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Transfer Switches Simplified

Transfer Switches Simplified

Practical tips for understanding and installing these devices in residential applications

As backup power sources become more common in residential construction (Photo 1), many homeowners have learned about the needs and benefits of transfer switches. They know in the event of an electric utility power supply outage it is at the very least desirable to power up selected circuits in a service entrance panel to provide limited light, heat, and refrigeration. Having the option of operating a double-throw switch — or sitting back and doing nothing while the switch operates on its own — is a very appealing and safe option.

Switch basics

In its most basic form, a transfer switch for a 120/240V single-phase service is simply a double-pole, double-throw switch of proper ampere rating with a middle off position (Fig. 1). By design, it is a physical impossibility for both power sources to supply the load simultaneously, unless, of course, that is the intent. It's also impossible for the two sources to mix — or for one source to backfeed into the other.

Photo 1. Interior of an 8kW LPG generator installed in residence for backup power. In the event of an electric utility outage, it starts automatically and comes online within less than a minute.


Installing a transfer switch is a relatively straightforward task. You install the switch upstream of the service entrance panel, which typically contains the main disconnect. You should locate it next to the entrance panel. For a retrofit, if the main disconnect is part of the service entrance panel, it's necessary for you to pull the meter (with electric utility permission) in order to de-energize the service entrance conductors. Then, reroute them into the transfer switch, wire in the generator, and refeed the service entrance panel. In all cases, you should use cable or raceway suitable for the environment and of proper ampacity. Make sure the neutral is run through unswitched, and the transfer switch enclosure is properly grounded. Where aluminum conductors are used, be sure to wire-brush the metal, apply corrosion inhibitor, and torque the lugs to the proper value.

For the electrician who has never retrofitted a transfer switch, the job is relatively straightforward, because it closely resembles the work involved with an ordinary service installation.

Manual versus automatic

A manual transfer switch must be actuated by the homeowner. After having ascertained that the electric utility outage is more than a momentary loss of power, the homeowner must manually start the engine, allow it to come up to speed and stabilize, and then throw the lever on the transfer switch into the generator position.

Fig. 1. A double-pole, double-throw switch is the basic transfer equipment when the entire load is supplied by either the normal source, such as an electric utility, or the backup source, such as a diesel generator. Backfeed or inadvertent connection of the two sources is impossible.


On the other hand, an automatic transfer switch (ATS) continuously monitors electric utility power. Fluctuations or serious power quality issues, which might precede an outage, trigger a start command to the generator. After backup voltage and frequency stabilize, the transfer switch brings the generator online. Once electric utility power is restored, with no fluctuations for a predetermined amount of time, the switch goes back to its normal position. After a cooling down interval, the generator is automatically shut down. All of these actions take place with no human intervention.

No matter which type of switch is being used, the foregoing discussion is based on the assumption that the generator is capable of supplying the total connected load, which is often not the case. Unless the residential load is unusually small or the generator is unusually large, successful operation depends on several factors.

With a manual transfer switch in place, the homeowner would first have to make his way to the service entrance panel. Many times, this would be done in the dark with a flashlight in hand. Prior to switching over to standby power, the homeowner would have to shut off individual breakers for nonessential branch circuits so as to shed part of the load and size it down to match the capability of the standby power plant. If the loads on their branch circuits weren't properly noted prior to this activity, it's easy to overload the capability of the generator right from the start.

Fortunately, this process is not necessary with newer transfer switches. Several manufacturers provide selective load transfer switches (click here to see Fig. 2), which provide alternate power to only those branch circuits chosen in advance by the homeowner. Because this type of transfer switch carries only a small portion of the total load, it is smaller and moderately priced. A typical 6-circuit model, suitable for a 5,000W generator, sells for about $350.

Moreover, the electrician that performs the initial installation does not have to cut incoming power and perform heavy service entrance conductor work during the install. As the installer, all you have to supply is a 60A double-pole breaker to match the service entrance panel make and model.

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 [email protected].

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.

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