From what I understand MV lamps can usually be be run on same wattage MH gear without issue. I am wondering about the reverse, MH lamps in MV gear. I have searched the topic and the biggest issue of concern seems to be that MH needs a higher open circuit voltage to start reliably.

My specific case involves a 1000w mercury vapor flood light.
I think the lamp current should be same.
The HID trouble shooting guide and according to it 400w and above MV and MH gear should have almost identical OCV.
Is there any other issues that I'm not aware of?

I'm sure most people here would rather see a MV lamp put back in and thats probably what will end up happening. The shorter lifespan is not a huge concern since the lights don't rack up a very many hours, but more lumens would be a plus.

Mercury lamps can be operated on CWA ballast for probe-start metal halide lamps, since only the OCV is different (Higher). But probe-start MH lamps may have ignition difficulties on MV ballasts.
Pulse-start MH lamps requires a different ballast, and requires an high voltage ignitor as well.
MV ballast OCV is 240V and probe-start MH ballast OCV is 380V.

An MH lamp firing on an MV ballast is contingent on a few situations. First, the MV ballast need be a CWA circuit, a typical HX or reactor ballast will have too low OCV. An HPF ballast with capacitor also improves starting of MH lamps. Second, the MH lamp needs to be one that has a low starting voltage. Some lamp manufacturers use a different starting gas fill that significantly reduces the voltage necessary to strike an arc, I believe argon or neon is used in these lamps. "Hard starters" or lamps that need the full OCV of an MH ballast will likely not start of CWA mercury gear, ive tried it. Sylvania's metalarc comes to mind first. Those lamps sometimes take forever to strike, even on MH gear. Ive had success getting Philips MH lamps, and many Chinese brand MH lamps (like Plusrite) to fire on mercury gear. A CWA mercury ballast will operate probe start MH lamps normally as long as the wattage matches of course.

For whatever reason, NPF HX and reactor mercury ballasts, like the kind found in cheap residential area lights will not operate an equivalent wattage MH lamp properly. Ive tried many of these ballasts (both 100w and 175w) with MH lamps (started using plasma ball), and the lamps operation is very unstable, flickery, and usually extinguishes if the lamp's position is changed during operation.

I highly doubt it is a reactor ballast but i will check that.

The gear in this situation runs 1000w MV. According to an HID troubleshooting guide the acceptable OCV for 1000w MV is 405-455, the OCV range for 1000w MH is 400-445. It does show lower wattage MH having a higher OCV then MV.

I will have to check the voltage in this fixture sometime and see what is is.

If OCV is the only concern with running a MH lamp I may have to try it out.

Thanks for the help.

If that 1Kw mercury ballast indeed kicks out 400-450v then you should be fine.

Well a bit of an update. I did re lamp about 8 out of 12 floods. The new lamps all light although some seem a bit slow to start.

I think some of the problem is the line voltage dips down to about 107-109V at the last few fixtures (120V ballasts). Over half of the voltage drop occurs on the utilities side, so I talked to the local guy about either a larger transformer or even just setting the taps on the existing transformer a bit higher. We'll see if they are willing to do anything.

New 1000w mercury lamps aren't really available so I thought I might as well try the MH lamps. I guess time will tell if they last, they are only used a few times throughout the summer.
I did have to get a new piece of glass cut for one that had been broken, hopefully they will continue to work for a long time.

PS, the Floods are some old GE's probably from the 60's or 70's. I think they were old utility owned lights that were donated when the utility switched to HPS.

Many of the lamps were GE also and with how dim they were I wouldn't be suprised if they were original. I posted a pic of one or two lamps in the gallery a while ago but I'm not sure if anyone could date it.

You can still get 1kw mercs online on websites like eLightbulbs.

I think some of the problem is the line voltage dips down to about 107-109V at the last few fixtures (120V ballasts). Over half of the voltage drop occurs on the utilities side, so I talked to the local guy about either a larger transformer or even just setting the taps on the existing transformer a bit higher

Use 120V to 12V ~150VA isolation transformer at the lantern (in the column base) to up the voltage back to 120V-ish

i tested a probe start MH 175W on my 175W MV NEMA and it worked... had a pretty cool start up.... first the start probe flickered blue about 5 times, then the two electrodes glowed orange and a blue arc flashed across about 2 times, then the start probe glowed blue and the electrodes white for a second then bam! it kicked on, pretty cool show...

The problem with MH in a MV only fixture will eventually manifest itself when the lamp wears a bit. As MH lamps age, they become harder to start and the lower OCV of the MV ballast will not be sufficient. You will then get a "short" lamp life. (I used quotation marks as the lamp will still be good, only when used with a proper MH ballast). Please note that this is for probe start only. Pulse start is a different game.

Use 120V to 12V ~150VA isolation transformer at the lantern (in the column base) to up the voltage back to 120V-ish

That would be an option, not in the pole base though since they're wood poles. Personally I think it would be better to run a bigger wire and get the utility to swap a bigger transformer to minimize the voltage drop. I figured I would try and get a bigger transformer first since they should swap it free of charge.

I second what other people have said:

The MV lamps work just fine on a MH ballast, I have a 175w CWA MH ballast that runs a standard 175w just fine.  (not sure I'd want to run it with one of those junky 125w-175w China junk lamps long term, but a "real" 175 will be fine.

I've successfully ran a MH lamp in a MV HX-ballasted area light before, it was an older GE Multi-Vapor.  It was an improvement- better CRI and much brighter than the clear 175w it replaced, but I didn't leave it long-term. I later bought a NOS `175w Metalarc thinking it would strike just fine, and be a brighter, nicer alternative, but it won't do anything on that ballast- the OCV is too low.