Myself, a long retired electrical power distribution design engineer (BSEE Purdue University, 1969/70) and my good sparky buddies Bus Driver, Mark and Buick n Deere n several other fine electricians and engineers n technicians HAVE BEAT THIS TO DEATH ON HERE BUT LOOKS LIKE WE HAVE FAILED TO GET THE MESSAGE OUT, BUT HEY I WILL TRY AGAIN........
If you wire up a "Sub Panel" fed from the "Main Distribution Panel" for 120/240 Volt Single Phase Three Wire Service..........
YOU RUN 4 WIRES (2 Hots L1 & L2, 1 Neutral a GrounDED Conductor, 1 Equipment Ground, a GroundING Conductor)
At the Sub Panel DO NOTTTTTTTT bond the Neutral Buss to the Equipment Ground Buss, keep them seperate and isolated
While metallic conductive Conduit with proper jumpers and connections etc. can in some cases be used for the equipment GroundING Conductor I WOULD RUN AN INSULATED GROUNDING CONCUCTOR IN THE CONDUIT AND NOTTTTTTTTTTTTTTT RELY ON THE CONDUIT ITSELF No wayyyyyyyyyyyyyyyyyyy
Sooooo the electrician who said run only 3 wires and RE bond Neutral and Ground at the sub panel I assume,,,,,,,IS IN MY OPINION WRONG AS RAIN PROVIDED!!!!!!!!! he is relying upon the latest NEC cuz yearssssssss ago things were done differently.........
NOTE..... I suggest you go with what LOCAL AUTHORITY HAS TO SAY versus myself or anyoneeeeeeee here, cuz if there is a local binding governing authority ?????????? its what they say and will approve that matters nottttttttt anyone here, regardless if we are right or wrong or the latest NEC or older NEC or for that matter ANY NEC has been adpopted in your juriusdiction, its ONLY one particular set of standards that may or may not have been adopted or may or may not have any authority where you live!!!!!!
That being said, as an engineer I could give you the engineering and safety reasons WHY you should run 4 wires, but that takes a lot of time to explain, and it may only confuse a non electrical person, no need to re invent the wheel all over again on here lol
Best wishes n God Bless,,,,,,Check with Local Authority and true competent professionals and experts before making a decision, cuz its you and your familys lives at risk, not ours..
John T in Indiana, BSEE, JD