Harold’s Interlocking transfer switch

Another DIYer Transfer Interlock done by Harold Madison, friend of Utterpower. Do note I had trouble wiht the form of the document he sent me, so there’s some cuts and pastes on my part. I love pop rivits, so I would haves used them 🙂


Harold writes:

This is my interlocking transfer switch arrangement:


Harold’s Interlocking Transfer switch







Note the main breaker is ON, to the right, and the 50 amp double-pole

breaker (Standby power in) is blocked in the OFF position by the slide.



Harold's Interlocking Transfer switch 2

Harold’s Interlocking Transfer switch 2




With the main breaker OFF, to the left, the slide can now be moved up, unblocking the 50 amp double pole breaker which can now be turned ON. 

Stand-by power is now connected to the load but cannot be connected to the grid.


Standby Power Cord Configuration


Plug end

(Has a matching hard wired receptacle to the 50 amp double pole breaker)


Any Questions? Harold might show up to answer them.. 
















        Hot     Neutral

                             115 volt supply from the genset.


Note that the two hot legs of the 220 V plug are jumpered together so both are fed from the same source. They are therefore “in phase” so there is no voltage potential between them.  With this configuration any 115 volt appliance in the house can be operated, none of the 220 volt ones can operate. There’s no need to turn off 220 volt breakers.


The refrigerator and freezer stay cold with no cords strung all over; the TV can be used to receive weather reports and monitor weather radar. Another up side of this is any 115 volt light in the house can be used so you can park the flashlight.  You do need to pay attention and not overload your standby generator.


My generator is a 2500 / 3000 watt unit that has served me well. With this power limit if I decide to use the electric coffee pot or the microwave oven I need to stop, inventory and make appropriate adjustments if necessary.


OK George; here’s my question:


If this configuration were inadvertently connected to the grid, what would happen?


I believe opposing secondary winding halves in the power company transformer supplying 220 Volt, 200 Amps to my place would smoke my generator with nothing out of the primary to the lines. However, since I never did finish my EE degree, I’m not sure; and I’m not about to try it.




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6 Responses to Harold’s Interlocking transfer switch

  1. Butch says:

    I have done a couple of very basic “installs” for people who had been back feeding their generators via an outlet or unplgging things and using 13 extension cords. What I have done is install an extra 220 breaker for them and hard wired and cord from it to out side where it can be plugged in to their generators. I wasn’t thrilled about not being interlocked but figured they were better off than with a “meet Jesus cord” back feeding a recepticle? I printed up instructions for switching and left them tied to the panels but I must admit to being VERY uneasy about not being interlocked so this and the article George did last week are very timely for me and gave me enough ideas about interlocks that I was able to come up with designs and install them, thanks much!

    Now,, what is the DIY fool proof method to shut the water heaters, well pumps and kitchen range when the grid goes down so they dont put the genny on it’s kness when they switch over? LOL

    • George B. says:

      Butch, the right way is to to prepare and have a safe plan that requires no thought, just execute the plan as written, interlocks, or the removal of the power meter itself is the only way to protect our lineman and others that make the assumption our part of the outside plant is dead and safe to touch..

      As for the way to assure no 240VAC loads start.. I’ve got that figured and actually took care of that at Zach’s house 🙂
      Note that we are using the same 120v leg on both sides of the 240 VAC circuits! This means we have zero potential across the 240VAC loads, and we don’t worry about them.

      We can use 120VAC to assure no 240 VAC loads run (WHEN) we have no critical 240 VAC loads 🙂

      As for the Meet Jesus plug.. I just used it last night to test a generator auto start function. I needed to bring over 240 VAC from my breaker panel to the unit under test, it monitors the 240 VAC and auto starts when it loses a leg. I used an extension cord, and added a male to male plug on one end. This gave me easy places to attach test leads with gater clips on each end.. at the breaker box end, and at the generator end.. Once I had it all set up with battery bank and start battery, I removed a leg, and watched the unit time and start 🙂 I then returned the leg, and watched it shut down 🙂

      • Butch says:

        I am becoming of the mind set that any person who is normaly on grid power should have to pass a test prior to purchasing a generator, LOL.
        Seriously those poor people are mostly clueless as to the amount of energy they use and the fact that whatever they pull out of an outlet requires a prime mover somewhere to provide for it. You call it critical thinking I believe? there is a lack of it in the ‘burbs let me tell you. ‘electric is nothing short of magic.

        On the bright side of the situation it does make for a steady supply of low houred engines taken from the burnt out gennys.. We have even made up some adapters to fix the tapered shafts for or uses. Now if they would only buy diesels instead of gas jobs I would be in engine heaven!

        • George B. says:

          There is another mind set that says we must protect all people from all dangers. Some people need pass a test before they buy Charcoal, as some do kill themselves NOT understanding they used it in a much tighter house than from which they came.

          We can all help the situation by assuring that people know the better ways, or have access to the plan. My reason for the posts and discussions here. Diyers are the Goto guys.. we can all help by sharing the safest ways..
          Those adapter shafts, I think several of the Surplus centers made them over the years, but things do come and go. If you are selling them, give yourself a plug here..

    • Harold says:


      If you jump the two hot leads in your bacl-up power cord together and feed both sides of your load center from the same 110 volt source that, in effect, turns off every 220 volt appliance fed by the load center. There is no need to turn off the breakers. Nothing powered by 220 volts will see a power source. The “genny” will not know and will not care.

  2. Butch says:

    Thanks for that 110V tip but I dont think it will fly for most households in this area, especialy those who have yet to power their energy needs for more than a couple hours after a thunderstorm.

    There is realy a mentality problem to fix prior to cutting them off from the 220 that runs the well, stove and water heater. Sometimes such is best self learned as it happened to my next door neighbor last June when the grid was down for 3 days.
    After day one of his wife ripping him for not having an emergency genny I offered him 30 amps from mine. He happened to have a roll of 10-3 so he paralelled two 10-3s from the 30 amp breaker in my box across the lawn to his main so he could run the well pump and some lights. Next day he drove 120 miles to buy a 12,000 watt gasoline powered screamer and wired it into his main. Once running I heard it belch followed by the outside unit of his central air coming on and everything was well in the house again (grins) Next day he comes over and asks how many gallons per hour we are burning? I said gallons!!??! Nearest I can tell about a quart an hour, We have a 1115 and a 15KW ST head. The math for him was $4 gallon time 24 times 1 gallon per hour. Cool $100 per day to keep a smile on mommas face.

    So,,, they want all the comforts of home and as long as the fuel bill hasn’t hit home every thing is A-OK in suburbia. But have a 3 day power oputage and a $300 fuel bill to pay?? Now that will get the most of them into the desired critical thinking mode REAL quick,LOL. How comfortable do you want to be is what I ask when ever I am asked to help size a generator plant. The answer is always the same, “if Im going to spend a dime I’m going to be as comfy as grid power” Then next question is how comfortable can you afford to be? Usualy quite the spread in the two KW figures when the calcs are finished LOL

    Thanks again,

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