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Quoting this because it is cool and I finally actually get to use a fact from my college education (which never happens). It isn't a CAT5 limitation it is a 100Mbit ethernet limitation. Before any ethernet frame is transmitted the physical layer transmits 56 ones and zeros, on off on off on off on off. At 100 mbit at the speed of light, a client on the far side of a 100m connection is guaranteed to see the first on off transition before the transmitter finishes the 56 bit preamble. This prevents anyone else from transmitting at the same time. CDMA - Carrier Detect Multiple AccessThe cat5 distance limitations only apply because of the fast signaling rate being transmitted across the wires.
Anyway, the distance of the CAT5 cable isn't a big deal when added in series to the thermistor because even at 500F the resistance is still in the kiloohm range and 100ft of cat5 wire is like 2 ohms.