Hi I have to run wire inside conduit for 400' to reach a shed from my home, I think do to voltage drop I have to start with a 8 or10 gauge. I am installing 5 outlets and a light ,my question is how do I transition from 8 gauge or 10 gauge to 14 gauge for my plugs?

Do I just install a jbox and do a splice from 8 or 10 to a 14 gauge? Or do I need a sub panel?

Ive installed outlets before but the wire is always there from the panel but this is thicker and don't
Know how to do it the proper way.

Whoa there, please!

Explain yourself just a tad bit more.

To begin with, 400' from a cost perspective, begs consideration of another feed method. For example, a utility supplied transformer or even Solar power.

Also, you are starting the process backwards. You need to decide what the voltage and amperage you need at the shed, then determine how to get it there.

NEC does not mandate voltage drop calculations. They make recommendations. Up to 3% on the feeder and up to 3% on a branch circuit with a max end-to-end of no more than 5%.

Not knowing the voltage or the amperage needed makes this an exercise in futility.

However, the way you would marry the feeder to the shed is either through a main disconnect switch and /or panelboard (best bet).

You are required to use a 4 wire feeder so bring 240 volts to the panelboard. If you want cable sizing, you need to give us the amperage.

You will need a separate grounding system at the shed, probably a minimum of 2 ground rods six feet apart. At the panelboard, the Neutral from the feeder cannot be bonded to the panelboard case or ground buss. Ground and Neutral at this panelboard must be isolated from each other. That means that you cannot place any equipment grounding conductors into the "Neutral" buss bar.

You use either 14 AWG (15 amp) or 12 AWG (20 amp) conductors for the branch circuits sourced by the panelboard to the outlets. Personally, I would use 12 AWG conductors and 20 amp breakers if you are going to be using motorized hand tools.

Are you just running a multi-wire (shared neutral) branch circuit, or a feeder to a panel? From your question I assume just a multi-wire circuit, or even just a 120V circuit.

If this is the case you do not need any grounding electrodes. You do however need a disconnect switch at the shed. What I would do is run to a small two-pole disconncet, like an A/C pullout. Then on to the first box with the same #10 or #8. I would splice at each box with #12 pigtails for the devices. I would not use the #12 to feed-thru. This way you can work with the easier #12 at the devices but you are not relying on it to carry the load so it will help with VD.