A debated and often misunderstood topic is designing a layout for installing
recessed lighting or high hats. To understand how the recessed can spreads
light can help in determining a layout. Another factor is the function of the
room they are being placed in. Kitchens, bathrooms and home offices
require more lumen's per square foot than a den or playroom.
 In the diagram to the left (click image to enlarge) You can see a typical recessed can with a 40 watt R-40 flood. The beam cast is 60 deg. In a setting with an eight foot ceiling the full light cover is about a 9 foot circle. The light is most intense at the center and gradually decreases going out from the center. (See Beam Angle Table below)
What we learn from this is the beam has a variance. In the diagram about 5 feet of
the overall is the most intense light. The middle 5 foot section is a fairly even
amount of light but once outside that circle the light rapidly diminishes till it reaches
the outside edge of the 9 foot circle. Would we install the lights 9 feet apart then?
Well if the room was large and perhaps a living room, then it would be adequate
I generally recommend that recessed lighting in a normal setting like this with 8 or 9
foot ceilings, be place no more than 5-6 feet apart. By planning the layout in this
way, the complete surface area will be illuminated to it's full potential. With dimming
and switching in sections the proper light for different situations is possible. In the
diagram below we can see how this applies to a room.
The first two lights on the left are placed at a little over 5 feet apart. The next two
are placed 6 feet apart. The B section is the area that overlaps the light from both
bulbs. We can see when the lights are placed 5-6 feet apart, with the overlap, the
floor area illuminated is complete. In an area where more light for detail work is
required, we can see from this diagram that 4-5 feet would give maximum
illumination.
When planning a layout you have to account for the walls. The first light off a wall
cannot be 5 feet or you will have almost no light nearest the wall. Using the same
formula of complete coverage we want to locate the first light about half the
separating distance we choose for the recessed cans. So at 5 feet between fixtures,
that first light should be about 30" off the wall. Now let's look at this from an
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In the above diagram we have a room about 19' x 13'. By placing 8 recessed lights
using the formula we outlined, the room has good general lighting. If in the above
case more illumination is necessary for the center portion of the room, we can place
a recessed light in each of the spots marked A. By adding these 3 fixtures the room
would be illuminated optimally. So now we have the layout whats next?
In this layout we have planned there are 11 fixtures. There is a multitude of
switching combinations we can implement here. Here are 2 different switching
scenarios below. They are only limited by your imagination.
The plan on the left separates the room into sections. The plan on the right also
separates the room into sections in a different way. The right plan also gives us the
ability to add that extra light we talked about in the diagrams above. In another
possibility we could take the middle seven lights on one switch with the outside
four on another.
As you can see there are different ways to light a room. To see switching options let's
take the above diagram. The outside wall washers can all be on one switch with
another switch for the 4 middle lights. Or we can have a switch for each row giving us
four switches. Another option could be the top four with the bottom six or vice versa.
Depending on the layout of the room and the function there are any number of
possibilities.
Below we will show how to take a simple bedroom and make it so much more
functional with some switches and  lighting. Wondering
what size cans to use.

See a 15' x 30' room layout with 2 ceiling fans
As I mentioned above, it is important to understand the aspect of the beam width and
angle, to lay out a room properly. Here is a Table to help you.
 Light Bulb Beam Angle
Height
(feet)
 10
 15
 20
 25
 30
 35
 40
 45
 50
 55
 60
5
.9
1.4
1.8
2.3
2.7
3.2
3.6
4.1
4.5
5
5.4
10
1.8
2.7
3.6
4.5
5.4
6.3
7.2
8.1
9
9.9
10.8
15
2.7
4
5.4
6.8
8.1
9.5
10.8
12.2
13.5
14.9
16.2
20
4.5
6.8
9
11.3
13.5
15.8
18
20.3
22.5
24.6
27
Here are some common bulbs and the beam angle for that bulb.
R-20FL - 45 deg.      R-20SP - 10 deg.
R-30FL - 60 deg.      R-30SP - 40 deg.
R-40FL - 60 deg.      R-40SP - 40 deg.
These may not be exact figures depending
on the type and brand of bulb used. Check
the manufacturers specifications to be sure.