@Ash
Alright I did some more figuring.

This equation works very reliably, and I would like to use it, but I have some questions.

I was using this equation in conjunction with ohm's law to calculate desired ballast impedance when given lamp voltage, circuit voltage, and lamp current.

This consistently overshot the specified impedance necessary for the lamp based on datasheets, but very precisely. For MV, it was almost exactly 20% overshooting, for MH it was more or less 15% overshooting, and for HPS it was pretty consistently 10%. I took the averages of all of the overshots and then the inverse and got these constants to multiply with the results to get the right approximate answer:
MV:  0.831
MH:  0.873
HPS:0.913

This is okay, all I have to do is remember these numbers when calculating. But I want to know where they came from. Is this the power factor of the discharge itself? That is what I am thinking, but I don't know for sure. I can't think of any other reason it would be so consistent across different wattages but so different between different HID technologies. I have yet to try this for fluorescent and LPS lamps but I might do that later.

Thanks again.

@Ash
Thanks so much for that equation! I thought that would be based on some super complicated concept that I would never understand, but I suppose not. I will play with this a little more later today and see what I can learn from it.

The ballast impedance is mostly inductive. So the approximate calculation would be :

Vballast = sqrt( Voc^2 - Varc^2 )
I = Vballast / Xballast

Not Vballast = Voc - Varc

Qualitatively the meaning is that the ballast current is a bit less sensitive to the lamp arc voltage than initially expected

In addition there are the ballast's resistive losses (mostly the winding resistance) and the lamp power factor (the lamp itself is a little lagging), but that does not affect the result much

The other important part is, that a Mercury lamp is not too sensitive to exact driving parameters. There are no halides to evaporate a the correct temperature etc. As long as it has enough power to evaporate the Mercury and achieve some pressure, and not enough to initiate failure mechanisms (by overheating the electrodes or the arctube wall) it'll be ok

Over here (230V, series chokes) i can confirm that 125W Mercury lamp (1.15A) works reasonably well on both 100W and 70W PulseMH/HPS ballasts (1.20 and 1.00A rated with their proper lamps respectively)

A question?, can foil seals on halide lamps oxidise with lack of use?
Digging through my halide box I found 2 Philips CDMs and an Osram power ball, all 70 watts. These lamps are practically new, the power ball has maybe 2 hours run time on it, however , looking at the pinch seals, their components are starting to go black!, on all 3 lamps! Not tried the Philips yet, but although the power ball fired up and runs perfectly, I m wondering what happened to the seals?, I’ve never seen this before on any halide lamp? See picture below…

I have a 100W MV lamp, and I have run it on a 100W MH ballast knowing that it will be slightly overdriven. I hear some people say that 100W MH ballasts are good for 100W MV lamps, and I hear other say that 70W MH ballasts are good for 100W MV lamps, but I would like a consistent answer. I did some calculations, but I know I am not seeing everything. Here we go:

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100W MH Ballast:
- 100W MV runs at .85A
- 100W MH runs at 1.1A
This means that 100W MH lamps use 29% more current than 100W MV lamps. Stay with me here.

70W MH Ballast:
- 100W MV runs at .85A
- 70W MH runs at .9A
This means that 70W MH lamps use 6% more current than 100W MV lamps. But wait.

I used to think from that quick calculation that a 70W MH ballast would be a much better fit to a 100W MV lamp. But we are not seeing the full picture. Magnetic ballast ARE NOT constant current sources, (especially not HX ballasts, which are the most common for 70/100W). Even CWA ballasts aren't truly constant current. If the arc voltage is higher (like with MV), then the current will drop slightly from where it was at with MH. Lets look at what I think is a more sensible approach to determining ballast compatibility:

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100W MH Ballast:
- 100W MV HX ballast acts like 169 ohms in series with 220V supply
- 100W MH HX ballast acts like 157 ohms in series with 220V supply
This means that 100W MH ballasts provide 8% less impedance than 100W MV ballasts.

70W MV Ballast:
- 100W MV HX ballast acts like 169 ohms in series with 220V supply
- 70W MH HX ballast acts like 188 ohms in series with 220V supply
This means that 70W MH ballasts provide 11% more impedance than 100W MV ballasts.

So now it seems like a closer match for both of them. 100W MH ballasts seem to be closer, but they will still overdrive slightly as opposed to 70W MH ballasts which will underdrive slightly. I suppose it is the user's choice as to which one works best for their situation.

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Am I seeing this right? Am I on the right track? I just need a sanity check. I am not at home right now so I can't measure the current on my MV lamp. Once I get the chance to do so I definitely will, and I will see if it matches these results.

What I have met, the interior lights just had their own switches, separate one for the front one or two and then one for the rest. So it was mostly up to the driver, how he operated these lights.
I was keeping them ON within the city area, where the sops were frequent and passengers were shuffling all the time, when there was sufficient road lights, I even left the front ones on. But when departing to the outer stretch of the lines, where there was longer drive between stops, I turned them OFF about a minute after departing the stop and ON again about a minute before arriving to the next one, to allow passengers to get seated or ready to leave. The front ones I then turned on only when at the stop and only if someone was using the front door (the busses had at least 3 or 5 doors, the front ones had quite narrow corridor around the driver cab, so people tend to not use them that much).

The glazing on insulators makes the surface smooth, so it is less tendency to hold moisture and contamination on its surface, so less risk of a flashover.
What the different colors mean, if anything at all, I have no idea.

Blackened arctube.

Sadly, I do not have access to a car, currently. So I am mainly looking for places I could buy off of that can ship it to my door.

I have a normal clear glazed (white) porcelain mogul socket, but I also have a yellow one. I have also commonly seen pink and gray ones floating around on Ebay. Why would they go through the effort of putting colored glaze on the sockets? What benefit does it serve? I thought maybe it could be an indicator of PSMH or HPS, but I see them used in situations where that doesn't make sense. Any thoughts?

Edit: Now that I think about it why glaze them at all? It it even necessary?