I recently spotted a pair of long disused 400w MV ballast cans and it turns out they were used for a interesting setup where the ballast cans were mounted on the side of the road and two overhead cables were used to feed a pair of 2nd generation Ovalite clamshells on a median mounted pole.

This vintage photo taken nearby shows this setup fully. You have a pair of clamshells mounted on the median and a pair of cables going back to the main power pole on the right to where the two pole mounted ballasts are. You can also see the more conventional setup for clamshells, with the ballast mounted at the base of the arm. Interesting they opted to mount the ballasts on the power pole instead of running a feed over to the median pole and putting the ballasts there instead.

Such a setup is possible as you can increase your ballast to lamp distance using thicker gauge cable but it seems like the setup would be a pain to troubleshoot as any issue with the ballast or PC (I suspect these were group switched back in the day) would require you to move over from the median to the side of the road.

It seems like back in the day people were pretty creative in remotely ballasting mercury luminaires, I've heard of mercury lamps designed to operate directly on a special series regulator so the series regulator serves effectively as the ballast for a string of luminaires. Line Material once sold such a system called the "Uni-Ballast".

 What a weird design ! Why not just put the ballasts on the median pole ? You'd think with that distance , it'd decrease starting reliability , especially on well used lamps on a cold evening .

 I wonder what the max ballast to lamp distance is anyways ?  The setup pictured has to be at least 75' !!

Agreed, it would seem a lot simpler to just run a line over to the median pole and put a PC and the ballasts there instead. I thought the distance would cause starting issues as well but it seems like with heavier gauge wiring, 75' isn't that big of a problem for 400w lamps. According to the chart I have, #14 is borderline for 75' and so #12 would probably be required to ensure reliable starting. You could only get away with mounting the ballast so far with probe start HID like MV or probe start MH. With HID lamps that required a ignitor, most "long range" ignitors max out at 30-50'. I suppose you could have a remote ballast with a local ignitor though if you use a two wire ignitor.

I think most typical clamshell installations were less than 10' away from the ballast, maybe 25-35' with a base mounted ballast.

I'm surprised that you could have the ballast up to 200' away with #10 wire. I've heard of crazy remote ballast setups where the parking lot lights were all remote ballasted with ballasts mounted in the building's electrical room. Or another case with MV lights in a stairwell remote ballasted from ballasts in a basement electrical room. I've also seen Holophane Lowbays in a library with MH with no visible ballasts so the ballasts were in a electrical room.

I'd imagine whoever had to convert this median pole to HPS had a fun time rewiring everything.

 WOW ! That's just crazy ! Still , you wonder just how many EOL lamps from the median would've still had some life if put into the ones on the side of the street .