Wires used in a Cat5e Cable

There are a total of eight wires inside a modern Ethernet twisted-pair cable which can accommodate an 8P8C (RJ45) connector or punched into a patch panel, which is Twisted into four pairs. All of the eight wires (or four pairs) can be used for 1000Base-T/GigE signalling at rates up to 1 Gbps.

Please see our other blog post How Gigabit Ethernet Works for additional useful info to assist you in understanding Ethernet, ethernet cabling systems, Ethernet categories and Ethernet wiring schemes. 

However, only 4 (wires not pairs) are needed for either 10Base-T signalling at rates up to 10 Mbps or 100Base-T/Fast-E signalling at rates up to 100 Mbps. Additionally, the blue wires alone can be used on re-crimped Cat5e cables for telephony, changing the connector from 8P8C to RJ11.

So, by using 100Base-T or lower will free up wires for other uses. 

In the 802.11af Power over Ethernet standard, wires 4 and 5 are ground wires, and wires 7 and 8 are live power wires. Ethernet cables can also be used with HDMI-over-Ethernet adapters as an in-place cabling method when lower signalling rates are acceptable. You could even peel off two pairs (for four total wires) to support 100Base-T Ethernet and two separate phone lines.