TRANSFER SWITCHES FOR HOME GENERATORS

 

WHAT IS IT?

When you supply electricity to the home from any external source, you need to avoid exporting power back into the grid. Therefore you need a device that can safely connect your house wiring either to the utility lines, or to your generator. Such a device is called power transfer switch. It comes in two flavors- manual and automatic. Automatic switches are used with fixed standby generator systems. They are either included in the set or the system manufacturer provides you with a few choices to pick from. Therefore their selection is pretty much straightforward.

Manual devices are normally used with portable generators, but they can be used with stationary systems as well. There are different types of manual switches to choose from. This guide will tell you what types are available, will explain their pros and cons, and will help you choose the right one for your home or business.

GENERATOR INTERLOCK.

This is the cheapest way to safely retrofit transfer equipment into your existing electrical box. The mechanical interlock is basically a system of slide plates. It ensures that the main service breaker and a back feed (generator) breaker cannot be in “on” position at the same time. Depending on the brand and type, the interlock kit can cost as low as $20 (of course you need to pick the one that is compatible with your breakers). You will also need to install a double-pole breaker for the generator line, and an outdoor inlet. The connections have to be done by a licensed electrician, which may cost several hundred dollars plus a permit. Once everything is installed, you can easily switch from utility to a genset in case of a power outage. The basic transfer procedure is as follows:

Generator transfer switch with interlock An installed Siemens interlock kit. Some models run under $20.
  • Start up the genset and let it warm up for several minutes. At this point it is still isolated from your home;
  • Turn off main service breaker;
  • Turn off all individual breakers;
  • Slide interlock plate;
  • Connect your genset to the twist lock receptacle by using a generator cord (see this page for the diagrams of different connectors);
  • Turn on the generator breaker;
  • Start turning on needed individual breakers one a time, and make sure you won’t overload the genset.

To switch back to the service you have to reverse the above procedure. Here is a conceptual diagram of wiring an interlocked switches. Wiring diagram for interlock transfer switch

Here are the main advantages and disadvantages of this method.

INTERLOCK PROS:

  • Lowest cost;
  • Allows emergency power to the whole house;
  • Does not require re-wiring of your existing circuits. You just need to install back feed breakers and wire them to a generator inlet.

INTERLOCK CONS:

  • You need 2 or 3 available slots in your load box right below the existing main breaker. You will have to relocate the existing branch circuits from these positions;
  • Some models may also interfere with adjacent slots;
  • Some kits are not functional when the cover is removed;
  • If you do not have enough free spaces in your breaker box, you will need to add a so-called piggyback breaker for each missing space;
  • You need to use only an interlock designed for the brand of your existing load center.
  • You may easily overload your genset if you don’t know exactly how much current each circuit consumes.

MANUAL TRANSFER PANEL FOR ENTIRE SERVICE

Manual transfer switchReliance manual transfer panel for around $100.

If your main service panel does not allow an installation of back feed switches, you can buy a separate sub-panel. Note that in practice, what is advertised as a manual transfer switch is actually a system that comprises of two double-pole switches and a locking mechanism. You need to operate this mechanism to shift from utility to your generator. If you want to transfer your entire service, you can buy a single-load switch for around a $100. Your installer will need to open the connection between the service disconnect and existing breakers box and install the changeover panel in between. However, if your generator is not big enough for the whole house, you will still need to turn off selected branches before starting up the generator. Such an option costs a bit more than a simple interlock kit and requires more re-wiring work.

A SUB-PANEL FOR ESSENTIAL LINES

This option is the most convenient though it is probably also the most expensive. You will have a separate box that includes both interlocked switches and individual breakers. It makes sense to choose this option if you don’t want to deal with finding in a dark basement the breakers you want to de-activate. You can pick in advance the circuits you want to backup and have your electrician relocate them from your main distribution box to the sub-panel. He will also need to wire the sub-panel to the main service disconnect and install an inlet. During normal operation your essential loads will pass through the transfer panel, while the remaining ones will be fed directly from the main box. With this method, the general procedure of activating backup power during a blackout is as follows:

  • Start up your genset outdoors and connect it to the installed inlet via a proper generator cord (at this point the genset is still isolated and is just warming up);
  • Toggle “Line” switch in the sub-panel to “OFF” position and turn off all individual breakers;
  • Slide the interlock plate and flip the “genset breaker” to “ON” position. By this time your genset has probably already warmed up;
  • Begin turning on individual breakers in the transfer panel one at a time.

Note that some consumer guides incorrectly state that with a separate transfer panel you can leave all your branch circuits “ON” when you are starting up the genset. This is not a good idea because the initial surge currents of your motor-driven appliances and lights may overload the genset and trip its overcurrent protection. You do want to power up your loads one at a time.

The branches left in the main load box will not be energized during a blackout. When the service is restored, the lights and devices connected to those lines will turn on, alerting you that the grid is back. Note that since the loads which are using backup power are isolated from the mains, there is no danger of back feeding. You will just need to flip the transfer switch back to “LINE” and shut down your genset.

GenerLink™

This is a special brand of redundant equipment. It’s like a collar that is inserted behind your electric meter. In case of a blackout you need to plug your portable genset to the GenerLink™ inlet and start it up. After that the system automatically switches your house from the line to your genset. Since the installation requires breaking the meter’s seal, it is normally done by a local utility. It may cost $500 to $1,000 installed. If you want, you may check if they allow it to be done by an independent licensed electrician. Some utilities may install GenerLink™ for free and charge some monthly service fee instead.

NOTE. The information in this site is provided AS IS for technical reference only without guarantee and liability of any type, neither explicit or implicit. It expresses only a personal opinion of the author, and does not constitute a professional advice- see complete disclaimer linked below.