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Consider 3/4" EMT running from a main panel to a workshop. It has THHN wires for a 240V 30A circuit and a 120V 20A. The EMT is the ground.

In the workshop, the EMT ends at a metal box, and from there on is Romex.

My question is: What gauge should a pigtail from the Romex grounds to the box be? Does this need two pigtails, at least one 10AWG and one 12AWG? Or one that's at least 6 or 8? Or just screw both Romex grounds to the box (though that's less convenient, esp. since you only get one wire per screw)? Or...?

The closest question I found was THHN through EMT - then convert to Romex?, but that doesn't address gauge, just the basic need to ground.

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One 10 AWG copper pigtail will do. In a shared conduit, (or in this case connecting to a shared conduit) one ground of the largest size is all that's required for all the circuit conductors. It does not need to be larger than that.

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The largest

The design presumption is that it's unlikely for two circuits to have a big problem (bolted fault) at the same time.

This only works for ground, however, not any active conductor.

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