Hey Guys! I’m back. I was curious, I need to light two fixtures with MH lamps, but I am only able to run one ballast wire in series. Does any wattage line up to run two smaller lamps in series for the correct current?

Are you looking to run 2 lamps off of one ballast? If so, I have been seeing a few dedicated 2 lamp 400W M59 probe start metal halide ballasts on eBay.

I'm was looking for some 175 of under for both fixtures. The bulb needs to fit a medium base? I guess a one lamp ballast can’t handle two smaller ones?

You mean running two 175W lamps on a single 350W (or 400W being close enough) ballast?

No, that won't work except some special case where it is explicitly rated for (where the lower power lamps are designed for exactly the same current as the higher power one, as I think some 1kW / 2kW MV/probeMH did, or two F20T12/F18T8 running on a single F40T12/F36T8 ballast choke in 230V world).
The ballast is a current source, designed to feed the lamp by a given current (it is a simplification, with certain open circuit voltage (that is important for lamp start)
But for this explanation good enough), so while most lamps are designe along very similar arc voltages (about 90..140V for most MV and probe MH) so higher wattage lamp usually need higher current feed

So connecting two 175W on a 400W ballast you first get the circuit current corresponding to the 400W (so 3.5A), assume the lamp did started.
But for the start, two lamps in series need twice the OCV and the snandard ballast won't deliver that.

Ah, ok. I wasn’t sure because sometimes I have seen people ran a HPS lamp on a MH ballast because one matches the same current and voltage. I was curious if you could for example take two 70 watt lamps and run them in series with a 150 watt ballast. I didn’t want to hurt the lamps.

I do know that in North America and Japan, there are some CWA and CWI ballasts that do run pairs of 400W mercury vapor lamps in series.

As far as running a pair of 70W high pressure sodium lamps in series, a pair of 70W S62 high pressure sodium lamps should be fine in series if you use a 150W M102 pulse start metal halide HX ballast.

In case you are curious, here is an example of a 2 lamp 400W H33 mercury vapor CWI ballast that runs both lamps in series:

https://ebay.io/m/3Qtcog

The main problem here is just finding two lamps that have the desired characteristics. This is hard to find, because the lowest voltage MH lamps are 85V, and 170V (85V*2) is higher than the voltage of most MH lamps. Larger wattages run at 250V or more, but those run at currents that are way too high.

The best bet here is HPS. US low voltage HPS lamps run at 55V, which is much better because 55V*2 is 110V which is a common MH arc voltage. Obviously HPS might not be very desirable for whatever your use may be but I don't know.

I would start with the following combos:
- Two 50W S68 HPS lamps in series on a 100W M90 MH ballast
- Two 70W S62 HPS lamps in series on a 150W M107 or 175W M152 MH ballast (M102 and M142 may work as well)

Other HPS wattages don't have desirable specs for doing this, two 100Ws might do on a 250W MH ballast but decently overdriven probably. 35W and 150W don't have any matches that I can see.

If you don't mind me asking, what situation are you in where one ballast must power two fixtures?

Basically, my dad has a little grilling area and the wires will have to go thru a single 1/2” conduit to get up to the two lights, which I would have to pull a second wire because the ballast was going to sit in a cabinet on the side of the house. I also only have one of each MH ballast wattage 150 watts and under, so I would have to go buy one which I was kinda hoping not too. It just might not work, I was curious if I could figure out something. Is it really bad for the lamps if the voltage is 5 volts + -?

The main problem here is just finding two lamps that have the desired characteristics. This is hard to find, because the lowest voltage MH lamps are 85V, and 170V (85V*2) is higher than the voltage of most MH lamps. Larger wattages run at 250V or more, but those run at currents that are way too high.

nothing a bit of lateral thinking cant solve


Ye Olde 2 lamp 215W F96T12/VHO ballast, loads of OCV to play with at a constant 1.5A, basically perfect for running 2 M107 150W or M57 175W Metal halide lamps in series if you must need to do that (perhaps with a small starting capacitor across the first lamp to help tickle the second lamp)

That sounds like a good idea, might be a lot easier to build a setup around that anyways.

How would I wire that up in series? With the two blues and a red?

Normally the lamp lead of the ballast will go to the center contact of the lamp, and then the screw shell would go to neutral/common. In series, it would be like this:
Lamp lead of ballast to center contact of lamp 1
Screw shell of lamp 1 to center contact of lamp 2
Screw shell of lamp 2 to neutral/common

So you're putting them in a string

How would that handle two 175 or 150 watt lamps if it only utilizes one 96 watt lamp lead? I get that is the series circuit, but wouldn’t it be really under driven?

Oh oh I see, you mean with the fluorescent ballast instead of an HID ballast my bad. Assuming you have a dual lamp ballast, you just ignore the yellow wires. Then you just pick one red wire and one blue wire (doesn't matter which one of the two wires of each color) and act like those are your lamp and common wire. So of the presumably 6 wires present, you are only using two of them. Then the same setup as I said before assuming that ballast will work for these lamps.

Ah ok. And that would be fine for two 175 or 150 watt lamps? Sorry, I want to make sure I’m not going to hurt my lamps.