How to Tell a CAT3 From CAT5 by Looking at the Wireby Oscar Sanchez
CAT3 and CAT5 wire are types of unshielded twisted pair wiring. Both types have 4 pairs of twisted wires that rely on the twisting to prevent noise. Category 3 wire is not used as much today but is much cheaper than Category 5. Category 3 can only handle a maximum of 10Mbps at 16MHz compared to Category 5 being able to handle 100Mbps at 100MHz. Most people use the faster Category 5 for networking since it can handle faster speeds. Category 3 wiring is mainly used for telephone services that use multiple lines.
Check the jacket of the wire for writing. Make sure you check at least three feet of the wire in case the printing is greatly spaced out. Most of the time, manufacturers will print the type of wire on the outside of the sheath.
Look at the width of the wire. A CAT5 wire will look thicker than CAT3 wire since the CAT5 wire is around 4mm thick. A CAT3 wire is a bit thinner and is sometimes flat like phone wire, depending on the manufacturer.
Check the connector at the end of the wire. CAT5 wire will have an RJ-45 connector at the end, which uses an 8-pin connector. A CAT3 wire may have an RJ-11 connector, which has 4 pins or an RJ-24 connector, which has 6 pins. Both connectors are smaller than the RJ-45 connector.