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From the 2015 UPC Illustrated Training Manual

408.3 Individual Shower and Tub-Shower Combination
Control Valves.
Showers and tub-shower combinations shall be provided with individual control valves of the pressure balance, thermostatic, or combination pressure balance/ thermostatic mixing valve type that provide scald and thermal shock protection for the rated flow rate of the installed showerhead. These valves shall be installed at the point of use and in accordance with ASSE 1016 or ASME A112.18.1/CSA B125.1. Gang showers, where supplied with a single temperature-controlled water supply pipe, shall be controlled by a mixing valve that is in accordance with ASSE 1069. Handle position stops shall be provided on such valves and shall be adjusted per the manufacturer's instructions to deliver a maximum mixed water setting of 120°F (49°C). Water heater thermostats shall not be considered a suitable control for meeting this provision:

Many reported incidents of serious burns and shocks received while showering have prompted the requirement for anti-scald shower valves, thermostatic mixing or pressure-balancing shower control valves (see Figure 408.3a). Both types of valves regulate the water temperature by tempering hot water with cold water. The code requires that shower and tub-shower combinations in all occupancies shall have individual shower control valves of the pressure balance or the thermostatic mixing valve type.

Where multiple or gang showers are installed, such as in athletic facilities or work places, in lieu of individually controlled pressure balance or thermostatic mixing valves, master thermostatic valves controlling all the showerheads in the system may be used (see Figure 408.3b). In the master thermostatic valve, a moving piston responds to temperature fluctuations from an expanding or moving bellows, a bimetallic unit or heat-conducting fluid effecting hot and cold water input. Set screws or limit stops are pre-adjusted to maintain a constant outlet water temperature not to exceed 120°F. Upon cold water failure, the hot water is instantly shut down.

In addition to the pressure balance or thermostatic mixing valves required by the code, there are numerous temperature-sensitive devices available for installation at the water outlet. Temperature-sensitive aerators, showerheads, etc., are all "aftermarket" products that offer an excellent safety function for those systems that are not otherwise protected. However, these devices are not alternatives to the code-required pressure balance or thermostatic mixing valves.

It must be noted that an abrupt change in temperature (to either hot or cold) may cause an involuntary reflex that can result in a fall when a person is standing on a slippery surface.
This hazard is not present when a person is sitting in a bathtub. Consequently, this code requirement is not applicable to bathtubs or other fixtures that are not also equipped
with a shower riser. Handheld flexible showers are associated with bathtubs and other vessels where the bather is seated and are also not required to comply with this section.

The 2015 Uniform Plumbing Code Illustrated Training Manual eBook is available for purchase here.