multimode
Angled ferrules are used where reflection disturbs a laser, which is mainly single mode telemetry and RF overlay. Multimode short reach links tolerate -20 dB reflection, so the extra cost of APC buys nothing and creates a mating hazard during moves and changes.
Matching the connector to the glass
Legacy 62.5 micron fibre
If the building is wired with om1 fiber at 62.5/125 microns, 1000BASE-SX reaches only about 220 metres and 10GBASE-SR will not run at all. New LC connectors can be spliced onto that cable, but no connector choice extends the modal bandwidth.
OM3 to OM5
OM3 supports 300 metres at 10GBASE-SR, OM4 400 metres at the same wavelength, and OM5 adds wideband operation for short wavelength division multiplexing. All three accept the same LC connector, so the connector decision is independent of the grade.
Inspection, cleaning and handling
Inspect with a 200x or 400x scope before every mating; a dry clean with a one-click tool then a second inspection is the only routine that holds loss figures. Where a link needs power at the far end, a hybrid powered fiber cable carries copper alongside the fibres, so the same LC end faces must be cleaned and the copper terminals checked for damage. Coil spare cable on a optical fiber spool at 100 mm radius or more, never in a tight figure of eight.
Practical scenarios
- Situation: a 288 port patch panel in a 42U rack – LC uniboot keeps the cable count halved and stays inside a 0.75 dB connector budget.
- Situation: a camera pole 180 metres (590 ft) from a switch – OM4 with LC at both ends, two mated pairs, total connector loss about 0.5 dB.
- Situation: a link that measures 6 dB instead of 1.4 dB – inspect the bulkhead adapter; a cracked alignment sleeve usually shows a 2-4 dB step at one end.
- Situation: a plant room with annual disconnect for maintenance – SC with its 2.5 mm ferrule survives rough handling better than a 1.25 mm LC latch.
FAQ
Can LC and SC be joined without a patch lead?
No. The ferrule diameters differ, so an adapter cannot align the cores. Use a hybrid patch lead with LC on one end and SC on the other, and count the extra mated pair in the loss budget.
What loss should a new multimode link show?
For 100 metres of OM3 with two LC pairs, expect 0.4-0.8 dB at 850 nm. Anything above 1.5 dB on that length indicates contamination, a bad cleave or a damaged sleeve rather than normal ageing.
Does connector colour mean anything?
It marks the polish and the fibre grade: beige for legacy 62.5 micron, aqua for OM3 and OM4, violet for OM5, green for APC. Treat the colour as a label, not proof; inspect the end face geometry before trusting it.
