I'm building a shed I want to add power to. To get power I'm planning to use the power that is going to my hot tub (50 amp).
The hot tub has a 50 amp breaker from the main to a 50 amp breaker box that is mounted on the side of my house. I plan to disconnect the hot tub and run the wire to my shed from this breaker box.
I can't find a 50 amp sub panel, but I can find 100 amp boxes online that are fairly cheap. I'm curious how I can wire this box to use the 50 amps that will be coming in?
Just wire it up. There is no problem using a sub panel that can take more current than you will ever feed it.
Points to note:
The cable to your hot tub will be rated for 50A (not 100A); therefore you must not upgrade the breaker in your main panel to 100A.
It would be sensible to add a note near the main panel that the cable is only rated for 50A, so not to change the breaker (so you, or a future purchaser, remembers in a year's time).
If you connect four 20A 240V devices to the sub-panel in your shed (and run them all at once) the sub-panel will be entirely happy with this, but the 50A breaker in the main panel won't be and it will throw - so don't do that then.
PS. This is not directly related to your question, but still. 50A is a substantial current. Do remember to torque the connection to the panel correctly.
Go shopping for tires. Almost any tire these days is rated 112 mph. *You're allowed to drive 65 on those, it is saying don't exceed 112 mph*.
It's the same with subpanels: The "100A" is a maximum rating. Do not exceed 100A.
However, you certainly should exceed your feed-breaker size of 50A. Even if 50A panels existed, they would be very small in terms of numbers of spaces. The most important job of a subpanel is to not run out of spaces. So you should be buying a subpanel with way more spaces than you think you're going to need, to assure you never run out.
So size your panel for the spaces and make sure its busing is at least 50A (which won't be hard).
Yes, that means you may have a 150A breaker in the subpanel being fed by a 50A breaker. That is fine.
The breaker isn't there to be a breaker. It's only there to be a disconnect switch, to satisfy a Code requirement for a disconnect switch in an outbuilding. Feel free to price separate disconnect switches if you want to, but you'll find "just getting a panel with a main breaker" is the cheapest way to get a disconnect switch.
"But I'm clever. I want the breakers to be the same size so the breaker in the shed will trip first, and save me a long walk to the house". Sorry, breakers do not work that way. Even if you found a way to match breaker sizes, realistically the feeder breaker in the house will trip most of the time. Especially when it is raining!
Using a larger sub panel will not be a problem but the breaker must stay a 50 AMP in the main panel. You can utilize the 50 Amp breaker in the hot tub panel as a connection point for the new wire run or just use that junction as a splice point. If you do that, I don't think you can double lug the wires and would have to use split-bolt connectors (buy a lot of electrical tape) or Polaris connectors. Wire nuts would not be a good idea.
50A is not a standard panel size, you will find a few more options if you search 60A.
You can use a panel rated for higher than the feeder, the panel rating is the maximum current allowed.
You may wan't to consider checking your wire size, the instructions for the last hot tub I installed specified #6 wire, most of the time #6 can be protected at 60A. (Actually confirming proper breaker for the feeder size is a good idea anyway.)
