If a fixture says it takes 75W PAR30, can I instead use a 50W PAR20? Only a few Halo trims specify both sizes. I'd assumed bulbs are non-interchangeable due to different physical configurations, but then texasredhead has mentioned there's an adjustment to lower the bulb.

Lots of missing info. If you are trying to design your own lighting in the middle of a remodel but asking these types of questions you might want to check out a book at the library. Attached is an online class. I hope the link works... There would be too much to type to explain all that you need to learn fast.

Here is a link that might be useful: More help

The size of the recessed can is determined by the light bulb(lamp)that you want to use.
As a rule the 5" is for 75par30 or less, used for general lighting.
The 6" is for 90par39 lamps that you would neec on ceilings over 15'.
The 4" is usually used for 50mr16 (or less) 12 volt lamps , used to direct light on art or task areas .
There is a inferior quality 50par20-120volt lamp that is used strictly to save money, but , because of it's relatively short life, extremely narrow beam spreads, and lower quality of light output, it costs you more in the long run.
Use 5" cans with 50 or 75par30/fl for general lighting, and 50mr16's for accent or task lighting.
It is entirely acceptable to use 2 different can sizes in the same room.

Thanks normclc  from a really vague question youÂve read my mind and responded with extremely concise answers.
The big-box stores are really pushing those "inferior quality 50par20-120volt lamp kits". A neighbor had these installed and I wasnÂt impressed. YouÂve explained to me why I wasnÂt impressed.
IÂve been studying the lamp data on the junolightinggroup.com. Taking your comments and re-looking at the tables, watt for watt, the par30 is just brighter, little wider beam spread, and has a longer rated life. So the 4" line voltage cans using par20 are a poor choice compared to the 5" cans using par30.
I only have 8Â ceilings so really big lamps (in a big 6" can) arenÂt required.
Could you explain why you specÂd the low voltage mr for task lighting?
IÂm putting recessed lights in a small 12 x 12 kitchen over the counter edge as task lighting. ThereÂs a pendant globe (100W) over the kitchen table for general lighting. The counters are 5 from the ceiling. Using the guideline that 50-100 foot candles are required for the task of food preparation. The lower bound on foot candles for kids that can see, and the upper bound for old people that canÂt see. IÂm middle aged and canÂt see well  went from "see fine" to "canÂt see squat" in low light in about 4 years.
I was considering 5" 75par30 on dimmers. Downside, I think, is heat output (will be fine in Great White North in winter, no so good in summer) and cheaper (initial costs and dimmers), versus low-voltage which is somehow better light and cheaper over a long time. Am I missing something here?
Aside, IÂve discovered that my joists are 7". (I would have thought they would be 8" trimmed down to 7 ½, but no). The 5" Halo cans IÂm considering are 7" high and would be flush with the subfloor above. Given that youÂre supposed to be ½" from joists, I assume that flush with the subfloor would be a no-no? The low-profile 5" cans donÂt come with the specular trims (which other posters think wonderful to cut glare), so maybe I get pushed to the MRs regardless.

Hi wildpack
I'll try to answer two of your questions

I specify the MR16 lamp for task or accent lighting because,
by using different beam spreads, and lower wattages, you can use 20 watt and 35 watt lamps effectively.Check out a 20 watt narrow flood or Spot for over the sink.
Secondly, by using the Halo shallow housing, you get more glare because the lamp approachs the plane of the ceiling, and causes glare.At my age, I need more light, but less glare
So perhaps, the low voltage recessed, with a clear alzak or haze trim would provide a better lighting job, using a VFL or FL lamp.
But a brand you recognize in the MR16 lamp, ie GE or Phillips, and you'll get 4000-5000 hours in life.
Do not but any "chinese" lamp.

thanks for the response.
So the housing is shallower because they've moved the bulb down. I've been surfing the net for recessed lighting info for so long that I'm missing the obvious!

Need to digest the other info. thanks.

Wildpack, a good post, as I too feel like I am missing something basic, despite reading reams on paper on the topic. I'm in a similar situation, but am getting conflicting advice on what to do. In fact, it's this competing advice that's doing me in.

I'm about to post on my dilemma ... if I get any responses, then there might be something in there to help you, too.

Wildpack, I strongly suggest that you go to a good quality lighting design store and get a consultation. I had done loads of Web and other research, and made some casual visits to a few stores, but until yesterday I had not done a "proper" consultation with a lighting designer. I am immensely glad that I did, and while I fully expect this to cost me more in the end, as I will buy from them and they're not cheap, it's worth every cent to me to have a properly and beautifully lit kitchen.

And, I am breathing so much easier now.

Let the experts help you. At a good shop, they'll be happy to do so. And, you'll get all the info you need to make your final decisions.