Where in the code does it say that the grounded conductor of a 2-wire circuit is counted as a current carrying conductor?

Does it need to? It seems patently obvious that if you get a reading on an amp clamp, it's a current carrying conductor.

Is this in reference to derating for the number of conductors in a conduit? If it is see 310.15(B)4

If the current travels to the load on the phase wire, the only path that it can use to get back to the source is the other conductor.
QED

For a 2-wire circuit, the circuit is always going to be unbalanced, and the neutral considered a current carrying conductor per 310.15(B)(4)(a).

No "Neutral" in a 2 Wire Circuit.

Same level of Current will flow on both Conductors -
(unless something is terminated incorrectly!..)

The same applies to:

1. L-N 120 Volt 2 Wire Circuits, from a 1 Phase 3 wire source,
2. L-N 120 Volt 2 Wire Circuits, from a 3 Phase 4 wire Wye source,
3. L-N 120 Volt 2 Wire Circuits, from a 3 Phase 4 wire Delta source,
4. L-N 277 Volt 2 Wire Circuits, from a 3 Phase 4 wire Wye source,
5. L-L 208 Volt 2 Wire Circuits, from a 1 Phase 3 wire source,
6. L-L 208 Volt 2 Wire Circuits, from a 3 Phase 3 wire source,
7. L-L 208 Volt 2 Wire Circuits, from a 3 Phase 4 wire source,
8. L-L 240 Volt 2 Wire Circuits, from a 1 Phase 3 wire source,
9. L-L 240 Volt 2 Wire Circuits, from a 3 Phase 3 wire source,
10. L-L 240 Volt 2 Wire Circuits, from a 3 Phase 4 wire Delta source,
11. L-L 480 Volt 2 Wire Circuits, from a 3 Phase 3 wire source,
12. L-L 480 Volt 2 Wire Circuits, from a 3 Phase 4 wire source,
13. L-L-L 208 Volt, 240 Volt and 480 Volt 3 Wire Circuits, from a 3 Phase 3 or 4 wire source.

Just wanted to make the subject clear.

Scott

BTW, I think it's time to draw up some basic circuit schematics, to demonstrate the flows + levels on 2 wire circuits
(also point out when a Common Grounded Conductor is a "Balanced Neutral", and when it isn't).