6-Ways to Select a Manual Transfer Switch


One does not have to go back very far to discover the riveting devastation numerous natural disasters have left in their wake. From Katrina washing over the gulf coast to severe snow storms freezing power lines across the Midwest to the most recent visit from Sandy which left over 8 million people without power, the increasing severity and frequency of inclement weather is matched by our growing reliance on the power grid infrastructure.

With America’s electrical grid growing older, power demands increasing, and costly upgrades being deferred, it is more important than ever to have a plan when the power goes out. As the Federal Emergency Management Association (FEMA) succinctly puts it: Plan, Prepare & Mitigate.

An important step in implementing an effective emergency backup system is selecting a manual transfer switch that can quickly and safely be connected to a portable generator should utility power be out for a prolonged period of time.

There are many factors to consider when determining which type of transfer switch is most suitable for your application:

1. Ampacity Draw/Usage
The first step in selecting a transfer switch is determining what ampacity is needed to provide power to the building. In the event of a prolonged power outage, will all of your electrical equipment need to be powered or can you get by with only a percentage being operational? If all equipment needs to be powered, then the ampacity should match the building’s service. In many cases, only a portion of electrical power is necessary for a facility to remain operational during power outages. In this situation, the transfer switch ampacity and subpanel would need to match ampacity proportion needed.

2. Permanent vs. Portable generator
A permanent generator works with an automatic transfer switch. An automatic transfer switch means the transfer of power from the utility grid to generator can be measured in seconds. While minimizing downtime is a huge advantage, an automatic system can be quite costly when considering the cost of the switch, permanent generator, fuel storage, installation, and regular maintenance of generator. In many regions various seismic permits will be required for a permanently installed generator and NERC requirements call for periodic testing.

A cost-effective alternative is a manual transfer switch which can allow you to connect a portable generator to your facility. Although the transfer of power is not immediate with a portable generator and manual transfer switch, connections can be established within a few minutes. In addition, the installation of a pre-wired manual transfer switch is easier than that of an automatic transfer switch and permanent generator. A manual transfer switch also has a much smaller footprint which is beneficial when space for permanent equipment is an issue. When needed, a portable generator can be rented or used for multiple facilities.

3. Safety Requirements from Local Jurisdiction
Local building inspectors typically require all new electrical equipment installed in their jurisdiction to be “Listed” which means the equipment has been approved by Underwriters Laboratories or another recognized test lab. For standby systems that allow portable generator connection, UL 1008 is the proper standard to comply to. If the equipment is not Listed, it may not be acceptable to the inspector.

4. Environmental Factors
Determine what environmental factors the manual transfer switch will be subjected to, i.e. heat, rain, rodents. A Type 3R enclosure is designed for outdoors and will protect components from rain. To ensure that insects, debris, and small animals do not enter the enclosure, the unit should be equipped with a removable cover over the cam openings in the bottom of the enclosure.

Once installed, the manual transfer switch is always in use. A padlockable door prohibiting unauthorized entry and lockable handle cover to prevent tampering with disconnect handles are important features.

5. Connection Options
“Direct wire”
A direct wire manual transfer switch requires a qualified electrician to make connections at the time power is needed, which can be inconvenient as they are in high demand during power outages. Connection time is longer with direct wire and there is an increased risk of backfeeding the utility grid since there is not a built-in interlocking mechanism. Backfeeding the utility grid can cause arc flashes which may result in bodily injury and switchgear damage.

“Single-pole cam-style connector”
This type of connection has grown to be the industry standard. 400A series 16 cam-style connectors are safe for handling and durable. A 180 degree twist ensures a locking connection. Cam-style connectors are typically color-coded for phase, ground, and neutral which simplifies the connection process and saves time.

6. Interlock Options
“Trapped key interlocking” (“Kirk” key system)
This type of mechanical interlock involves a series of keys that are locked and later released in a particular order to prevent utility power and generator power from being energized at the same time. The drawback of the key system is if a key is lost at the wrong moment it can render the whole system useless.

While the double-throw utilizes an interlocked lever system which visually indicates on/off positions, it does not offer circuit breaker protection.

“Circuit breaker safety-interlock”
A lever system with circuit breakers is fail-safe as interlocking mechanism prevents cross-connecting utility and generator power sources. Circuit breaker transfer switches, such as StormSwitch, can also be service entrance rated. In addition, there is no chance of losing or duplicating a key.

Evaluating a backup system that is suitable to your needs is an important step in emergency preparedness. Installation of a manual transfer switch requires pre-emptive installation, but it can be the difference in your facility being operational versus out-of-business in the aftermath of the next disaster.

For information on ESL’s StormSwitch manual transfer switch product line, visit stormswitch.com.

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2 thoughts on “6-Ways to Select a Manual Transfer Switch

  • September 15, 2013 at 2:49 am

    I am looking for a manual transfer switch so that I can hook up a 20kw portable diesel generator to my house in the event of a storm.

    Can you help me select an appropriate model? Where in the Fort Worth area would I buy it?

    Brian Gaddy

    • September 16, 2013 at 5:36 pm

      Thanks for your inquiry, Brian. Our hats off to you for thinking ahead on preparing for power outages. Unfortunately, we don’t work in the residential sector, however some good general guidelines when selecting a manual transfer switch are: evaluate the power needed, ensure the MTS has an interlock mechanism to prevent cross-connecting power sources, and that it’s listed to perform its intended function.

      You may want to try touching base with a local electrical supply house or retail distributor for a residential manual transfer switch. Good luck with your search.


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