I want my kids' tree house to have a switched light. I do not want to run any underground wire back to our house. I want to run an extension cord out to the tree house.
Can I plug in the extension cord to the exterior outlet on the side of my house, then run the extension cord to the tree house, cut off the end of the cord, and wire it to a metal box with a switch, and then run regular conduit up to the ceiling for the light?
Cutting off the socket end of the extension cord is not a great plan, I would use an inlet instead.
Actually, I would go a completely different way. It isn't 1930 anymore and we're not forced into one technology. Instead of having this genuinely dangerous thing in a kid's tree house where you have to tell them "Now children, do not touch this! This technology is not for you"… I would instead pick a technology which is very much for them, and take advantage of the astounding advances in battery, solar and LED lighting and go with a 12 volt battery system, perhaps with lithium or a 10-cell stack of NiMH D-cell batteries if you want to be super safe.
If you're not familiar with this tech, don't worry - your kids will surpass you! LOL Before you know it, they'll have LED strip lighting in the ladder rungs, and probably a WiFi hotspot. But as long as they're playing in a 12 volt regime, they're fine.
The system would be entirely local to the tree house, and would not connect in any way to AC mains electrical.
Solar panels are so cheap and efficient these days that you should be able to get reasonable recharge even in the shade.
No: permanent use of an extension cord, or modification of the cord, are not allowed by electric code unless the cord is labeled as allowing that.
A similar question was asked not long ago; the answers there might be useful for you.
In EU - no. All outside cables shall be weatherproof and connections outside house shall be weatherproof. You cant make permanent connection from outlet to box especially using standard cables. Only connections from box to box with proper cables are allowed.
But.
You can connect any extension cord to outside weatherproof outlet and connect loads on end of this cord. Remember that maximum cable length is 50m from breakers, deducting 5m for any outlet - if use cord this is 10m (5m for each end) and connecting cables in tree house counts. Cord shall be removed when weather exceeds this clearance or when is not supervised by adult.
Your could also install an in-wall inlet at the treehouse. This would allow you to disconnect the extension cord when mowing, during the winter, etc
Instead of laying the wire on the ground or burying it. You can suspend the cable. Make it at least 3 m above the ground level at the sag point. That will keep it out of the way in all seasons.
Get some outdoor rated cable and a catenary wire then you can run the catenary from the house to the treehouse and string the cable along it with hangers. Don't just use plastic cable ties, eventually even the UV rated ones will fail.
Make sure that wire is GFCI protected. Or make it a low voltage circuit so playing children are in no danger of getting shocked.
You are basically making a custom power strip. As long as it has only single plug, it is fine.
Since this is outdoors, and kids, make sure the work is well secured and insulated, especially against rainwater.
Use orange 'garden' extension cord so you don't run it over when moving grass.
Make sure that the cord has three wires, so you have something to ground the metal box with.
Also make sure that the outlet at side of your house is gfci protected.
If you can be bothered for extra safety, consider using a 24V doorbell transformer and 24V bulbs, these are standard parts and use same wires, but lower voltage.
