Differences Between US and UK Residential Electrical Wiring
There are many differences between US and UK residential wiring, most obviously that UK wiring is supplied at 240VAC line to neutral, single phase, 50Hz. This is as opposed to the US electric service rated at 240VAC line to line, 60Hz, supplied from a center tapped transformer which provides two legs at 120VAC line to neutral. There are a number of other differences in wiring techniques and safety devices as well.
I am not an electrician. In fact, I'm not even British. However I am an electrical engineer and have worked closely with electricians, as well as having done some residential wiring in my own place. All my UK wiring information is second hand, most of which I got from hanging around UK DIY forums on the internet. I will not be held responsible for injuries, damage, or code violations for work done following my advice. Consult an electrician or your local codes.
Incoming power to the US is provided from the distribution network at 13.8 kV (average, this varies from place to place), ratcheted down from hundreds of thousands of volts used on the massive continent spanning transmission network. These 13.8 kV lines are fed into center tapped transformers, sometimes called a pole pig, before being run to the house as a pair of large cables at 240VAC. Sometimes these are run up in the air, but underground service drops are becoming more popular. As with the UK system's guaranteed 50Hz signal, the North American Electric Reliability Council ensures the 60Hz signal is precisely 60Hz when averaged over time.
Supplier owned property on site
The electric utility owns the service drop and the electric meter in the United States. There is no separate fuse block for disconnecting the power when service is discontinued, this is done at the meter. Power consumption is also metered in kilowatt-hours in the US. The utility owns pretty much everything up until the two legs of the system reach the main circuit breaker in the electrical panel. From that point on, it belongs to the customer.
The main electrical panel (or fuse box)
The main electrical panel is a rather large box when compared to the UK consumer unit. First of all, it contains two bus bars for the two legs of the 240VAC line to line power, protected by a main 100 or 200 Amp double pole circuit breaker which protects all the other circuit breakers in the panel. Most circuits only use one of the legs, running off a 120VAC line to neutral connection. Only large equipment like the clothes dryer, the air conditioner, the oven, and the range use both legs for 240VAC power and will be fed from a dedicated circuit. This cuts the current draw in half so that smaller wires can be used for these applications.
Furthermore, the generally larger houses in the US have more circuits than a UK installation, powered from much smaller circuit breakers (15 or 20A). Rather than one circuit which powers multiple rooms, each circuit in a US house will generally power a single room, both the outlets and the lights.
Finally, while a single Residual Current Circuit Breaker will protect multiple circuits in a UK consumer unit, a US Ground Fault Circuit Interruptor breaker will only protect a single circuit. More often, GFCI protected outlets are used rather than providing the protection at the panel with the circuit breaker though.
One further difference is that sub-panels are much more common in the US than they are in the UK. In the generally larger US houses, basements and rooms clear across the house are pretty far away from the main panel and have sub-panels fed from the main panel as a convenience.
The concept of the ceiling rose is unheard of in the US. A hot wire simply comes off the switch near the room's door (or doors for three-way switches) to the light socket, along with the unswitched neutral wire. Typically these are connected with wire nuts, unlike the typical UK terminal block installation. The connection may be continued on to the next light fixture with a third wire in the wire nut.
The same circuit that powers the lights will generally also power the outlets in a room, although the outlets will be unswitched except for the ones that might have a lamp plugged into them.
Radial wiring and electrical outlets
Unlike the UK ring main, in which the hot, neutral, and ground wires are run from the consumer unit jumping to each outlet, and then back to the consumer unit in a loop, the US wiring scheme is radial. This means that the three wires jump to each outlet but do not return to the circuit breaker panel. Because of this, there is only one wire instead of two powering each outlet so the wires must be larger to handle the Amp load by themselves.
Individual outlets are not fused, although some may be protected with built-in ground fault circuit interruptors. This is required outdoors, and in kitchens and bathrooms near plumbing fixtures. Nearly all outlets are duplex (having two sockets), and ones that appear to have four sockets are just two duplex outlets side by side.
All sockets are rated at 240VAC at 15 or 20 Amps, the 20 Amp versions having a T-shaped neutral slot to allow high current draw appliances to be plugged into them and not the 15 Amp versions. They are rated at 240VAC even if they are intended for 120VAC use. The 240VAC use outlets have one of a variety of special shapes that only allows their intended plugs to be used in them. US outlets are not shuttered like UK outlets, and it is recommended that houses with small children use plastic outlet protectors that plug into the socket to cover and insulate them.
The standard US 15 Amp plug will have either two or three prongs, depending on if a ground pin is present or not (unlike UK plugs which always have a ground prong). Polarised plugs will have a wider neutral prong than hot prong to prevent people from plugging them in backwards in cases (such as lamps) where this makes a safety difference. The ground pin is round, and the hot and neutral pins are flat and vertical. In high current draw appliances intended for 20 Amp outlets, the neutral prong is horizontal to prevent someone from plugging it into a 15 Amp outlet. Most US plugs are unfused, with the notable exception of Christmas lights.
US wiring uses different colors than UK wiring. The neutral is always white or light grey and the ground is always bare, green, or green with yellow stripes (very rare), but the hot can be any color except the ones reserved for ground and neutral. Typically black will be used for the hot wire coming from the circuit breaker and red will be used for the switched portion of the wire, and these are the standard colors used in cables found in the hardware store. In complicated installations, the different wire colors help differentiate the different circuits in a cable run.
Bathrooms in the US allow standard wall mounted switches (as long as they are a minimum distance from the sink, toilet, and bathtub/shower) and GFCI protected outlets. There is no need for a special current-limited shaver outlet and optional pull cords are a fashion or convenience decision more than a safety issue. Power showers are rare in the US because of the standard, large, and convenient (but inefficient) US-style water heaters. However the demonstrated efficiency of on-demand water heater units is trying to take away market share from the bulk water heater style home.
Outdoor outlet regulations are similar to UK regulations. GFCI outlets with waterproof covers are required on all accessible outlets.
Special thanks to benjya and Transitional Man for help with this writeup.