Running a ground wire in conduit to plumbing

[Juan for the road]

This metal box will need to be bonded to the largest equipment grounding conductor. This will be a #10 that is either in the 10/3 or the 8/3 cable.

I don’t understand just how you are installing these cables. If this box is a junction box then there should be an even number of each cable unless you are pulling the cables straight through the box without a splice or joint.
In this case either the 10 or the 8 will need to be opened to access the equipment grounding conductor.

Yes,these wires were run into an attic for future use and just terminated into a large metal box and wire nutted.(the reason for the odd number).

If a box is not equipped with a ground screw or other means of grounding,what is the proper way to ground it?

 

[JWelectric]

250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Grounding conductors and bonding jumpers shall be connected by one of the following means:
(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed as grounding and bonding equipment
(4) Exothermic welding process
(5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less than two threads in the enclosure
(7) Connections that are part of a listed assembly
(8) Other listed means