Using Cat5 cable for powering doorbell and wire gauges

Question

The house I moved into had a cat5 cable running from their video doorbell to their base unit instead of a "regular" 2 conductor wire (that I've seen in previous places).

I want to upgrade to a Nest doorbell and don't want to run new wire through the house so plan to use the same existing cat5 cable to do so. I've been advised to "double strand" the wires in the cable to ensure that it has enough power.

One issue I have is where the cable comes into the house, it is not long enough to reach the junction box where I plan to put the 16V transformer. This leads me to my 2 main questions:

To double strand, is it literally just twisting the copper ends of two wires within the cable together and hooking that into the screws?

Because the cat5 cable isn't long enough to reach the transformer, can I attach a similar gauge of wire (or more accurately, a gauge that matches two 24 or 26 gauge cat5 wires twisted together) with a wire nut to create an extension to reach the transformer?

Does the gauge of the wire from the transformer to the chime have to match that from the doorbell to the transformer?

Is there anything else I should know or am blatantly ignoring?

Here are some photos if it helps:

Current doorbell wiring outside: https://imgur.com/Rb7vmDX

Base unit (in kitchen): https://imgur.com/njvSTkS

Junction Box (in kitchen): https://imgur.com/DQcHxhl

Answer 1

If your doorbell (I'm not looking it up, presumably it doesn't use the cable for data) needs only 2 wires, might as well do 4&4 since you have 8, and they are much smaller than standard doorbell wire, while the video doorbells tend to be much more power-hungry than a normal doorbell.

Specifically, do all the white-with-color as a bundle, and all the solid colors as a bundle, and that way, your current-carrying wires end up twisted together just like real doorbell wiring.

Probably easiest to strip the 4 wires and wire-nut them to a single heavier wire to connect, rather than trying to get 4 twisted-together wires under one screw. Where you need to extend the wire to transformer, wirenut each group of 4 to one wire in a two-conductor cable of a more suitable size, such as 16 or 18 gauge. The wire gauge for the bits at the end does not need to match the Cat5 wire gauge.

Even though a junction box for connections is not required for low voltage wiring, it's a very good idea, and tends to promote the junctions staying together, rather than falling apart.