finishing our shed to create an office space
From a very practical standpoint, you can't do that with one circuit. That leaves you with two options:
Work within the rules and add individual circuits of other types, including fixing up this cable.
You can only have one circuit of each "type" - so one 120V circuit (which you already have), one 240V circuit (which you could use for HVAC) and there are some ways you can add additional circuits. But it gets tricky. And messy. And extremely limiting.
Forget this circuit - remove it altogether. Add a much larger cable (or wires) and put in a subpanel.
If this were just "I want to do a little more in the shed than I was before" then making do with one circuit would probably work. Power a few tools while also having lights is quite doable, particularly since LED lights require very little power.
But you're not doing that. You're making an office. Which typically means:
- One lighting circuit - so a problem with your receptacle circuit(s) doesn't turn out the lights.
- One, preferably two, receptacle circuits. Particularly if you have a laser printer or other high current (even if relatively limited usage) devices. Even more so if you want a mini-fridge and a coffee maker.
- One circuit, either 120V or 240V depending on your specific needs, for HVAC, typically a small mini-split heat pump.
You may have other needs as well, depending on how big the shed is and how you want to use it.
How do you do this? A subpanel. The basics are:
- Conduit (buried 6" to 18" deep depending on type) + wires or direct burial cable (24" deep).
- Cable or wires should be aluminum to save money. Exact size will depend on capacity needed, though oversizing is OK. Typical is to use 2/2/2/4 AWG aluminum for up to 90A, though if fed with a 60A or 70A breaker that's just fine.
- A "main" panel. Why? Because it includes a disconnect (a subpanel doesn't, unless you use a locked-down backfed breaker for the incoming feed). At least 12 spaces, but more is better and doesn't cost much more. A 20 space panel is perfectly reasonable and 30 space won't cost much more.
- GFCI on most circuits (depends on NEC version), either GFCI breakers or (generally much cheaper) deadfront or receptacle on 120V circuits.
- Ground rods needed. Yes, even though you will also have a ground wire going back to the main panel (either in conduit or as part of the cable).
- Get this thing permitted and inspected properly. You can, in many places, get away with little things without permit/inspections. Wiring up a shed can be a DIY project, but if you don't get it permitted then you could run into serious problems in the future.