Why Does Cat5e Cable Max Out at 100 Meters? The Physics Behind the Limit

Many network engineers know that Cat5e (超五类) cables generally have a transmission distance limit of 100 meters; beyond that data cannot be guaranteed to transmit stably, with severe packet loss or no connectivity. But why is it limited to 100 meters? Few can explain.

Before discussing this, we must clarify that the 100 meters refers to the distance between the two electrical ports of directly connected network devices, including network modules and patch panels.

We know digital data is transmitted in binary, i.e., 0 and 1. Ethernet cables carry current that creates voltage; we transmit data via voltage. Voltages between +5 V and +15 V are defined as “1”, and between –5 V and –15 V as “0”. Voltages between –3 V and +3 V are considered discarded. There is a tolerance margin from 3 V to 5 V, because at ±3 V the signal is discarded, so it cannot be counted as 2.

Based on the above principle, we can roughly calculate the maximum cable length. The national standard specifies the conductor diameter as 0.5 mm, giving a single‑wire cross‑section area of 0.1965 mm². At room temperature, copper resistivity is about 0.0175 Ω·mm²/m, and the characteristic impedance of the cable is |Z| = 100 Ω.

Using the formula resistance = resistivity × length / area, we can estimate the maximum length of the cable to be about 112 meters.

Of course, this is a simple calculation and not very rigorous. In practice, one must also consider the data rate and the capacitive effects of longer cables (greater parallel capacitance slows speed), as well as cable bending and compression, which affect actual length. Considering redundancy for patch cords and future aging, it is advisable not to exceed 90 meters, especially for PoE installations where extra margin is needed.