Cat 5

Cat 5 is a four-pair twisted copper cable designed to carry high-frequency signals over short distances, typically in local area networks. The designation comes from Electronic Industries Alliance (EIA) Category 5 specification, which sets performance standards for the cable's bandwidth, impedance, and signal integrity. Each pair twists at a specific rate to minimize crosstalk and external interference, making it suitable for Ethernet, telephone, and other structured cabling applications.

The cable consists of eight 24-gauge copper conductors arranged in four pairs, each pair insulated in different colored jackets: white-blue, blue, white-orange, orange, white-green, green, white-brown, and brown. The entire bundle is wrapped in an outer plastic sheath, typically PVC or polyethylene. Cat 5 supports frequencies up to 100 MHz and can transmit data at 100 Mbps over distances of 100 meters, the standard run length for horizontal cabling in commercial buildings.

Two variants exist: unshielded twisted pair (UTP) and shielded twisted pair (STP). UTP is far more common in office and residential installations because it is cheaper, easier to terminate, and generates less heat. STP includes a foil or braid shield around the pairs and is chosen for environments with heavy electromagnetic interference, such as factory floors or broadcast facilities. The shield adds cost and requires proper grounding at both ends to be effective.

Common failure modes and limitations

Cat 5 cables degrade when bent sharply, crushed, or exposed to moisture. The twisted pair geometry can be damaged during installation if the cable is stretched or pulled too aggressively, and the unshielded version is vulnerable in environments with strong radio frequency sources or high-current switching equipment. Modern networks typically use Cat 5e (enhanced) or Cat 6, which support higher frequencies (250 MHz and 500 MHz respectively) and lower crosstalk, but Cat 5 remains functional for legacy 100 Mbps installations and is still manufactured and stocked.

Cat 5 connectors use the RJ45 standard, an 8-pin modular jack. Proper termination requires careful unwinding and arrangement of the pairs to the correct pinout, as poor termination introduces signal loss and bit errors. The cable is rated for maximum use in uncontrolled indoor environments between 0 and 40 degrees Celsius; exceeding these temperature limits can cause the plastic jacket to become brittle or the impedance to shift out of specification.