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I know the current code. However, in the past, many of these were done with three usually common sized conductors. This question has come up on many job sites with existing underground conductors where they are not dug up and replaced with the 4th grounding conductor. My question was what to do when there are only the existing three and they need to be used as is. Code also requires specified grounding conductor for branch circuits, but when I install a light or receptacle on circuits without ground, code doesn't require rewiring the circuit.
Yes, they are common 250 kcmil aluminum conductors with direct burial rated insulation. Thanks for reviewing.
.I see Mike came back and gave you the answer you needed.