Business
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

Business
The Meter Base, Service Entrance Cable and Where Responsibility Splits
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Between the utility’s wires and the panel inside the house there is a short stretch of equipment that confuses almost everyone, because ownership changes somewhere in the middle of it and the exact point varies.

Working from outside in, the utility runs conductors to the house either overhead or underground. On an overhead service those wires terminate at a point of attachment, usually a hook or bracket bolted to the structure, and the utility’s responsibility generally ends at or just past that connection. The mast or conduit holding that attachment, the weatherhead at the top, the cable running down and the meter enclosure on the wall are normally the property owner’s equipment.

The meter itself is the utility’s. The box it plugs into, the meter base or socket, is usually not. This is the distinction that surprises people when a meter base is damaged, because the utility will pull the meter and disconnect power but will not repair the enclosure.

From the meter, service entrance conductors run to the main disconnect. Until they reach that disconnect they are not protected by any overcurrent device, which is why the run is kept as short as practical and why the code treats it with particular care.

Underground services follow the same logic with different hardware. The transformer, the buried cable and the connection at the pedestal belong to the utility. The conduit riser at the house, the meter base and everything inward belongs to the owner.

The practical consequence is worth knowing before there is a problem. Storm damage to a mast, a meter base pulling away from a wall, or a corroded service cable are all repairs that have to be arranged and paid for privately, and they require coordination because the utility has to disconnect and reconnect around the work.

Exact boundaries differ between utilities and are published in each one’s service requirements document, which is the only authority worth relying on for a specific address.

Anything touching this equipment is also a coordination job rather than a scheduling one. The meter has to be pulled and the seal broken by the utility, work happens with the house de-energised, and an inspection usually has to clear before power is restored. Planning around a day without power is part of the job.

Related reading: source