Can a medium-size cobrahead take a 750 watt HPS bulb/ballast? I want to replace the ballast in one of my M400A2's, but I'm worried that it can't handle 750 watts.

GE offered the M400A2 in 750w HPS at one point, but it would get really hot (similar to a 400w HPS in a small cobra) I wouldn't advise using it indoors if you do do this. Here is a picture of one (source:galleryoflights.org)

Do you have a 750w HPS ballast?

I don't have a ballast right now, but I found one here.

Never heard of a 750W sodium lamp.

Never heard of a 750W sodium lamp.

The only applications they are really used in anymore are highmasts. It is quite rare to find them used for anything else.

Odd wattage even for there, considering that 1kW lamps are much more common and can be used instead (with fewer luminaires on the mast)

Speaking of weird wattages, what about the 310W ones? You could just use 250 or 400 watt versions instead.

I think those are for retrofitting mercury vapour fixtures without having to replace the ballast, it motivates the consumer more if they know they're getting considerable energy savings from retrofitting existing fixtures.

What are you attempting to light up, 750 watts is a lot of light.. 

Speaking of weird wattages, what about the 310W ones? You could just use 250 or 400 watt versions instead.

Not the same thing as you cant vary the number of cobraheads on a pole while you can with luminaires on a mast

@ Homebrew: Nope 310W HPS uses its own ballast. The idea behind 310W HPS is that it is a lumen match for 700W MV, which was a fairly uncommon wattage even back in the day. Most places that used 700W MV probably converted to 400W HPS. They'd still cut power consumption almost in half and get a little more light to boot.

200W HPS is actually the lumen equivalent of a 400w MV but 250W HPS won by a long shot at being far more popular. 250W HPS is brighter than 400W MV and still saves quite a bit of power.

And 50W HPS is the lumen equivalent of 100W MV but 70W HPS is far more popular to replace 100W MV except for the New England area, where there's a lot of 50W HPS street lights.

From what I've seen, HPS "lumen matches" for MV often appear dimmer than the MV equivalent they're replacing, so more often than not, the 70, 250, or 400W HPS lamp is going to appear closer to the 100, 400, or 700W MV it's replacing.