The SCI 301 Puzzler

A schematic of a service panel is shown with two 120 VAC phases coming in from the top. The circuit breakers are arranged as shown. The wires shown are the red and black "hot" wires and the yellow neutral wire. (The bare copper grounding wire has been omitted for simplicity.) The phase on the right was wired by a professional using standard house wiring for each of the branch circuits. The phase on the left was wired by a well-meaning do-it-yourselfer who saved money by using single strand wire that was on sale. The single strand wire has the same AWG rating as the household wiring and is capable of carrying 50 A of current. Each circuit has its own hot wire, but they share one neutral return wire. Can you explain why there's a potential hazard with the do-it-yourselfer's wiring scheme, even if the breakers operate correctly? What should the do-it-yourselfer have done, still using single strand wire?

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