Transfer Switches Simplified

Jan 1, 2009 12:00 PM, By David Herres, Master Electrician

The retrofit

Photo 2. Automatic selective circuit transfer switch retrofitted to 200A residential service. Because it is for an optional standby system, the owner designates the circuits to be supplied.

Begin a retrofit by mounting the selective circuit manual transfer switch next to the service entrance panel (Photo 2). A length of flexible raceway, in many cases electrical nonmetallic tubing (ENT), is often furnished with the transfer switch. However, you should replace this with metallic raceway if the building is three or more stories above grade — or if ENT is prohibited by local codes. Note: ENT is permitted per the 2008 version of the NEC.

Feed the prewired harness through this raceway. It includes heavy conductors for the new double-pole breaker in the existing service entrance panel along with designated ungrounded branch circuit conductors. Splice these new conductors from the harness to the preselected branch circuit conductors with twist-type wire connectors inside the service entrance panel. The raceway also includes a green equipment grounding conductor, which is provided for the purpose of grounding the transfer switch enclosure. The wiring harness is preterminated in the selective circuit transfer switch so the only connections that have to be made are those in the service entrance panel. Although the original branch circuit breakers are no longer used, you can leave them in place as spares, or remove two of them to make space for the new double-pole 60A breaker.

With increased arc fault and GFCI branch circuit mandates, it may be necessary for you to upgrade some of the breakers in the transfer switch. Some models do not allow for this change. It's important not to overlook the whole notion of breaker compatibility. Some breakers from different manufacturers may appear to interchange and fit the box properly, but they are not listed for this purpose. One problem could be a mismatch of the metal alloy with the bus bars. Accordingly, you must address this detail.

In addition to the cost of a double-pole breaker, another item of significant expense that has to be furnished to complete the job is a flexible cord of sufficient ampacity and suitable to the environment to bring power from the generator to the transfer switch or — if hard-wired — to a power inlet box mounted on the outside of the dwelling.

Know the Code

The installation we've been discussing is, of course, governed by the NEC. The transfer switch is defined in Art. 100 as: “an automatic or non-automatic device for transferring one or more load conductor connections from one power source to another.” The reason this definition does not mention keeping the sources isolated from one another is that, in certain instances, one or more electric power sources are permitted to operate in parallel with a primary source of power, notably the electric utility. This situation is covered in Art. 705 (Interconnected Electric Power Production Sources) and is seen in homes with wind or solar power systems where there is a cogeneration setup with the electric utility. For this type of hookup, a synchronous inverter is required so that DC power from the storage batteries is converted to AC power of the proper voltage and frequency, and phase-synchronized to the electric utility supply.

The requirements in Art. 445 (Generators) is very brief and makes no mention of the required transfer switch. This and other details are covered in Art. 702 (Optional Standby Systems).

Emergency systems are those that supply power and/or light in the event of an electric utility outage to designated areas where such electrical supply is essential for safety to human life. These systems require ATSs, and a minimum time interval is specified for the switchover to be complete.

Legally required standby systems are those mandated by municipal, state, federal, or other codes or by a jurisdictional government agency. These also require ATSs. There is also a minimum time requirement, but it is longer than for emergency systems.

Optional standby systems supply power to facilities where life safety is not an issue and where they are not mandated by codes or government regulations. Transfer switches may be manual or automatic. In addition, there is no specified minimum transfer time. In most cases, it is this Article that governs residential backup power systems, including transfer switches. Before performing such an installation, you should carefully review this Article to make sure your work is NEC compliant.

Optional standby power is useful for supplying those loads which, when not served due to an electric utility outage, could cause discomfort or financial loss, but are not life safety related. If only certain loads are picked up by the optional standby power supply, then it is the responsibility of the homeowner or designated manager to select those loads. Typical choices are heating and refrigeration (especially freezers where thawing could be costly), data processing and communications, water supply and sewage disposal (where electric pumps are involved), selected lighting, garage door openers (so that a vehicle is not stuck inside), some cooking equipment (including gas stoves with electric ignition), an electric clock, and the like. Choices can be based on need and amount of power consumed. It should be possible to supply everything essential to a normal existence and even include TV and computer without overloading a moderate-sized generator.

Article 702 requires that audible and visual signals shall be in place to indicate failure of the optional standby source and to indicate when the optional standby source is carrying its load. An exception allows these signals to be omitted if the optional standby source is portable, but not if it is permanently installed at a given location.

Another Art. 702 requirement is that a sign shall be placed at the service entrance panel stating type and location of the optional standby equipment.

To properly size the backup generator and transfer switch, follow the procedures covered in Art. 220 (Branch Circuit, Feeder and Service Calculations). If two loads will not be used concurrently, the smaller of these can be omitted in calculating the total load. The classic example is heat and air conditioning. This calculation will influence the size of the generator and transfer switch.

As we have seen, a transfer switch is a key element in a residential backup power installation. However, it's simply a basic double-pole, double-throw device with a middle off position.

Herres is a licensed master electrician in Stewartstown, N.H. He can be reached at electriciansparadise@hughes.net.

Sidebar: Safety First

There really isn't a fail-safe way to ensure every building on the electric utility grid has its main disconnect opened before its on-site generator is fired up. For this reason, some utility supply lines will receive a backfeed during outage periods. In this situation, the generator's 240V output is applied to the electric utility transformer's secondary windings. From there, it is stepped up to line voltage, invariably in the kilovolt range. This situation endangers the lives of electric utility workers.

In the event a backup generator is supplying power to a service entrance panel with the main disconnect in the ON position, and electric utility power is restored, the two power sources will not be in phase. In this case, the smaller of the two power sources (i.e., the home generator), with its much higher internal impedance, will be destroyed