From the other forum;
"As provided in 250.130(C) the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.
An example; a furnace in a utility room in a basement wired with a 2 wire Romex without ground. Electrical service is upstairs at far end of house.
Copper water main enters basement, is located directly beside the water heater and furnace. Customer replaces furnace that requires an equipment ground, 250.134(B) would allow an equipment grounding conductor to be run to the water service within 5 feet of entry."
When I bought my house I had a aluminum exposed #6 attached to dryer from cold water pipe. I later ran a new circuit and recycled the AL.
I thought one could only run this on exposed flex and bonded on one end.
Living the code.