Running a ground wire in conduit to plumbing

jwelectric said:

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.

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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?

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