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What color should the return wire in a 24vdc two wire transmittere be? Black or some other color than black? Before you answer go lift the hood and see what color the return on that 12 VDC system. If its like most system its black. In more than 30 years of experience I've seen most industrial system use black as the return. However, in the utility and commercial power plants it seems the black wire is the return if it was wired by and instrument tech and its the hot if wired by the electrcian.