How-to: Make A Mains Crossover Cable

Update: This How-To was written for April Fools’ Day. It is not advised  you attempt to make or even use this cable. The comments have made it very clear how dangerous to you and others using this cable can be. The image above is not of a full male-to-male cable, only the ground is connected, and the generator is not running.

We all know Ethernet has the crossover cable, cars have jumper cables, and RS232 has the null modem. Well, it is about time our wall sockets get their own crossover cable. This crossover cable is great for running power to a circuit disconnected from power. Maybe you are out of fuses, the breaker is broken or you just don’t want to go check the fuse box when there is a murderer about. This cable makes a great gift for even the most loathsome of acquaintances.

This an April Fools’ Day post. You should not do this!!!

Warning: These are Mains voltages and currents we are working with. If you do not know why you should not attempt this, you should not attempt this.

You will need:

  • 1 Extension cord.
  • 1 Male extension cord repair connector.
  • Tools to strip the extension cord, and attach the connector: Knife, wire strippers, and screwdriver.
  • Respect for mains voltages and currents.

If your extension cord has warning labels, read them.

Cut the extension cord to the desired length.

Strip about one inch (2.5cm) of the outer insulation from the part of the cord that has the male plug.

Strip about 3/8 inch (1cm) of each of the conductors. If there is a strip gauge, use it for a guide instead. Twist each of the stranded conductors to prevent fraying and possible shorts later.

Slide the shroud over the cord.

Screw the conductors into each of their respective terminals on the connector.

  • The Green or bare conductor goes to the ground conductor terminal. It often has a green screw.
  • The White conductor goes to the grounded conductor terminal. It often has a silver-colored screw.
  • The Black conductor goes to the ungrounded conductor terminal. It often has a gold-colored screw.

Use a multimeter to check continuity and make sure there are no shorts.

Slide the shroud up to the connector and fasten the two parts together.

Secure the cable clamp.

There you have it, a finished Mains crossover cable.

There you have it, a Mains crossover cable. How might you use this crossover cable?

396 thoughts on “How-to: Make A Mains Crossover Cable

  1. It’s still poor taste.

    And some moron who comes here later in the year and thinks this is great without spotting the date will wind up killing or injuring himself or some line monkey.

    I worked with those guys for a few years back and this happens far too much.

  2. another excellent extension cord mod is to cut off the female end, wrap one wire around the doorknob and the other wire to strategically placed metal. for barefoot situations, a floormat. other situations, something metal the hand will touch while the knob is turning. but if you put that second contact in the wrong spot, you gonna have some bodies to deal with.

  3. Yeah, I’m BAWW’ing because it’s an absolute joke. An absolute joke that something as morbidly stupid as this can be made fun. April fools is about intelligent hacks, witty hacks, funny hacks.

    Yes it’s CLEAR (I hope it is?!) that this is an April fools ‘joke’ but it’s just a brute of a killing machine. In no way should this be made light of. A few years ago I would just be like.. yeah ok so they posted a really bad hack and it’s funny, roll on next years April fools.

    But I’m genuinely scared some of you FUCKING RETARDS would try this.

  4. 1. It’s not illegal everywhere. We don’t all live where you live.

    2. The post specifically says not to do it. There’s no liability. Any jerk could google this and murder his family. Since when is information a bad thing?

  5. This is just an auxillary supply how do you think hospitals work when there is a power cut. Put a circuit breaker on it to switch when there is no power coming into your home with a multiple earth breaker just in case and a normal fuse.Then upgrade the cable to handle 20-30 amps and your done safe. I agree this thing would be damm dangerious if it got turned on with another supply, but lets be honest do we rely need those people who do that in this world?


    1) Mains Voltage Can be Back Fed into the National Grid – and anyone who’s working on it!

    A solution to this would be to use a DPDT Generator isolator to switch from your mains supply to a generator as it does not allow both to be run at the same time.

    2) There are no circuit protection or electrical failsafe devices as you have now bypassed your fusebox/consumer unit.

    A solution to this would be to get a generator that has a Circuit Breaker/fuse AND a Residual Current Device at the generators outlet.

    3) You are supplying your ring main through a spur. This is dangerous because the flex will be rated to carry a certain current. Under BS 7671:2008 (British Standard Regulations) a 1.5mm2 conductor can carry a max of 20A and 2.5mm2 (the biggest size typically available of flexible cable) a max of 27A.

    This is more applicable to America but the UK too;

    In America as the supply is rated at 110V, the same wattage applicance will draw roughly twice the current than the UK at 230V.

    Im from the UK, and i don’t know what the regs are in the US, but there probably similar.

    Trust me. I’m an electrical engineer–I Know!!!

    Don’t feel too bad Devlin, too succeed we must first learn to fail. Think of this as a learning experience, and ENSURE that you always think about electrical safety.

    Remember, people are rarely affected by your acts, but always your ommisions!

  7. But again, its not just the moron who gets culled.
    It often is the Linesman who gets sent out to fix Joe Morons power who gets it.

    This happens with alarming regularity when some bright spark comes up with this idea himself, and doesn’t isolate the breakers.

    That is not amusing. That is not a joke.

  8. Wow this is great… This site posts tesla coils and people with diy’s to rail guns with big boxes full of caps. They all can kill you, same goes for this. you commenter’s are golden and yea thats sarcasm.

    Yes this does work but please take into consideration that everyone isnt as stupid as you and dont need to be warned or bitched at to have this taken down. quit being low-voltage pussies.

  9. @RBRat3 – Yeah, but the Tesla coils aren’t posted as an easy how to guide!

    Plus you wont find high power caps laying around most peoples garages.

    @Phelps – Free country to let someone else fry your ass whilst only doing your job! Right on! F- you buddy, I need my cool beers!

  10. I always have an extra 15 amp breaker on the one side of my panel I want to power and use that to backfeed power into the panel after shutting off the main.

    This post IS DANGEROUS and at least should tell people to shut off the main.

    In my opinion it should be done at the power box (AND ONLY WHEN THE MAIN IS SHUT OFF) and not feeding who knows how many amps into a wall socket that rated for 15 amps (yeah I know some are more but in homes its usually 15)

  11. for all you naysayers, this is a simple case of use at your own discretion. I have done many similar things in my life. Simply flip the breaker before using this “hack” and you’ll have a room with power as stated earlier. Take no risks and get no rewards.

    just use some practical common sense (although I do believe it to be a dying breed) and you should still be alive tomorrow to try something just as dumb again.

  12. Wow Crappy design. at least throw in a inline box with a single breaker of fuse in there to make it somewhat safe.

    Hack-a-day Showing Noob’s how to kill themselves in new ways.

    Honestly, whoever wrote this up was a complete idiot.

  13. And what if you kill a repairman just so you could watch your favourite program.

    Its all too easy to forget. Better to ensure that it can’t happen in the first place.

  14. Hah that is funny. My dad did this once by accident after he cut our extension cable with a hedge trimmer. I zapped my hand with 120 Vac, didn’t hurt much but I could see how it might be dangerous (just heed the warning at the top of the article!)

  15. Funny joke, but poor taste. I love the fact that it encourages reading the safety labels before totally disregarding them.

    I have this set up at my house, but the right way. Critical circuits set up in a second fuse box with the AC Jack for the generator installed on the outside of the house near the covered porch, so the generator can vent properly.

  16. In case it hasn’t been made clear enough already:


    You could kill yourself, you could kill someone you love, you could kill others and you could burn your house down.

    I know it’s already been said a lot, and that this is an April fools joke, but seriously, it’s Mains power, I’ll screw around and do tons of dangerous shit, but one of the few things I /will/ respect every damn time is Mains Voltages and Currents.

    I’ve known enough people who have died from screwing around with this kind of stuff, and heard enough horror stories from people who do it for a living, that whenever I even /hear/ about this kind of thing, it gives me the creepin’ willies.

    To all those saying it isn’t illegal:
    While nominally true, it /is/ probably against code (and for good reason) and if some one dies because some idiot didn’t flip the main breaker or something equally retarded, then it WILL become a matter for a criminal court.

    for what it’s worth, I /HATE/ April Fools day, I dont even know why I turned my computer on today >:(

  17. It is very rare that I will ever dump on somebody else’s hack… I laud those who go forth and build, tinker, and experiment. If anything, I have mocked those who whine “that’s not a hack,” or “I could/did do it better.” I have poked fun at those who bitch about projects with arduinos and blinky LEDs. And, I have *never* advocated the removal of a post, as I am against the notion of censorship.

    Having said all that, this “hack” is so stupid and so dangerous that it *really* needs to be removed from this site. There is no fundamental difference between this post, and one that “instructs” a person to load a revolver and shoot it into the air. The author might “know what he’s doing,” but either way, sooner or later, either the operator, or worse, some innocent and unassociated party, will end up dead.

    The author may be a nice guy but this doesn’t change the fact that he is an ignorant fool. A FAR better article would have described how to install a transfer switch at the fuse box… something comparatively cheap and easy to do.

    I’ve said enough.

  18. Sounds to me like we are talking about incompetent linemen. Who grabs a main without checking it for voltage first? “Durr, I flipped a switch, so let me just start grabbing stuff.” And he’s supposed to be the professional?

  19. This may not be as far fetched as it looks, This was published in Popular Mechanics some years ago.

    The biggest problem other than the MAIN MUST BE SHUT OFF is that most outlets in an older home are WORN OUT this causes a poor connection and HEAT. Heat causes burned wires and FIRE.
    After the ice storm in Northern NY a number of years ago, my partner and myself spent almost a year repairing burned walls and outlets from these being used.
    For a little over 50 bucks you can buy a plate that forces you to turn off the main while turning on your breaker for your generator.
    It is called a generator interlock kit.$file/RetaiLinkDec2002small.pdf

    Have it done right. Call someone with some knowledge of the subject.

  20. I have to echo the other “pansies” here. It would be one thing if it only endangered the moron that actually used it, but when it has a high possibility of injuring/killing outsiders it’s a bit too close to the line IMHO.

    You can do better than this HaD.

  21. Phelps what is the lineman checks for power and finds none, while he’s working on the cables you come along and start up your generator forgetting to switch off the breakers.

    Or lets say the lineman comes along and fins power on the grid that isn’t coming from the powerstation, how does he find where it is coming from??

  22. Jeah, it’s April 1st, but this one is a really bad joke.
    I think here at HackaDay are enough clever people who could make a safer and cooler Crossover Cable.
    This one is really simple and easy to use, but it is quite deadly, too.
    So if you are so bored that you have to try this Howto out, a fail in assembly or usage maybe will bring some action in your life.

  23. Most generators I know have one of these cables built in. Certainly in the middle east you get power from generators using something like this.

    So all he’s done was cut an extension cable open.

    NOT A HACK in anything but the most literal sense of the word!

  24. The outcries have already been done against this so no point joining in, however the real danger of making something like this and storing it with your generator is what if you have a power cut but you’re not there so someone else drags out the generator and sees the cable and thinks “oh that’s neat” and doesn’t know all the safety precautions involved with doing something so potentially dangerous.

  25. The title of this post made me laugh. Then I got really annoyed reading all the no humor net nannies getting their panties in a wad over this. PS The disclaimer was funny too.

  26. Years ago i made a similar cable and used an outdoor knife to strip the insulation. We were camping in italy and wanted to use a laptop to watch movies before going to sleep. But our old VW Bus had outdated connectons but was parked near a mains station. I knew it was wrong and did it anyway…

    Anyway: It is stupid, dangerous and i agree that this post should be deleted. A hack should not be something that can get you killed in many ways.

  27. I hope a lot of people in my area build one of these…. because the roads are just too damn crowded already, you know what I mean?

    Population control indeed. :D

    j/k… duh

  28. Wow. For a bunch of hackers and people who enjoy experimenting, I am surprised by the amount of cry babying. This is a valid hack and has its place. Of course there are better ways. Of course it’s dangerous. If plugged into a house line that has not been disconnected from the circuit being powered up this way you will know it immediately as the genset power will go to ground. The chance of hurting a lineman is beyond the scope of this (lineman will have his tiny “Multi Tester” with him, wont he)?
    I am stunned by the lack of ingenuity some of you “hacker, experimenters” possess. Please be careful taking your bubble bath kids, you might drop that toaster…..

  29. Kill, kill, kill??? I’ve been shocked by 110 Volts like 1000 times, and it was never more than an “ow”. And I climbed telephone poles for 8 years, and never met a lineman that didn’t wear high voltage insulated gloves when working on the mains. All you fellow EEs out there, you ever heard of Ohm’s Law? IE: 110 Volt divided by the resistance of your body from hand to toe = not-so-much current. Figure it out(accident by reflex, not withstanding)…

  30. it has some good uses, like using a generator when the power’s out, but the description’s disclaimer is not NEARLY long enough or detailed enough. someone could get really hurt trying to use this to do the wrong thing.

  31. There is nothing wrong with this post. It is a methodology that is used often for temp emergency power. Is it the safest way to do it? Absolutely not. But it is cheap, and it works in a pinch. And just like anything else that has to do with mains voltage, if used correctly, is perfectly safe.

  32. I was waiting for that one, too. My grandfather is a master electrician, and I’ve seen him have to grab 110 twice to tell if it is hot. “I think that might be live…” second grab “yup, that’s live.” (And then he keeps going)

    And he’s seen guys wiring up 3-phase 480 bare handed and hot. (They knew he was a lightweight and warned him not to touch it, though.)

  33. @Pete:

    They use a special type of box that manages power both ways. When their [solar/water/wind/whatever] power generation system is producing more than the house needs, it allows power to flow to the grid, and the power company’s meter runs backward (or another meter runs, depending on the system). When the house is using more power, or it’s cloudy or for whatever reason the house power system isn’t producing enough, that box allows power to flow from the grid into the house.

  34. We did something like this back in 2004 in FL when the hurricanes bailed through. We turned off the main breaker and backfed our generator into the circuit panel to power some of our electric items (fridge, lights, fans). Couldn’t power AC though, pulled too much power. This works as long as you cut the main power coming in.

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