The panel is an ohmmeter
A supervised zone is a resistance measurement with a threshold on it. Wire a resistor across the far end of the loop, put the contacts in series with it, and the panel can tell the difference between a closed loop, an open loop and a cut cable, because all three read differently. That is the whole idea, and it is a good one. What it means in practice is that anything else with resistance in the loop is added to the reading, and the panel has no way to know it was the cable rather than the resistor.
On the defaults — 500 ft of 22 AWG out to a detached building, eight devices — the cable and contacts add about 17 ohms to a 2,200 ohm resistor. Against the resistor that is under one percent and looks like nothing. Against the 100 ohm window in the form it is 17 percent, and that is the reading that matters. Run the same distance in 24 AWG and it is 26.5 ohms; run 24 AWG copper-clad aluminium and it is 40.6. Shorten it to 150 ft of 22 AWG, which is a normal indoor zone, and the whole thing drops to 5.6 ohms and stops being a question at all.
Where the margin actually goes
Alarm loops rarely fail on the day they are installed. They fail years later, when a junction in a crawl space gets damp and adds a few hundred ohms, or a screw terminal that was tightened onto insulation finally loses the last strand of contact it had. If the cable already used most of the window on day one, there is nothing left to absorb that, and the zone starts reporting a fault at four in the morning in February and behaving perfectly when somebody comes to look at it in the afternoon.
So the useful reading of the number this page gives you is not whether it works now. It is how much of the budget is already spent before anything has aged.
The dual resistor case
Put a second resistor across the contact and the panel gets a third reading, which lets one pair of wires report normal, alarm and tamper separately. The two resistors in parallel give a lower value than the end of line one alone, and the gap between those two readings is what the panel discriminates on.
Cable resistance shifts both readings up by exactly the same amount, so it does not close the gap. What it does is move both of them toward whatever the panel expects, which matters if the panel is checking absolute values rather than a difference. Most do check absolute values, which is why the offset is still the number to watch.
Powered devices change the problem
A contact is a switch and draws nothing. A motion sensor is a powered device and pulls current down the same cable, which turns the run into a voltage drop question as well as a resistance one. On the defaults — two sensors at 20 mA over 500 ft of 22 AWG — the drop is about two thirds of a volt. Put four sensors on the end of that same run in 24 AWG and it is over two volts.
The part that catches people is that the panel does not hold its stated auxiliary voltage on battery. It falls as the standby battery discharges. So the worst voltage at the end of the longest thinnest run happens during a power cut, an hour into the exact event the system exists for, and it happens to the sensors furthest from anybody who would notice.
What this page will not do
It will not tell you the run is acceptable. The window, the resistor value, the auxiliary voltage and the minimum operating voltage are all numbers that come from the panel documentation and the device data sheets in front of you, and they differ between makers and between models from the same maker. The page takes yours, does the arithmetic, and prints both sides of every comparison so you can make the call. Where a jurisdiction, an insurer or a monitoring agreement imposes requirements on any of this, they take precedence over anything worked out here.
Questions people ask
How long can an alarm zone cable run be?
It depends on the gauge and on the resistance window your panel works to, and the page prints the figure for both. On a 100 ohm window with 22 AWG copper and eight devices, the wire alone can run about 3,070 feet one way before it fills the window. Drop to 24 AWG copper-clad aluminium and that falls to around 1,250, a factor of about two and a half. The panel documentation is what sets the window, and panels differ by more than gauges do.
Does wire resistance affect an end of line resistor?
It adds to it. The panel measures the whole loop, so it sees the resistor plus the cable plus every contact and termination in series. On a short run that is a fraction of a percent and nothing. On a long run of thin wire it can be a meaningful part of the band the panel reads as normal, and it is entirely invisible until something else adds a few hundred ohms years later.
What size wire should I use for alarm sensors?
This page states no requirement. It shows what each gauge does to the loop resistance on your run and to the voltage arriving at powered devices, and prints the run length each one allows for the window you entered. The heavier gauges cost more and pull harder through a stud bay; the lighter ones spend more of the margin. Which trade is right is a judgement about the building and about who has to maintain it.
Why does my motion sensor drop out during a power cut?
Because the panel auxiliary voltage falls as the standby battery discharges, and the device at the end of the longest thinnest run has the least voltage to give away. It works perfectly on mains, when the panel is holding its full output, and stops when the supply sags into the drop the cable is already taking. The page shows the arriving voltage at the output you enter, so the same arithmetic can be run at a lower supply figure.
Is copper-clad aluminium alarm cable a problem?
It has about 55 percent more resistance than solid copper at the same gauge, so every number on this page gets worse by that factor. It is also more brittle at terminations, and terminations are where alarm cable fails. Whether it is permitted at all in your installation is decided by the rules in force where you are and by whoever inspects the work, not by this page.