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Sizing a Generator Transfer Switch

Here are the Steps for Sizing a Generator Transfer Switch: The rating and size of all the electrical components of the system should be sized properly.

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Hooking Up a Generator to the House Panel
Using a Circuit Breaker Interlock Kit
for Backup Power

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The Importance of Sizing Generator Transfer Switches
Electrical Question: If I have a 50 amp inlet can Reliance Model 31410B 10 Circuit Transfer Switch, is it rated for 12,500 watts? My electrician installed a Reliance Pro\Tran Model 31410B 10 Circuit Transfer Switch that says its a 30AMP, but he says it is the same as the 50AMP 51410B. Is this true? Will I still be able to pull in 12,500 watts? The inlet does plug into my 50AMP out on my generator but I don’t have a watt meter to verify

This electrical question came from: Steven, a Homeowner from Washington Township, Michigan.

Dave’s Reply:

Thanks for your electrical question Steven.

Sizing Generator Transfer Switches

Steven, lets do some basic math here and you will see for yourself what the rating is:

The Steps for Sizing a Generator Transfer Switch

To calculate watts, the formula is amps times volts.

Steve, I will just say that you better have your  electrician explain his calculations to you, then compare the results to the rating of the equipment, along with the rating and size of all the electrical components to make sure your system is sized properly.

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FAQs Frequently Asked Questions and Comments

4 Responses to “Sizing a Generator Transfer Switch”
  1. Gene says:

    Bid to add a Generac-6462-16,000W + 200A Service Entrance Transfer Switch (RTSY200A3) into an existing house wiring system. Electrical contractor’s bid states: ‘Current utility service is 100Amps and the generator transfer switch is 200Amps. We will have to install a new 200Amp Service Entrance Meter’ Where is this scenario described in the NEC documentation? Would you agree this is a required step? The matching of utility service and transfer switch ampacities is necessary to meet code and inspection?

  2. Dave Rongey says:

    Hi Gene,
    Installing a Generator and Transfer Switch
    Without knowing all of the details about this project, it would be interesting to note that the size of the transfer switch should be based upon the maximum capacity of the generator. A quick calculation shows that this generator produces under 100 amps, (16,000 watts / 240 volts = 66.66 amps) so my question is why a 200 amp rated transfer switch when 100 amp transfer switches are available.
    It is always a good idea to get two or more bids for a project such as this, no doubt you will receive more clarification.
    I hope this helps,

  3. Joe says:

    I have 200 amp service to the house and a 6000 watt generator. The generator needs to only supply 40-50 watts to the house. What size manual transfer switch do I need? The generator and the electrical service does not go directly to the house but to a farm pole where I split the incoming power to three out buildings and then to the house.

  4. Dave Rongey says:

    Hi Joe,
    For your safety, and to make sure the installation is up to current NEC Codes, generator projects such as this are best planned out with an on site evaluation by a licensed electrician who can work with you to make sure all aspects of the project are taken into consideration.
    Be Safe,