Your breaker needs to match the load!
You didn't mention what the load is. If the load is general 15/20A receptacles, you need to change the breaker to 20A. (because kitchen receptacle circuits must be 20A, and 15A receptacles are allowed on 20A breakers, that is the only exception).
If you have an oven whose breaker says "30A" then the breaker must be 30A.
Etc.
The most you can get out of that #8 copper NM-B will be 40A. If I am incorrect and the cable is a type other than NM or UF, then you may be able to get 50A out of that, but only if it's going to an appliance which is asking for a 50A breaker. Note that since 40A receptacles are impossible (not enough pin arrangements available), 40A circuits use 50A receptacles. This is not an exception to a rule. 40A appliances need 40A breakers.
That box is too small to do anything with
That "Octagon box" doesn't have enough cubic inches to splice anything to it. You need
- 3 cubic inches for each #8 wire
- 2.5 cubic inches for each #10 wire
- 2.25 cubic inches for each #12 wire
- Grounds are "4 for the price of 1" based on the largest. Expect #10 ground with #8 conductors.
- I assume there will be no receptacles, but each yoke (receptacle assembly) needs cubic inches same as 2 "wires" (the largest size attached to it).
So if you are tying the #8 cable to two 12/2 cables, you have 3x#8 (9 c.i.), 4x#12 (9 c.i.) and 1x#10 (2.5 c.i.) for ground. That is 20.5 cubic inches.
If you attach the #8 cable direct to a 40A receptacle, you need 3x#8 (9 c.i.), 2x#8 (6 c.i. for the receptacle) and 1x#10 (2.5 c.i.) for the ground. That is 17.5 cubic inches.
How do you obtain the cubic inches you need? The simplest way is to stack on an octagon "extension box" which is the same shape/size of octagon box with an open back. If you need a low-profile solution, change the octagon box to a 4x4x1-1/2" box (same thickness slightly wider dimension) which is 21 cubic inches, possibly with a blank domed cover (1/2") to gain 6 more cubes. If you need more still, go to a 4-11/16" square box (42 cubic inches).
The box needs a cover.
The ground wire must go to the metal box.
There is a #10 ground wire coming in with this cable. It must be attached to the metal box FIRST, do not attach it to a receptacle or other cable and bypass the metal box. You are welcome to use 6" of #10 wire as a "pigtail" to the metal box, then join that to the other with extra-large wire nuts.
There will be a hole in the back of the box tapped for a (screw size) #10-32 machine screw aka bolt. They actually make special green screws made for this purpose, but plain old silver ones will do. Do not use the wrong thread pitch and do not use a sheet metal screw.
Pigtails do not count when figuring cubic inches. So the #10 ground pigtail can be ignored.
Panel numbering is incorrect
Someone who doesn't know what they're doing went down the sides of the panel and numbered all the breakers. They were already numbered (stamped in the metal) and the Sharpie is wrong. They should be numbered 1A, 1B, 3A, 3B, 5A, 5C down the left side for instance. That is a big deal because of phasing.
https://youtu.be/jMmUoZh3Hq4?t=538
The breakers in actual space 1-2 are on phase L1.
The breakers in actual space 3-4 are on phase L2.
The breakers in actual space 5-6 are on phase L1.
The breakers in actual space 7-8 are on phase L2.
GE's are weird in that they let you stack 2 thin breakers in one space. But with great power comes great responsibility.
The amateur numbering destroys this important information. 240V breakers span 2 spaces (e.g. the 30A breaker in 1B and 3A) which gives it access to 240V. That is essential. Especially for MWBC circuits, where mis-phasing it could cause it to fail "silent but deadly".
If you'll be placing a 2-pole breaker, or moving breakers around to fit a 2-pole breaker, you need to know about that stuff, particularly MWBCs, as it's very easy to get those wrong in a GE panel.
