Electrical

Distribute electricity from the service to protected branch circuits.

Follow the service, panel, protective devices, cables and outlets as a connected system.

Context. An educational model. Geometry, material assumptions and the reference building are described in the illustrated study.

Electricity reaches a house the way water does: from a main in the street, through a meter, to a central point that splits it into branches. But it is far less forgiving. Too much current heats a wire until its insulation fails; a loose connection arcs; a nail through a cable or a hair dryer in a sink can kill. So every part of a house’s wiring is sized, protected and placed by rule. The figure follows the reference house’s power from the utility’s transformer at the corner of the lot to a receptacle over the kitchen counter.

The utility’s primary line runs at thousands of volts. A transformer, here a green box on a pad at the corner of the lot shared with the neighbours, steps it down to 120/240 V: two hot conductors 240 V apart, each 120 V to the neutral between them. From it the service lateral runs underground in PVC conduit 24 in (610) down, bedded in sand, with a warning tape a foot above, and sweeps up the front wall into the meter base. The meter belongs to the utility; everything after it belongs to the house.

Inside the wall behind the meter, the service entrance cable drops to the main panel in the basement. It is kept as short as it can be, because nothing protects it: the main breaker is at its far end. That is why the panel sits on the basement wall straight behind the meter, and why the meter is on the side of the house facing the street.

Main panel

A 200 A main breaker, two hot bus bars, a neutral bar and a ground bar, 40 spaces. A 120 V breaker clips to one bus; a 240 V breaker spans both.

Grounding

Two 8 ft (2.4 m) rods driven 6 ft (1.8 m) apart outside, and a bond to the metal water pipe within 5 ft (1.5 m) of where it enters. They hold the house’s wiring at the earth’s potential and give lightning and utility faults a path into the ground.

Bonding

Every metal pipe, box and frame is tied to the ground bar, so that a live wire touching one trips a breaker instead of waiting for a hand.

The code sizes a house’s service by a standard calculation, not by adding up nameplates: not everything runs at once. General lighting and receptacles count 3 VA per square foot; the two kitchen circuits and the laundry 1,500 VA each; the first 3,000 VA of that at full value and the rest at 35 percent. Fixed appliances (dishwasher, disposal, microwave, furnace blower) count at 75 percent, a range at 8 kW, an electric dryer at 5 kW, and the larger of the heating and the cooling.

For the reference house, 2,592 ft² (241 m²) on three levels, that comes to about 26.7 kVA, or 111 A at 240 V. A 150 A service would do. It gets 200 A, the usual size for a new house, because the next owner may add an electric car charger, a heat pump in place of the furnace, or an induction range, and replacing a service is far dearer than buying a bigger one at the start.

From the panel, branch circuits run to the rooms. Each has a breaker sized to protect its wire, not its loads: 14 AWG copper on a 15 A breaker, 12 AWG on 20 A, and heavier for the range and the air conditioner. Plug in too much and the breaker’s thermal element heats and trips it, as the figure’s does: a hair dryer and a space heater on the bathroom circuit draw 25 A, and a 20 A breaker will not hold that for long.

GFCI

A ground-fault circuit interrupter compares the current going out on the hot wire with the current coming back on the neutral. If 5 thousandths of an amp go missing (through a person to a wet floor) it opens in a fortieth of a second. Required wherever water is near: kitchens, baths, laundries, basements, outdoors.

AFCI

An arc-fault interrupter listens for the ragged current of an arc, from a crushed cord or a loose connection, and opens before it can start a fire. Required on most circuits that serve living spaces.

Voltage drop

Wire has resistance, so a long run loses voltage under load. A branch circuit should lose no more than about 3 percent; the readout gives each circuit’s drop for its length and what is plugged in.

Cable goes in at rough-in, with the frame open and before insulation. Like the ducts and pipes, it must pass through the structure without weakening it, and stay where a nail cannot find it.

Joists

Cables crossing joists go through holes bored at mid-depth, far inside the limit of a third of the depth and 2 in (50) from either edge. Cables along a joist are stapled to its side.

Studs

Holes bored at the centre of the stud. Where a hole’s edge comes within 1¼ in (32) of a face, a steel nail plate 1⁄16 in (1.6) thick covers it, so a drywall screw or a trim nail stops at the steel.

Staples

Within 8 in (205) of a box that has no cable clamps, within 12 in (305) of one that does, and every 4½ ft (1370) along the run.

Boxes

Nailed to the side of a stud, standing out to finish flush with the drywall. Receptacles at 12–16 in (305–405) above the floor, over counters at about 44 in (1120), switches at 48 in (1220) beside the latch side of a door. No point along a wall may be more than 6 ft (1830) from a receptacle, so no cord has to cross a doorway.

Outside walls

Every box in an outside wall is a hole in the air barrier. Airtight boxes with a gasketed flange seal to the drywall, and the batt is cut round the box and split round the cable rather than crushed behind it. The wall’s control layers are drawn in Wall assembly.

The rules quoted are those of the US model code for houses; local codes add to them, and the utility sets its own rules for everything up to and including the meter.