In . com, dean wrote:
I heard that two of the following HIDs can run off the same balast.

Anyone know which two?

Mercury vapor
Metal halide
High Pressure sodium

Assume the wattages are the same.

Mercury lamps of 175, 250, and 400 watts (H39, H37, and H33 ANSI ballast
compatibility codes respectively) can be run from same-wattage
non-pulse-start metal halide ballasts (M57, M58, and M59 ANSI ballast
compatibility codes respectively).

H36 1000 watt mercury lamps and not H34 ones can be used with M47 metal
halide ballasts.

I have heard of some sodium lamps for use on some metal halide ballasts
- use only ballasts indicated as compatible by the lamp manufacturer.

There are 150, 215, and 360 watt "mercury retrofit" sodium lamps that
work on at least some H39, H37 and H33 mercury ballasts respectively.
Read the lamp packaging - I have known these to be restricted to ballasts
of "reactor" and "lag autotransformer" types. Newer versions may be
compatible with more ballasts including M57, M58 and M59 metal halide
ballasts.
CAUTION - unless rated specifically for use with metal halide ballasts,
a "retrofit sodium" on a CWA type (usual type) metal halide ballast can be
significantly overpowered when its characteristics change due to aging.

Other combinations are known to often work but are advised against
because any or any combination of the following are possible:

1. Lamp may have trouble starting in cold
2. Lamp may have trouble starting after some aging
3. Lamp and/or ballast and/or socket may fail in an unsafe manner

- Don Klipstein )