The Extension Cord From Hell




How to build a hybrid extension cord/outlet strip.

DANGER: This project involves household outlet voltage. DO NOT ATTEMPT IF YOU ARE NOT FAMILIAR WITH 120V HOUSEHOLD WIRING.

When the project is completed you have an adaptable extension cord. I used 5 outlet boxes, you can use as many or as few as you want, just change the length of the cord and parts count accordingly.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Required Tools

To build this cord you will need:

A set of wire cutters
A wire crimper
A wire stripper or knife (or both)
A small and large flat blade screw driver
Pliers (needle nose or regular) (not pictured)

To test this cord you will need:
A Digital Multi Meter (not pictured)
An Outlet Tester (not pictured)

Step 2: Required Materials

To build this you will need:

1x 25ft 14ga medium duty extension cord
10x duplex outlets (buy the contractor's pack)
10x entry cable clamps 3/8 inch (fits half inch hole)
5x metal electrical box
5x metal box cover
70x 16-14ga 4-6 stud spade lugs (buy the 100 pack)

Step 3: Box Prep

To prepare the box for use you will need to do the following:

1). Use a screwdriver to pop one of the small holes on the top and one on the bottom out.
2). Use the pliers to bend the small round cover until it breaks off.
3). Install the cable entry clamps into the holes. Orient them so the screws face sideways compared to the box (as seen in the pictures)
4). Build all of the boxes the same way.

Step 4: Wire Prep

This will be the most boring and tedious section of the project. Do not skip the steps of crimping the spade lugs onto the cable! You will have a much easier time using the cable with the lugs on it. This also gives the cable a more professional build.

1). Identify how far from the plug to the first box (about 10 feet for me). Measure that length from the plug and cut the cable.

2). Strip about 6 inches of the outer insulation from the cut end of the cable. Be careful not to damage the inner conductors.

3). Strip about 3/8 inch of the insulation off of the black, white, and green wires.

4). Place a spade lug on the stripped wire and crimp it on.

5). Measure about 2 feet from the receptacle end of the cable and cut.

6). Strip about 6 inches of the outer insulation from the cut end of the cable. Be careful not to damage the inner conductors.

7). Strip about 3/8 inch of the insulation off of the black, white, and green wires.

8). Place a spade lug on the stripped wire and crimp it on.

9). Cut the remaining wire into 4 sections that are about 2 feet long and 5 sections that are about 6-8 inches long.

10). Strip about 6 inches of the outer insulation from each end of the 2 foot cable. Be careful not to damage the inner conductors.

11). Strip about 3/8 inch of the insulation off of the black, white, and green wires.

12). Place a spade lug on the stripped wire and crimp it on.

13). Strip all of the outer insulation off of the 6-8 inch sections (you want only the black, white, and green wires).

14). Strip about 3/8 inch of the insulation from each end of the 15 short wires, then crimp a spade lug onto each end.

Step 5: Outlet Prep

Prepare the outlets for installation into the box

1). Remove the screws from the tabs.

2). Depending on the box that you choose, remove the mounting ears.

3). Attach a black and white wire to each outlet in pairs. Attach the black wire to the gold screw, and the white to the silver screw. Do not use the green screw yet. I attach the wires to the bottom screws on the first outlet and the top screws on the second outlet.

Step 6: First Box

This is the first box in the chain.

1). Take the cable with the plug end on it, and insert it through the "top" cable clamp on one of the boxes prepared earlier, then tighten the clamp evenly.

2). Insert a 2 foot jumper cable through the "bottom" cable clamp on the box, then tighten the clamp evenly.

3). Attach the black and white wires from the cables to a pair of outlets, one cable to each outlet.

4). Attach a green jumper with and the cable's green wire to the first outlet's green screw, then attach the same green jumper to the other outlet in the pair with the other cable's green wire.

5). Install the outlets onto the box cover using the hardware that came with the cover (mine came with screws and nuts).

6). Install the cover onto the box. NOTE: Depending on where and how you plan to mount the box, it might be worth not fully installing the cover at this time.

Step 7: Last Box

Now we build the last box in the series. I then build the intermediate boxes.

1). Insert a 2 foot jumper wire through the "top" cable clamp of the box, then tighten the clamp evenly.

2). Insert the receptacle end of the cable through the "bottom" cable clamp of the box, then tighten the clamp evenly.

3). Attach the cables to the outlets as you did for the previous box, including the ground wires.

4). Install the outlets on the box cover.

5). Install the box cover onto the box. NOTE: depending on where and how you plan to mount the box, it might be worth not fully installing the box cover yet.

Step 8: Intermediate Boxes

Now we assemble the intermediate boxes. These are very much like the first and last box, but you only use the jumper cables between the boxes.

Step 9: Checkout and Finished Project

Before you plug this into the wall, some basic safety checks.

You need a Digital Multi Meter (you do know how to use one, right?)

Using the OHMS settings, check for shorts between the conductors on the cable. Neutral to ground, neutral to hot, and hot to ground. If you find any, check all your work and repair the problem. Check that the metal boxes are connected ONLY to the ground conductor! YOUR SAFETY DEPENDS ON THIS CHECK!

If the unit passes the short check finish installing the unit where you want it, make sure all the covers are fully tightened down.

Plug the unit into power. If you pop the breaker you plugged into, check your work for shorts again.

Use an outlet tester to check all the outlets, including the cable receptacle. If the outlet tester shows correct wiring in all the outlets, you can use the project. If not, find the error and repair it before use.

Be the First to Share


    • CNC Contest

      CNC Contest
    • Make it Move

      Make it Move
    • Teacher Contest

      Teacher Contest

    56 Discussions


    8 months ago

    I would advise skipping the spades, and instead LEARN how to make a pigtail using a wire nut and some solid-core wire (14ga is fine and keep it under 1500 watts). That's how you'd do real wiring (of a lamp box, at least... stranded wire in outlet work boxes is against NEC code).

    And others have said: put GFCI in the first outlet. Not just another opportunity to learn how this stuff works, but your life could depend on it! GFCI is sometimes a pain, if the work-box is not deep enough...)


    1 year ago

    I have made cords like this before
    Use it alot.
    Just dont plug in more thAn 15a at one time like 13 microwave ovens or fridges. Dont trust breakers i have personally melted wires and breakers and they still didnt trip (it wasnt assembled right even tho it was installed by a pro) you never know.
    Some have asked about using houshold wiring
    I would say no you should not use houshold wiring
    It is solid copper and It isnt made to flex very often it could break in very little time and short.


    Reply 3 years ago

    In most cases in my experience, 8 outlets on one line are enough...

    however, if someone is in a band, 8 outs would only cover one person, or maybe 2 at most.

    so it all boils down to what you want this for, and how many items will you have plugged in at one time...

    Before you make something like this, draw up a list and/or a plan of what you need, what you want, and then go from there.

    But for safety sake, ALWAYS have a GFI outlet first. It just might save your life.


    9 years ago on Step 8

    Could I use standard house wiring then add a three prong extension cord end? The wire is on sale here for 2$ a meter.

    2 replies

    Reply 3 years ago

    the safest way to make a cord like this is to have a GFCI as the first outlet & then have everything wired so if the GFCI pops, the whole line(outlets on the cord) shut down.

    In America, the standard wire for homes is 14 gauge solid....

    personally, I would rather use 12 gauge solid or strand. the heavier wire increases safety.

    What kind or what gauge of wire is used, or is the standard outside of America, I have no idea, so I can not even guess as to what you should or could use.


    10 years ago on Introduction

    Even if you had large guage wire, the breaker for the outlet you plug into is likely not rated for the kind of pull an outlet mess like this *could* attempt.

    And the *wiring in the wall* between the outlet and the breaker would *also* not be rated for the enormous pull of a cord like this. This thing will start a fire for someone, somewhere.

    8 replies

    Reply 9 years ago on Introduction

    Outlets do not pull any amps when they are not being used. Even if all of these were plugged into the wall, but nothing was plugged into them, there would be absolutely no usage at all. Also, the wiring between most outlets in your home is probably 14 gauge wire, just like the extension cord this person used.


    Reply 8 years ago on Introduction

    The concern is that standard 14 gauge wire is rated for 15 amps. If you have this many sockets, generally, you're going to be using some/most/all of them, possibly exceeding the 15 amp rating. Also, bear in mind MOST outlets are not on isolated circuits, generally all the outlets from a room or several rooms are tied together, facilitating to potential overload.

    General use extension cords are 16 gauge stranded wire, which will support even less. More likely a tripped circuit breaker before a fire, but fire is always a possibilty, especially with the on/off switch.


    Reply 3 years ago

    that is why it is ALWAYS a good idea to have a GFI on a home made extension cord


    Reply 7 years ago on Introduction

    Don't have such faith in things, The breaker may not be tripped until after the wire catches fire.

    They only trip after a particular critical mass, and if your wire happens to fall in the zone where the household circuit is fine, but it's not. *shrugs*

    This isn't a very safe instructible, there's a reason why extension cords are limited to the number that you'll find on power boards.


    Reply 4 years ago on Introduction

    It's only as dangerous as a person's use of it. You can just as easily daisy chain two or three surge protectors. If you have several tools being used at the same time then yes, this cord could be dangerous. I wouldn't use it at a job site or shop where several people would be working at the same time.


    the average household does use AWG 14, however that is solid. stranded as are in cords would be AWG 12 to equal the same electrical flow as AWG 14 solid.


    Reply 4 years ago

    actually, you get a higher current rating with stranded. it's kind of weird but I'm an apprentice electrician and it has the table in my expensive code book for all the wire ratings. Just sayin'. I'd probably do it all different tho!


    3 years ago

    I noticed you did not use a GFI with this...

    Is there a reason why?

    I like this idea BUT, I would have used a GFI as the 1st outlet...that way if there is an overload, the GFI will pop before anything gets overheated in the wall or gets short circuited.

    And the female on the end? personally I think it is a bad idea. kind of like an accident waiting for a place to happen. but generally speaking, this is a good idea.


    4 years ago on Introduction

    This is great, despite the naysayers. I move a lot, so my shop has to be SEMI-mobile. I may have 15 things plugged in, but I'm only using one or two at a time. Well, an LED task light, a small stereo, a cell phone charger, and a corded tool. That's all I can juggle at one time. All of the other tools are turned off, waiting to be used.