Category 5 cable is a twisted pair high signal integrity cable type often referred to as Cat5 or Cat-5. Most Category-5 cables are unshielded, relying on the twisted pair design for noise rejection. Category 5 has been superseded by the Category 5e specification. This type of cable is used in structured cabling for computer networks such as Ethernet and ATM, and is also used to carry many other signals such as telephony and video.

Category 5 cable includes 4 twisted pairs in a single cable jacket. This use of balanced lines helps preserve a high signal-to-noise ratio despite interference from both external sources and other pairs (this latter form of interference is called crosstalk). It is most commonly used for 100 Mbit/s networks, such as 100BASE-TX Ethernet, although IEEE 802.3ab defines standards for 1000BASE-T – Gigabit Ethernet over category 5 cable. Each of the four pairs in a Cat5 cable has differing precise number of twists per meter based on prime numbers to minimize crosstalk between the pairs. On average there are 6 twists per 5 centimeters.  

CAT5 Cable Standard

The specification for Category 5 cable was defined in ANSI/TIA/EIA-568-A, with clarification in TSB-95. These documents specified performance characteristics and test requirements for frequencies of up to 100 MHz.

United States T568B Wiring

Pin Pair Wire Color
1 2 1 Pair 2 Wire 1white/orange
2 2 2 Pair 2 Wire 2 orange
3 3 1 Pair 3 Wire 1 white/green
4 1 2 Pair 1 Wire 2 blue
5 1 1 Pair 1 Wire 1 white/blue
6 3 2 Pair 3 Wire 2 green
7 4 1 Pair 4 Wire 1 white/brown
8 4 2 Pair 4 Wire 2 brown

Connectors and other information

The cable exists in both stranded and solid conductor forms. The stranded form is more flexible and withstands more bending without breaking and is suited for reliable connections with insulation piercing connectors, but makes unreliable connections in insulation-displacement connectors. The solid form is less expensive and makes reliable connections into insulation displacement connectors, but makes unreliable connections in insulation piercing connectors. Taking these things into account, building wiring (for example, the wiring inside the wall that connects a wall socket to a central patch panel) is solid core, while patch cables (for example, the movable cable that plugs into the wall socket on one end and a computer on the other) are stranded. Outer insulation is typically PVC or LSOH.

Cable types, connector types and cabling topologies are defined by TIA/EIA-568-B. Nearly always, 8P8C modular connectors, often incorrectly referred to as "RJ-45", are used for connecting category 5 cable. The specific category of cable in use can be identified by the printing on the side of the cable.

The cable is terminated in either the T568A scheme or the T568B scheme. Canada and Australia use the T568A standard, and the United States commonly uses T568B scheme. It really doesn't make any difference which is used as long as you use only one of the standards so all connections are the same at your location to avoid confusion and potential problems. Mixed cable types should not be connected in series as the impedance per pair differs slightly and may cause signal degradation. The article Ethernet over twisted pair describes how the cable is used for Ethernet, including special "cross over" cables.

Conductors Required

10BASE-T (IEEE) and 100BASE-TX (IEEE) Ethernet connections require two cable pairs. 1000BASE-T (IEEE) and 1000BASE-TX (TIA/EIA-854, requiring category 6 cabling) Ethernet connections require four cable pairs. Four pair cable is by far the most commonly available type.

Bending radius

Most Cat5 cables can be bent at a radius approximately 4 times the diameter of the cable.

Maximum Cable Segment Length

According to the ANSI/TIA/EIA standard for category 5e cable, (TIA/EIA 568-5-A[5]) the maximum length for a cable segment is 100 meters (328 ft). If longer runs are required, the use of active hardware such as a repeater, or a switch, is necessary. This 100 meter limit is all inclusive. According to the standard, the practical limit for permanently installed cable is about 90 meters, leaving 5 meters at each end for the CAT5 patch cables that connect the end equipment to the wall panel.