MH lights - bulbs in parallel?

[BFBOB]

I'm a fan of metal halide lights, and am in the process of adapting some old-timey green enameled shades to use them. I'm using Medium Base porcelain sockets which limits my choices in lamps, but that's fine. For my application 100W is about the maximum I'd want anyway.

I know that you're supposed to match the lamp to the ballast size (wattage) and start type. Some have reported using lamps rated less power than the ballast with no ill effects, but the consensus seems to be that at best it cuts the efficiency. How about putting two (or more) lamps in parallel on one ballast? Say, two 100W bulbs (identical bulbs, same manufacturer, correct start type for the ballast) on one ballast rated 200W?

The reason I ask this is that what I'm doing requires the ballasts to be remote from the light fixtures already. I'm going to have to fab up a box to hold the ballasts and run wires to power and the fixtures (luminaires to you purists), so it would cut the work by 1/3 to run two lamps off one ballast. It would also save some $ since one big ballast costs less than two little ones.

The ballast will not be very far from the lights - ten feet max.

Anyone tried it? Know for sure? Have a plausible theory?

 

[Norcal]

Not going to work & the fixture needs to be enclosed w/ 4KV rated lampholders, under 175 watts MH lamps are pulse start which means a high voltage pulse is needed to start the lamps, the reason they need to be enclosed is the arc tube operates at around 1800 degrees F, & a broken hot lamp can cause a fire, the best advice I can offer is do it right.....

Note: there is a limit on how far a remote ballast can be from the lamp, check & follow manufacturers instructions.

 

[BFBOB]

Good advice. My first attempt was 70W MH's in ordinary cheapie clip lights plugged into remote ballasts with normal outlets and the plugs that came on the clip lights. They work fine with 15' of lamp cord, though I fully expect the insulation of the plastic sockets to break down at some point. So far, so good, but these lights are only used for an hour or less at a time. The new ones will have to run 10 hours or so. That probably qualifies as "continuous duty" I'll change to porcelain sockets as soon as I find some that fit, and I am using them on the new lights.
The porcelain sockets I'm using are rated 250V. That strikes me as odd since the wire that feeds them is rated 600V! But, if a 4kV socket is necessary, wouldn't the wire need to be 4kV also, right back to the ballast? The internal ballast wires aren't. They are 600V.
Your comment on the high voltage pulse start got me thinking - for arc initiation (plasma, ionization,) it's the volts that count, not current. That means a high impedance drive circuit. Probably two bulbs would load it down to the point it wouldn't produce the voltage necessary and neither bulb would start.
I hadn't thought to check MFR spec for distance to remote ballast. I'll do that.
Finally, I am using bulbs rated for open fixtures. I'll just have to be careful not to break one. Shouldn't be too tough; they are made of MUCH thicker glass than normal bulbs or CFL's

 

[nehog]

As mentioned the voltages are far above what you can reasonably 'remote' without using so very expensive and difficult to find wire.

Second, you cannot parallel bulbs, as MH bulbs are very unique (unlike other lamps such as halogen and incandescent) and won't equally share the ballast.

The mismatch between the socket maximum voltage and the wire maximum voltage is to be expected. Most wire is rated at 600 volts insulation. The main issue on the sockets is heat--porcelain is necessary. I'm not sure the voltage rating required for pulse start MH bulbs however, though probe start ones are around 400 volts, and pulse is higher in the start-up phase.

As you mention, there are bulbs rated for open fixtures (I use these, as my fixtures are open on the bottom) and that is safe. However you need to understand the issues of a broken outer glass envelope when using them in open fixtures. If the outer glass envelope breaks you must immediately shut off the lamp or evacuate the area due to incredibly high UV light output of the inner lamp. We're talking sun burn and other risks in a few minutes in some cases.