Why are the Cooper 400 watt HPS ballasts bigger than the 250 watt ones? My Multi-Tap 125 400 watt HPS ballast is the same size as my Cooper 250 watt ballasts, and both my 400 watt MH/MV ballasts. And my 250 watt MH ballast (that is 205 volts so I can't use it)

But it seems the cooper 400 watt HPS ballasts are a little bit bigger. I wonder why.

GE is totally different so they have different size ballasts.

Well it is pretty common for a 400 watt ballast to be larger than a 250 watt ballast, I don't know of a single case where a 250 watt one would be bigger, from the same manufacturer that is, the ballast on the right is a Westinghouse ballast.

There are some cases where a 250 watt Ballast is bigger than a 400. My M-400 A2 250 watt ballast is bigger than my M-400 R2 400 watt HPS ballast. Why? Because the M-400 A2 is a regulated lag while the M-400 R2 is a CWA.

Well sure different types of ballasts are going to be different sizes, I have a CWA 1kW MH ballast multi-tap (120/277/347V) and a CWI 1kW MH 600V ballast, the 600 volt ballast is about a 1/3 bigger.

What I am trying to say is.. Okay I forgot the 125 400 watt ballast is a bit THICKER, but has the same square dimensions as the 250 watt Ballasts. But is a little thicker.

But this one seems to be bigger the other way around.

Can different winding techniques could result in a different ballast size? Some companies keep their ballasts similat sizes so they can use the same outer frame and just do the windings differently.