You seem to misunderstand something basic about resistance and wire! Let me list some details to illustrate what to do.

Let's say 24 ga wire has 0.025 ohms per foot. That's about what it has.* Let's say 18 ga wire has 0.005 ohms per foot, because that's about what it has.

If you have a ten foot run of 24 ga wire, its resistance is about a 0.5 ohm. If you had a ten foot run of 18 ga wire, its resistance would be about 0.1 ohm -- 20% of the resistance of the 24 ga wire. But you can't figure out how to put an RJ12 on an 18awg wire.

Don't. Instead make two 6 inch pigtails of 24 ga wire coming out of RJ12s. That's a foot of wire. Splice this into 9 feet of 18 gauge wires. The resistance becomes

1 foot of 24 ga = 0.025 ohms
9 feet of 18 ga = 0.045 ohms
The resistance of this combination of wires is 0.025 + 0.045 ohms = 0.07 ohms. You have to double that as the circuit has two wires, so the resistance of the circuit wiring is 0.14 ohms.

We started with a 24 ga wire with 0.5 ohms of resistance. We now have a cable of 0.14 ohms resistance.

Two things can be seen:
Those pigtails make it possible to drastically lower the resistance, and
Any system of batteries and parts that's susceptible to failure with resistances WAY under one ohm has design problems!


Tell your relative you have to call on a professional since you can't figure it out. Get the freakin' locksmith back out there.

You will suffer momentary shame because of being powerless to help your relative, and he will never call you to waste your time on something for him for free again. How sad!






*Retrieved from AWG Characteristics file that's been in my "Tech Info" directory for ten years

Thanks Ernie, I thought splicing cat at each end would kill any advantage of a heavier wire in the middle.

The locksmith is supposed to be coming back to try to solve the problem. But that doesn’t prevent me from getting multiple phone calls asking if I can do something about the wire not being big enough.

If the lock system and keypad both have RJ12 connectors, then there is something else going on.
I can not think of a single manufacture ever in 26+ years, that would have RJ12 connections at both ends, but would require you to splice a heavy gauge wire in for a short run. No, no, no... something else is going on.
There is a problem somewhere and it is NOT wire size related.

If you are going to help your relative out, you are going to have to get the make and model of everything involved and do some research. If you can't find copies of the installation instructions online, then the locksmith HAS to supply you with them.
After you have familiarized yourself with the system, you will need to go thru it and check every detail of the locksmith's work. Good chance he screwed up connecting something or not using the right part. Or something not configured right. At the very least, you might discover the wrong power source is being used.
9V lithium batteries have a slightly higher voltage (9.6V typical) and almost 3x the mAh (1200mA). Where as a 9v NiCd only has 7.2V and 120mA.