How To Design A Custom Generator Interlock Plate

If you connect a generator to your home’s main electrical panel when the power goes out, you need to make sure the main breaker is shut off. Otherwise, when the power comes back on, you (or the linemen) are going to have a bad time. There are commercial interlock plates which physically prevent the generator and main breakers from being switched on at the same time, but since they tend to be expensive, [HowToLou] decided to make one himself.

The hardest part of this project is designing the template. It needs to be carefully shaped so its resting position prevents the generator’s breaker from being switched on under normal circumstances, but once the main is turned off and out of the way, you should be able to lift it up and have the clearance to flip the lower breaker. Spending some quality time at the breaker box with tape and a few pieces of cardboard is going to be the easiest way of finding the proper shape.

Making a template with thin cardboard.

In the video after the break, [HowToLou] demonstrates the ideal shape for his particular application, which should help you get your mind wrapped around the idea. There are a lot of variables involved, not least of which the size and placement of the breakers, so taking the time to get the template right is critical.

Once you have the shape, you could really make the plate however you want. [HowToLou] cuts his by hand out of a piece of thin aluminum, but you could certainly 3D print it or even CNC it out of a thicker piece of metal. The important thing is that its stiff enough that somebody can’t just bend it out of the way if they’re fumbling around with it in the dark.

It probably goes without saying that a homemade interlock isn’t going to be up to code, but even if you don’t have any inspectors sniffing around your electrical panel, it’s a sensible precaution to have something like this installed. The middle of winter is a bad time to realize you don’t have any way to safely power your home when the grid goes down, so the key is getting something like this ready to go before you actually need it.

84 thoughts on “How To Design A Custom Generator Interlock Plate

    1. Well, you could easily convert the units yourself if you felt like learning something new, while coming to the realization that there is more than one standard used around the world. I know it’s a bit more complex than the simple base-10 SI system, but well, history and politics are kinda weird.

      In all fairness, it’s probably easier to introduce articles like this in terms of easy-to-underdtand units. That’s clearly an American box – The average American will have an inherent understanding of how long an inch is and whether or not he can bend a quarter.

      And it is a true shame that we adopted the “inelegant” naming system for our metals. That extra “i,” so important to the greatest of royal scholars, was just so important to make important science just that much more legitimate. I do wholeheartedly apologize for not truly respecting that convention for the game-changing revelation that it must surely be.

      Oh, and if you truly cared about anything, you’d surely be concerned that this dude just derated his panel by drilling a couple holes in it and adding an unapproved interlock to it. Goodbye homeowners insurance. But no, you clearly had more important matters to address. Bravo, good sir, for enlightening us once again.

      1. For me it looks like (zinc) galvanized steel, independent how you write aluminium. Of course I really prefer metric units, but inches at least do not have a zero offset like this Fahrenheit scale.
        I just wonder, if it would not be better, to make the interlock from a stiff insulator, e.g. like unclad epoxy PCB Material.

          1. Celsius is Centigrade. And if that “doesn’t fully mesh with SI.” then Fahrenheit is miles (or kilometer :-) off. The SI unit for temperature is the Kelvin and it has the same “size” as Celsius.

        1. Celsius is just as arbitrary as Fahrenheit with regards to a zero point. Kelvin (or eV for particles) are proper units for temperature. I’m too lazy too search, but I think 0 F is when blood freezes and 100 F is (supposed) the human body temperature.

          1. Celsius is based on water and is not arbitrary. 0 is when water freezes, 100 is when it boils. Fahrenheit is based on a combination of the freezing point of a brine solution for 0, ice water was determined to be 32, and 96 was thought to be the same as ideal human body temp at that time. It’s kind of a mess.

      2. On the other hand, it is really fitting that this article is tailored to an american audience. The rest of the world simply isn’t in the ridiculous position of having enough resources to afford a generator that is neatly integrated into your home and at the same have such a shitty power grid.

        For the rest of the world, if you are connected to the grid, it works. Period.
        If it doesn’t, you live in such a poor part of the world, that owning a generator is a wet summer dream anyway.

          1. The frequency dropping is annoying, but pretty harmless. A blackout is much worse.

            In Europe I have only seen generator changeover switches, never an interlock like this. Probably different code.

        1. France, Spain, Italy, Germany…… I’ve experienced power outages in all of these nations. In “poor” nations, the power is only part time to begin with.
          Yes, we Americans fp live for excess.

          1. Europe generally doesn’t get hurricanes… It’s not that the infrastructure is bad: the environment is quite hostile at times no matter how robust you build infrastructure. FWIW, power outages are still extremely rare in the US. The uses for generators are for times when utility companies are sending convoys of utility repair trucks across state lines in the aftermath of a huge storm.

    2. “Is that Bananas?”

      The unit is “template fit” from what I see.

      Neat idea and never seen just a breaker used as a transfer switch or the lock design. Super practical and easily improvised.

      Great comment also noting the difference in National Electrical Code (NEC) article 250 requirements for Floating and Bonded Neutral Generators.

      Guessing what is demonstrated is for a Floating Neutral Generator design.

      Can do a check with your generator off with a continuity tester and if there is NOT continuity between the neutral plug and the metal frame, the neutral it is floating.

      1. That’s why you can’t buy a generator with a bonded neutral after about 1965 when grounds became commonly used. Every generator made today has a floating neutral and a separate ground which bonded to the frame. It costs an extra wire but it is safer. Modern generator plugs are always four conductor.

        1. That’s right, The difference is the installation size nowadays. Typically size dictates the need for a bonded neutral in a generator. Once you get over a certain KW most generators become Bonded type.
          Average Joe isn’t going to have a bonded neutral generator unless it’s attached to the house permanently.
          If you have a Bonded neutral generator and it’s “portable”. It’s built on a trailer.
          Use cases are tor things like theme parks, outdoor venues, pop up events, medical centers, casinos… where power loss might not be an option.

          We break the neutral for a few reasons with a transfer switch in these circumstances. One of the big reasons is to keep our hydro workers safe.

        1. Negatory. Someone in the throes of a disaster can “attempt to help” and fire up a generator, FAILING to note the need to prior deactivate and activate the switch/ switches, depending on the modus-operendi, put in place. It must be idiot proof, not merely idiot resistant.

  1. A 200 Amp panel can handle 48 KW. That generator is only 6 KW. Now you have to switch on and off various breakers to power selected appliances. Breakers are not switches. There are transfer switches made for this purpose which switch selected circuits over to generator power and disconnect them from the main panel. He saved a few bucks and did a major hack job and I don’t mean that in a complimentary way. Do it right.

    1. – It’s not ‘save a few bucks’ – it’s quite possibly a 2-3x budget increase for transfer panel/switches/’select circuits’ route, quite possibly breaking the project budget. Personally, I’d rather be able to have my furnace and well work if an extended outage does come up, but I’m also not going to drop a few grand on it (not saying I’m going to do this either).
      – If you’re capable enough to do this, I think you also wouldn’t have that hard of a time keeping your house load from popping the generator or gen feed breakers, without needing to play the ‘flip the breakers’ game. Worst case if you don’t, you trip a breaker. The panel being capable of 48KW is irrelevant – there’s protection on the feed… Not really seeing a big difference from a solar branch circuit here… Just nice enough to have a grid interlock.

    2. You put the generator on a breaker just under what it’s rated for, giving you adequate protection. There’s a bit of common sense needed, to not to try to run everything at full tilt off this. As they make commercial versions of this that pass code, it’s not as crazy as you think.

      1. Yeah… technically I’d want the generator peak continuous at no more than 80% if I recall correct. I know that’s not accurate since gets more convoluted with the code and is something better to write out for the situation possibilities and typical general use. I’ve never installed a generator before… though guessing same NEC logic applies.

        Reminds me of recently looking at the logic for a stove and oven circuit that seemed strange at first glance and seemed seriously underrating the max continuous range and oven power requirements for the NEC required circuit wiring and breaker.

        Then was like… yeah is starting to make sense logically drawing out and doing the math… you know how often will everything be running at max power?

        1. Some of the math is definitely non-intuitive. Part of it, at least on a US install, is the 220 vs 110 voltage. A generator on a 220v 30 amp breaker can serve 60 amps of 110 devices. Same thing kinda goes for a stove, as only part of it actually runs at 220v.

          1. One way to think of it is that the circuit has two phases: one wire that is producing an alternating voltage waveform, and the other wire producing that same waveform mirrored over the x-axis (180 degrees out of phase from each other). A third neutral wire grounds to earth. Devices needing 120V potential tap one of the two hot wires and use the neutral to complete the circuit. For 240V potential, the neutral is ignored and the circuit uses the potential across the two opposite phase hot wires.

          2. I am not an expert on this, but my understanding is that your service panel’s neutral conductor is grounded to earth by using either a grounding rod or underground water pipe. As a safety precaution, the green grounding wire and conduit are also electrically bonded to this ground. There are three wires that then run from your service panel to the transformer servicing your building. The two hot wires are configured to attach to the opposing wrappings around the transformer core to give 180 degree out of phase voltage. The neutral wire straddles the middle of this configuration and is intended to have no voltage differential. There is no neutral back to the generator through the distribution grid: the Earth serves that purpose.

  2. If you have a generator other than a Honda sitting on your back deck with an extension cord, you need to have a proper transfer switch. It’s irresponsible for HAD to be promoting unsafe bodges like this.

    1. That’s exactly what I thought. This is a terrible idea.

      Another one I’ve seen is people walking into my local electronics store to build a male to male electrical cable (known as a suicide cable) to hook their generator into a wall socket. The staff often explain to them why it is a bad idea and refuse to help them if the customer still wants to go ahead with it.

        1. Yeah, but the problem once you use a cable like this to plug a generator into your house, you could electrocute a worker up the line working on the power cables, or blow up the generator and burn your house down once the mains power comes back on.

          1. If there is no meter, there is no connection to the upstream line, which is specifically what he is talking about. It’s still not a great idea, but if you’re careful it’s better than freezing to death.

        1. You should move to the U.K. I don’t think the electrical specialist at B&Q could tell you what a widowmaker cable was or why it’s a bad idea.
          Then again, that might be because we have reliable electricity…

          1. Everyone’s asking the point of a hack like this.

            HERE IT IS: [HowToLou] is 100% competent and capable of powering his house during an electrical outage. He’s not always going to be home when that happens though, so this is to protect his wife or older children who may need to fire up the generator and power the house by themselves. They are by no means idiots, but not everyone is trained and how these components work together. This is a safety measure to protect the innocent.

            Comments about generator inlet plugs built into the house are partly irrelevant because they ignore the fact that you still have to interlock the main breaker and generator breaker to safeguard upstream lineman.

    2. – So what’s unsafe about it? – At least he’s being nice to linemen and preventing accidentally leaving the grid connection up. Heck of a lot better and safer than those dual-female ended cords to backfeed a 220 welder circuit from your garage from a gen, with no grid interlock. His gen has breaker, panel had another breaker for gen feed with interlock, and individual circuits still have protection. Not sure where the ‘real’ safety issue is here (but I’d be happy to hear about it if there is one).

      1. In my country it is theoretically illegal to replace a wall socket or a light switch, because a certified electrician must at least inspect the electrical installation, and sign the document that everything is up to code. Any DIY work done on it is basis for insurance company not to pay insurance in case of fire caused by electrical installation. I imagine the same goes for USA. So yeah, nice hack, but bad idea in a long run…

        1. I live in the USA. It is perfectly legal for me to do my own electrical work on my personal residence. House was built in 1965 and still has an Edison fuse panel. As long as I don’t touch the panel, I have no requirement to have wiring/outlet/switch changes inspected either. However, the moment I pull a permit to have the meter pulled so I can upgrade the meter pan, service entrance wiring, and panel to modern 200A standard breaker panel, I have to pass a full inspection to current code standards. I can still do all the work myself, just have the inspection requirement before they will reinstall the meter.

          Mind you, electrical code requirements not set on a national lever, but are dictated by either the state, or the locality (city/county), depending on the state, and are an absolute patchwork mess. The next town over from me (same state) requires all electrical work to be done by a licensed electrician.

          Lesson to everyone who is unfamiliar with how things work in the USA is, the federal government is only responsible for inter-state related things. States are independently responsible for all things intra-state, and the laws regarding just about anything can vary WILDLY from one state to the next. Please do not assume that anything is “standard” in the US. Two states next door to each other might as well be two different countries as far as a large amount of everyday common things are concerned.

      1. That product is made by these folks:

        I just installed one of their interlocks the other day and am very pleased with it. I thought about making my own, but was thrilled to find a well thought out kit that has all the bits (interlock, drill template, drill bit, screws, threadlocker) in one bag.

    3. How this works in the real world is that you can’t afford to do it to code (explicitly not saying “right” because there are plenty of functional, safe ways to do it that aren’t up to code; the difference between a compliant installation and a code violation around here – all else being equal – can be as simple as whether the installer was blessed by the state or not) so you don’t do it at all, and then when your power’s out and you can’t flush the toilet and all your food is about to spoil, and your pipes are about to freeze you rig something up.

  3. While I agree with the general sentiment of “doing it right”, I’d like to point out a couple things:

    First, it’s OK (and expected) if the panel is rated more than the generator. Go add up all the ratings of the breakers in your main panel — in just about every home, the total will be more than the main. But, not everything is on at the same time, and if it all happened to be on at the same time the main would trip, protecting the panel’s busbars and the upstream feeder. In this case, as long as the generator breaker is sized correctly for the wire connecting the generator to the panel, it’s fine — you just might need to think a little bit (ie, don’t run the electric dryer, oven, and stove all at full power when you’re on the generator.) I have CTs on my main feeder, and the most I’ve ever seen was around 6-8kW, with “everyday” (not using the electric dryer or AC) use topping out around around 2-3kW. THink a little about what you’re turning on, and honestly I prefer the flexibility of having the whole panel backed up rather than automatically transferring only certain circuits.

    Second, I one bought an (official and expensive) generator transfer panel (manual, not ATS). It was literally just a standard Siemens busbar assembly, in a metal enclosure, with a piece of bent sheet metal interlocking two standard breakers.

    Third, at least this hack is “how to make a proper(ish) interlock for a hardwired generator” not “how to make a double-ended extension cord and backfeed your neighborhood and kill your generator and/or yourself”

    Sure, it doesn’t have a UL label, so it’s (probably) not going to pass an inspection, and if something goes wrong your insurance company will likely not be pleased. But, it’s also far from the most dangerous thing I’ve seen here.

  4. Don’t interlock kits need to be UL Listed? A DIY one wouldn’t be. Off-the-shelf kits cost what they do because they’ve gone through the UL testing and listing.
    $75 isn’t much to avoid a potential malfunction with a homemade-one and your home insurance denying a claim (or lawsuit from an utility) after finding it as a potential root cause of a problem.

    1. No, panel parts should not be UL listed. (nor internal computer parts like a PSU)
      UL Certification can mean several things.
      UL Listed Certification is for consumer end products like a hairdryer.
      UL Recognized Certification is for components, like a circuit breaker, transformer, relay, etc.
      Then People (like me a long time ago), Facilities, and Processes can be UL certified to build safe stuff with UL Recognized Components, like control panels.

      Seems like the NEC code pretty much says anything with two circuits and components, like circuit overload devices (breakers), is considered a control panel. Some Insurance/Customers wanted all the Control Panels UL Certified (eg Recognized). PITA/$ 20 years ago, pretty much mandatory now, from what I hear from old friends.

      The big problem I saw with the generator/line interlock switches from Home Depot (a lineman working there clued me in) is that they are only manufactured and approved for a few panels. Older units just are not supported. If you have to swap the whole box out anyway, why not just upgrade to a proper (more capable) system.

      Oh how I long for a Whole House UPS, with generator start capabilities after about 5 min.

  5. Of course suicide cords are dangerous, hence the name. Problem easily solved with a male power inlet receptacle, standard with generators. Male plug to generator outlet, female end to power inlet wired to generator breaker. Male power inlet can’t be energized if breaker interlock does its job.

  6. Now if you write “aluminium”, you’d tend to write “galvanised” and not “galvanized”?

    NB: not a native speaker here. Just trying to learn my way around. Curious, as most of the audience here :)

  7. When my solar was installed they also installed a 200A emergency transfer switch for the generator. Switch was about $400, labor and inspection was “free” because they were already mucking around for the solar and that bit was financed.

  8. Simple idea, but they’re sometimes the best.
    Are those USD 75 variants really based around the same idea?

    What I like most is the use of making prototypes or mockups from cardboard, and I’m glad to see it has its own tag.
    Some other materials I use for similar mocups is the (around 0.2mm) sheet from aluminium cans, and thin MDF for stuff that’s later to be made out of thick sheet metal.

  9. kind of dismayed by all the people griping about this…given that you’d do anything to get power during a severe outage, this is relatively on the safer side of “anything.” it’s not like he’s burning charcoal in his livingroom…

    but my concern, which i don’t know anything about, is this 220 / 110 thing. it’s my understanding that if the electrician did a decent job balancing it when you set up your house, then under your normal kind of loads the two phases will tend to be balanced. and if they’re a little unbalanced, you can run a little current on upstream neutral it’s not the end of the world.

    but i’m worried that under this sort of exceptional scenario. if i’m trying to fit the essentials under a tight budget, i feel like i’m pretty likely to go around the house unplugging all the regular loads and i might just literally have two 1500W heaters on outlets that never saw any load before in regular life. i trust myself to figure out to get those on two separate 20A circuits but if they wind up on the same phase, what happens?

    i don’t know how these little consumer-grade generators work…does it burn up the generator, or does it let neutral drift to one side, or what? honestly, i assumed they all just output 110 and never thought about it before

    1. Every receptacle on a portable generator has a breaker. If it is outputting 220V it has a 2pole breaker. So if it is a 20a 2pole breaker feeding a circuit with a neutral (3 wire circuit) , either line can draw up to 20A before tripping the breaker at the Generator receptacle.
      if you are drawing (3A on Line 1) and (3A on line 2) your breaker isn’t seeing 6A against the 20A rating it is seeing 3A against the 20A rating. So the more you balance it the more things you can potentially run.
      This is where a sperate panel comes in handy. Your electrician will take the circuits out of your main panel and put them in the generator panel in a balanced manor. Sparky’s don’t go around balancing your house circuits like people imagine because it’s typically not necessary. If a Load would be large enough to require balancing as a 120V circuit typically it becomes a 220V circuit. Example your stove. If you had a stove as 120V the amperage would be so high you would need to balance the entire house against it. So Electric stoves are 220 instead and 220V circuits are naturally balanced because they use both lines.

      I’ve changed a lot of panels and I would highly recommend against putting screws blindly into the front of a panel. I would also recommend highly against opening the area that houses the Main breaker. The large cables coming from your meter are always live and the next nearest fuse will not trip before the power kills you.

      1. Also, the reason a stove, dryer (other high power item) uses 220v is the amount of current required. The larger the amount of current, the larger the gauge of wire…too small a gauge of wire (dismissing other things for simplicity) would cause the wire to become a fuse or fusible link. Think fire in this case.

        P=I x E (power = current x voltage)
        Power is watts and watts is heat (all the words are interchangeable). Joules is the metric equivalent to the word Watt.

        Lower the voltage…up the current…increase the size of the wire. Stove, dryer (other high power items) already have large gauge wire at 220v, imagine (do the math) what the current rating would be at half the voltage (110v). The math will say: double the current at half the voltage. That’s a huge wire (expensive and almost impossible to manhandle).

  10. Not sure of this need, other than to make it idiot “proof”, which well yeah where there is a will to be an idiot there is a way.

    My generator connector is also my motorhome connector so locking one or the other out like this is not an option simply because it makes no sense to have 2 50amp TV outlets into the same main panel.

    Do it the right way and get a proper transfer switch if you think you need to protect yourself from yourself when it comes to using a generator on your house.

  11. Not too sure why this sort of bodge is necessary.

    I had a generator input installed when getting the wiring done on my current house. Generator and main are connected to the house wiring via a 3 pole double throw switch [main is 3 phase]. Impossible for generator & main to be connected at the same time.

    1. For a new build this is a few dollars more and a great idea even if it’s never used. To change out the mains panel for this “newer type” panel is not a trivial expense. From a regular panel to your very nice newer panel is a few dollars. Throw away the existing panel and buy new is the price of a whole new panel plus your electrician.

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