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I want to wire an 8 kw standby generator to power my home furnace (120 v, 20 amp) automatically in case of a power outage.

The generator can be purchased with and without a transfer switch.

Is there important circuitry in the transfer switch (e.g., that starts the generator every week so)? Or, can I just wire the furnace to a double pole double throw relay, for $20 or so?

(I plan to run a few extension cords to power lights, refrig, etc. but I will hook those up manually.)

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    This is not a place to cowboy it. If looking for "automatic" operation, get a listed transfer switch. For manual operation, an listed interlock can be used. In either case the device used must be listed for the application, as improper operation creates a hazard to life. As an aside, it can also cause a hazard to your power being restored, if the linemen notice an improper generator hookup to the system. – Ecnerwal Nov 1 '15 at 1:13
  • @Ecnerwal I want to know my options before hiring an electrician. I have had licensed electricians propose very different work on past projects, with very diferent prices. (My town requires such work be done by a licensed electrician.) I do not want to buy more work or equipment than necessary. Is there another name for "listed interlock"? I did a web search with no success. I am located in the USA. – Joel W Nov 1 '15 at 1:52
  • Try UL Listed Generator Interlock, perchance. Or a better search engine. Here be wikipedia en.wikipedia.org/wiki/Generator_interlock_kit and here be one for a Square-D QO or Homeline panel: static.schneider-electric.us/docs/Electrical%20Distribution/… – Ecnerwal Nov 1 '15 at 2:00
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The input to the house should be an "INLET" not an Outlet.. The power cord from the generator has a connector (MALE Plug) to connect to the generator, and a FEMALE to connect to the house mounted inlet. The inlet should be connected to a DEDICATED receptacle inside the house to allow use of an extension cord to power individual loads OR to a manual double pole double (SPDT) throw transfer switch if feeding the main service panel. Properly installed and listed SPDT transfer switch prevents connection of the generator to the utility. Alternates using UL listed circuit breaker interlocks are also available. Reliance makes a 6 circuit panel that includes an inlet, and six interlocked pairs of circuit breakers that facilitate power transfer. Do it correctly, use a licensed electrician.

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Keep a flashlight in a handy spot and check the batteries regularly. Find space in the breaker box for a 240VAC two pole 60Amp breaker or higher. Run appropriate wiring that matches the breaker and more than the generator output to the outside and terminate into an appropriate 240VAC 60Amp outlet or bigger. Connect the output of the generator to the 60+Amp external outlet. Turn off the main circuit breaker from the utility feed to the distribution box. Turn off all the circuit breakers except one that has a running load. Start the generator. One at a time turn on the circuit breakers making sure they are loaded up before you go to the next circuit breaker. This should give you an idea as to how much of your house load the generator can handle. Before going back on utility power Unplug the generator. NEVER leave the generator plugged in for stand-by, and always turn off the 60+Amp circuit breaker when not in use. Unless you have an automatic transfer switch, the generator and the main breaker that interrupts the utility power supply must NEVER be connected to the load at the same time. If you want an automatic test start weekly you will need an automatic transfer switch. Also, if the generator is started by battery, you may also want to run a 120VAC outlet for a battery trickle charger to keep the battery charged for emergency and programmed test starts.

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    Doing this without a UL Listed interlock device to GUARANTEE that the main and the generator input CANNOT both be on is both illegal and stupid. -1 – Ecnerwal Nov 1 '15 at 14:32
  • So, the "code" forces consumers to buy an overpriced metal sheet called 'interlock kit' for around $70, and you can't make that yourself because it doesn't have the UL certification and is not "up to code"... that is even more stupid! Furthermore, the "up to code" interlock kit does not protect you from the scenario that the main switch becomes broken and is closed even in the OFF position. I would think the best way is to using one of those large DPDT manual switches, but for some reason I don't see that recommended anywhere, instead they push the $300+ transfer switch, even more stupid!!! – Jose Cifuentes Sep 18 '17 at 15:32
  • ...and I say even more stupid because you are duplicating breakers and infrastructure while throwing money away. From an electrical engineering perspective, the most safe and cost efficient way to accomplish this would be using a proper manual double (or triple for neutral) pole double throw switch. You connect the main panel to the common terminals, and the generator and main power lines to the remaining commuting terminals. There's no room for error there and you're reusing all of the existing breakers. Now, don't know if this is "up to code" as I don't see this solution listed anywhere. – Jose Cifuentes Sep 18 '17 at 15:47
  • @josecifuentes the code is there to protect those who want to be safe from those who don’t. The code doesn’t care if you think your solution is better, it cares for the linesman who have to work on the line feeding your home. Their lives shouldn’t depend on how highly you think of yourself. – user81649 Dec 13 '19 at 11:41
  • @user1512321 "Thinking highly of myself" was not the intention behind my comment. I was questioning why the solution to use a DPDT switch is not seen anywhere, being a safer solution, linesman included. – Jose Cifuentes Dec 24 '19 at 15:11

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