On the main breaker for my house I have a 50amp circuit which until recently was being used for a hot tub. I would like to repurpose the circuit by running it to a sub-panel with two 15amp circuits and one 20amp circuit (still 50amp total) which will be used for a shed (outlets, lights, and mini-split ac unit). Is this something which is doable?
This is absolutely doable. A few things to watch out for:
- Breaker Size
The breaker in the main panel can be the existing 50A breaker. Or you may be able to use a larger breaker. With most panels, larger breakers cost very little until some particular point - typically somewhere around 100A or so the price will double or triple (or more). The size is limited by the wire.
- Wire Size and Type
You can, generally, use copper or aluminum wire when going between panels. That is because most panels and breakers (unlike receptacles, etc.) are rated for use with both copper and aluminum. You can often (especially with current copper prices) find large aluminum wires available for less than equivalent copper wires. By equivalent, I mean that the physical size (AWG) for a particular current (Amps) for copper will be smaller than the physical size of aluminum wire for that same current. But even though the aluminum is physically larger, it is often much cheaper. There are a number of tables available showing wire sizes - beware that there are a few gotchas to look out for, so ask here before buying the wire, just to make sure it will work.
The wires can be individual wires in conduit or can be a cable with all the wires together. Cables save having to use conduit. However, for outside and/or buried use, only certain cables will meet code. In addition, buried cable has to be buried deeper than conduit with individual wires.
For your use, you will need 3 main wires - 2 hots and a neutral - and, unless you are using appropriate metal conduit, a ground wire.
The subpanel can be a small subpanel, like you may have had for your hot tub. But far better is a big panel, often listed as a "main panel". You will need a disconnect switch at the shed - that can be a separate switch but it is often cheaper and easier to use a main breaker in the subpanel. That main breaker can be any size from the size of the feed breaker in the main panel on up. A 100A breaker (especially if bundled with a panel) can be used even if the feed is a 50A breaker with 50A-rated wire.
You will need GFCI protection for most, if not all, circuits in the shed. Keep that in mind when buying the subpanel as GFCI typically requires a full "space" for each circuit where marketing for the panels is often based on "circuits"/"spaces" with "circuits" as much as 2x the number of spaces.
You will need grounding at the shed. Typically this is 2 ground rods. This is in addition to the ground wire (or metal conduit) going back to the main panel.
Generally what you are planning is fine. Really it is a bit risky to assume the hot tub was properly wired so to deliver a safe answer we would need to know the wire type and size of conductors including the neutral and ground. It may even be rated for more than 50A because installation instructions for hot tubs sometimes require oversized wires to accommodate high ambient heat.
It sounds like your load is fine if the wire is properly sized for at least 35A. When calculating load the load is available and should be calculated separately for each opposing 120v legs, and 240v loads contribute to both legs. So your 240v minisplit will load about 20A on each leg, and then you can put your two 15A circuits on opposing legs and your load on each leg will total 35A. (Actually probably less, the minimum circuit ampacity required for minisplits is usually less than the overcurrent protection required.) Additionally you can install load breakers that total more than 50A when diversity issues are taken into account. For instance if your minisplit is cooling only and you have a baseboard heater you only have to add the bigger load.
There are lots of details, most significantly you will need a disconnect switch in the shed, best accomplished by using a main breaker panel. You will need to drive ground rods at the shed, and you will need to float the neutral at the shed, keeping it isolated from the ground.