Not every wire in a house carries a lethal voltage. Doorbells, most thermostats, irrigation valves, alarm sensors and a good deal of garden lighting run on low-voltage circuits that behave under entirely different rules from the 120 volt system around them.
These are almost always Class 2 circuits, which means the supply is limited in both voltage and available power by the transformer feeding them. A typical doorbell transformer produces sixteen or twenty-four volts and is limited to a few volt-amperes. That limit is what makes the circuit safe to handle and is why the thin conductors used are permitted despite looking far too small for anything.
The transformer is usually mounted on a junction box near the panel, in a mechanical room or occasionally on the side of the furnace. Its line side is a normal 120 volt circuit and needs a proper enclosure. Its low-voltage side is what runs through the house on thermostat cable, commonly eighteen-gauge with several colour-coded conductors in a jacket.
Because the power is limited, low-voltage cable does not need the same protection as branch circuit wiring. It can often be run without conduit, stapled loosely and passed through framing without nail plates, though it still needs to be supported and kept out of harm.
What it does need is separation from line-voltage wiring. Running low-voltage cable in the same box or the same hole as 120 volt conductors risks a fault crossing between them, and it also picks up interference that makes sensors behave unpredictably.
Troubleshooting these systems is mostly about the transformer and the terminations. A doorbell that stopped working is far more often a dead transformer or a corroded button contact than a broken wire, and the transformer is the cheapest thing in the chain to test.
Modern thermostats have added a wrinkle worth knowing about. Many now need a constant common conductor to power themselves, and older installations frequently ran only the conductors the old thermostat required. That is why a straightforward-looking thermostat swap sometimes turns into pulling new cable back to the furnace.
More on this: wiring basics for homeowners
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