Hello,

First and foremost I am sorry if this kind of topic already exists, but I have few newbish questions:

Will 240 volt mains 80 w mercury lamp run normally on 240 volt mains 70 watt sodium ballast (without ignitior)? So far I've read that operating voltage of both lamps differe in more than 80 volts. Will this kind of operation hurt the ballast or bulb?
I ask this because 80 watt ballasts are almost impossible to get these days and I have quite a stock of that kind of lamps.

Thank you for your answer!

The 80W mercury lamp, will be slightly overdriven at 70W sodium ballasts. In Israel, most of the internal glow starter sodium lamps, operates from the lanterns original 80W mercury ballasts, and are slightly underdriven. But this shouldn't cause much damage to the lamp.

It will overpower the lamp and shorten its life severely

What works for sure is :
 - 70w HPS lamp on 80w mercury ballast (the opposite of what you want)
 - 125w mercury lamp on 70w HPS ballast

It might work on 2 36w T8 chokes in parallel however - looks like it'll be overpowered a bit but idk if it is going to affect the lamp much

Will 240 volt mains 80 w mercury lamp

First the mercury lamp isn't anything like 240V or so. It is rated to be fed by 0.8A current (which is normally dictated by the ballast and not the lamp, so "80W MV ballast" would feed ~0.8A into any lamp, regardless what the lamp is rated for) and it have arc voltage about 110V (and that is dictated by the lamp, so it would stay the same 110V, whenever you feed the "80W MV" lamp by 50W or 250W).

80 w mercury lamp run normally on 240 volt mains 70 watt sodium ballast (without ignitior)?
So far I've read that operating voltage of both lamps differe in more than 80 volts.

The US market 70W HPS lamps have arc voltage of ~55V and are rated for about 1.5A
The European 70W HPS are rated at 1A with arc voltage of ~77V.

The ballast output current does not depend as much on the load voltage, so when the ballast is designed to feed the 77V lamp by 1A, it would feed almost the same 1A even into the 110V MV lamp.
And that mean running the lamp at about 100W, what is quite a lot for the 80W rated lamp, mainly because the lamps are designed quite on the limits, what the materials could endure, in order to boost the efficacy as much as possible. So such overpower would send the design over the limits...

So you have two options:
Either use two "36W" fluorescent ballasts in parallel (those would feed about 0.85A, so about 85W, what is acceptable), but their losses would be about 20W
Or use 125W MV lamp, this is rated for 1.15A current and have about 120V across the arc, so it would operate at about 105W. It would be a bit underpowered, but not as much to really hurt the lamp.

Thanks to all of you for your answers and explanations! It appears that I will be sticking to 125W MV lamp till i get another HPS with internal ignitor (since they arents really used around here, it takes time till they order them).

Or you can add ignitor