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Subpanels: When You Need One and How They Differ
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A subpanel is a smaller distribution panel fed from the main panel, providing additional circuit positions somewhere else in the building. Garages, workshops, finished basements and additions are the usual reasons for one.

The case for a subpanel is usually one of two things. Either the main panel has run out of physical space for breakers, or the new circuits are far enough from the main panel that running each one individually would mean an unreasonable amount of cable. One feeder to a subpanel, with branch circuits distributed locally, is cheaper and tidier.

The important technical distinction is how neutrals and grounds are handled.

In the main panel, the neutral and the equipment grounding conductors are bonded together at a single point. This is the one place in the system where that connection is made.

In a subpanel, they must be kept separate. Neutrals land on an isolated neutral bar, grounds land on a separate ground bar bonded to the enclosure, and the bonding screw or strap that ties the neutral bar to the can must be removed.

The reason matters. If neutral and ground are bonded in both panels, normal return current has two parallel paths back to the source, and part of it will flow on the grounding conductors and on any metal bonded to them. Grounding conductors are not intended to carry current in normal operation. This is one of the most common errors in owner-installed subpanels, and it does not produce any obvious symptom.

The feeder itself needs four conductors: two hots, a neutral, and an equipment ground. Three-conductor feeders were permitted in some circumstances historically and are not acceptable for new work.

Sizing is straightforward arithmetic. The feeder breaker in the main panel protects the feeder conductors, and the subpanel must be rated at least for that breaker. There is no requirement for the sum of the subpanel breakers to be less than the feeder, since not everything runs at once, but the actual expected load should be calculated rather than guessed.

Detached structures bring additional requirements, including a means of disconnect at the structure and a grounding electrode system at that location.

Related guide: home electrical wiring

Real Estate
Low-Voltage Wiring for Doorbells, Thermostats and Sensors
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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