Transfer switches are the unsung and often neglected heroes of emergency, legally required standby and optional standby systems. The much larger generators and other alternate-power sources often overshadow these switches, but ultimately, these devices sense normal power loss and activate the generator or alternate power source to critical loads.


Proper installation and maintenance ensure transfer switches function correctly and the generator starts when it needs to. However, many electricians and electrical contractors know only what is in the National Electrical Code (NEC), such as the information on transfer switches in Article 700 covering emergency power, Article 701 covering legally required standby power, and Article 702 for optional standby power. The listing and labeling information for these devices contains many more requirements, and the electrician and contractor must understand them to enable a proper installation.


Underwriters Laboratories’ 2015 Guide Information for Electrical Equipment, normally called 2015 UL White Book, contains important information pertinent to electrical designers, installers, inspectors and engineers. Within the guide, Category WPTZ covers transfer switches, which can have a maximum rating of 600 volts (V) alternating current (AC), can be automatic or nonautomatic, and can have associated control devices. Transfer equipment can also be rated in excess of 600V AC but cannot exceed 38 kilovolts.


If a normal supply fails, automatic-transfer switches are intended to transfer a common load to an alternate supply. They also are intended to automatically return the load to the normal supply when the normal supply source is re-established. Transfer switches rated for total system transfer (100 percent load transfer) are marked, “Suitable for control of motors, electric discharge lamps, tungsten filament lamps, and electric heating equipment where the sum of motor full-load ampere ratings and the ampere rating of other loads do not exceed the ampere rating of the switch and the tungsten load does not exceed ____ percent of the switch rating.” The manufacturer would fill in the blank with the percentage value based on the switch’s continuous rating.


Transfer switches have been investigated for load switching and in-rush capabilities as well as the number of operation cycles based on the switches’ intended use, which includes scheduled test operations where the full load is switched. Most transfer switches are designed so the load cannot be simultaneously disconnected from both normal and alternate-power sources where either power source is available. The exception is if the transfer switch is suitable for use as service equipment and the disconnecting means is externally accessible so each power source (service or other) can be independently disconnected. Transfer switches that are rated for use as service equipment may be provided with ground-fault protection as part of the transfer switch and may contain feeder overcurrent protective devices.


Transfer switches are typically intended for use with copper conductors, unless marked to indicate terminals that are suitable for use with aluminum conductors. This additional marking for aluminum conductors is independent of any marking on the terminal connectors and is indicated on a wiring diagram or other readily visible location. This requirement is different from most other electrical equipment because it requires a specific marking on the switch or on the wiring diagram indicating aluminum conductors can be used. Unless marked otherwise, wiring space within the switch enclosure and the terminations is based on 60°C wire for switches rated 100 amperes (A) or less and 75°C for switches rated more than 100A.


Transfer switches that contain integral overcurrent devices are suitable for continuous loads at 100 percent of the switch-rated current, unless restricted to not more than 80 percent of rated load current. The switch will be clearly marked as “Continuous Load Current Not to Exceed 80% of Switch Rating.” If the transfer switch does not contain integral overcurrent-protective devices, the transfer switch is rated at 100 percent of rated current.


Most transfer switches are not permitted to be installed on the line side of the service overcurrent device and must be marked with the maximum ampere and class of fuse. If a circuit breaker is used for protection, the switch manufacturer must provide the breaker’s specific size, specific manufacturer and maximum current rating.


Transfer switches may be required to be marked as follows: “When protected by a circuit breaker rated no more than ____ amperes, this transfer switch is rated for use on a circuit capable of delivering not more than 10 kA rms symmetrical amperes, ____ volts maximum.” The first blank is filled with the circuit breaker’s maximum current rating, and the second is filled with the maximum circuit voltage.


There is much to know about transfer switches in addition to the information that the NEC contains.