electricallightingnecwiring
Ok so this paint photo is roughly what I want to do to my basement. The lights are 6 inch recessed lights. The fireplace blower was on the old 15 amp circuit so I dont see a problem there. It currently has the old black wiring that is on a 15 amp breaker. I plan of replacing the wiring maybe leaving the home run. Can I place all of this on one 15 amp circuit or should I split the lights and the receptacles to two 15 amp circuits? Or Should I use one 20 amp circuit?
NEC Article 210.52(A)-(H) tells us that, the maximum distance (measured horizontally along the wall) to a receptacle should be no more than 6'. There are two important exceptions to this rule. doors, fire places, and other openings do not count as wall space. Also any wall less than 2' wide, does not count as wall space. The idea here is that if you have a lamp with a 6' cord, no matter where you put it (along the wall) you should always be able to plug it in.
Lets take this 20x20 room for example.
We'll need a minimum of 6 receptacles, in this room to meet code. You'll notice the wall with the door and closet, only has 1 receptacle. This is because the doors, and the 1' 11" wall between them do not count as wall space.
Keep in mind, however, this is a minimum code. You can always install more receptacles, if you want to.
Once you've determined where your lights and receptacles will be placed, you'll have to determine how many and what size circuits you'll need. For this, we can reference NEC Article 220.
Lights
For dwelling units, we'll use 3 Volt-Amperes/ft² to figure out how much lighting we might want. When measuring area we must measure from outside to outside, so we'll have to include the wall thickness in our calculations. So if we have a 20'x20' room, with 2x4 walls and 5/8" drywall on each side we'll get.
3 1/2" + 5/8" + 5/8" = 4 3/4"
20' + 4 3/4" = 244 3/4" = 20.4'
20.4' * 20.4' = 416.16 ft².
416.16 ft². * 3VA = 1248.48VA
We know that a 15A circuit will be 1800VA (15A * 120V = 1800VA), so we can see we'll only need one 15A circuit for lights.
Receptacles
When calculating loads for receptacles, we'll use 180VA per receptacle. Using this value, we can determine that we can have 10 receptacles on a 15A circuit.
15A * 120V = 1800VA
1800VA / 180VA = 10
For each 20A circuit, we can have 13 receptacles.
20A * 120 = 2400VA
2400VA / 180VA = 13.3333333333
It's a good idea when wiring up a new room, to keep the lights and receptacles on different circuits. This is not required, but it does make practical sense to do so if you can. As an example, lets say you have the lights and receptacles on the same circuit. Every time you vacuum the lights dim, then the breaker finally trips. Now you're left standing in a dark room, trying to get to the door without stubbing your toe. If the lights were on a separate circuit, you wouldn't stub your toe.
∴ lights + receptacles on same circuit = stubbed toes.
Now I'm not saying you should have a bunch of circuits with a single light on them, just that it's a good idea to have receptacles and lights on different circuits. You could always share a light circuit across a few rooms, to decrease the number of circuits required.
The best way to figure out how many, and what size circuits you need for a room. Is to plan out how many consumers you'll have first. Decide how many lights and receptacles you want, then determine what size/type of wiring you'll need.
As far as the NEC is concerned, there is no limit to the number of outlets (lighting outlets in this case) that are allowed on a single circuit assuming that the attached fixtures don't exceed the rated capacity of the circuit.
However, @Edwin raises exactly the correct point in the comment - what matters is what the fixtures are rated for, not what they are currently being used for. The general rule of thumb that electrical inspectors tend to use is around 8 boxes for a 15A circuit and 10-12 for a 20A circuit. Keep in mind that they are counting boxes, not what gets put into them. The idea is that you can put in a receptical rated at 5W, decide you don't like it, and throw something with a 150W load in the same receptical. The only thing that makes something a "light" in the eye of the inspector is that it is on the ceiling and has a switch running to it.
Outside of the load consideration you actually bring up a good point in the question, which is safety. While the NEC doesn't specify a minimum number of lighted branches (other than the panel lighting requirement), your local authority very well could. I know my jurisdiction only allows 2 rooms to share the same branch for lighting purposes, precisely for the reason you mention - you pop a breaker, you're in the dark.
Best Answer
I'm going to leave minimal for someone else and answer smart....
Put the lights on their own 15A breaker.
Put receptacles on a couple of 20 amp breakers.
If you MUST share (or think you must) put half the lights and some receptacles on one circuit (probably 15 amp unless you really want to run 12 gauge to lights) and half the lights and some other receptacles on another. Having lived with shared outlets and lights, I'm opposed to being left in the dark when a receptacle is overloaded. A lighting only circuit almost never trips, IME.
Being a basement I think you need everything to be GFCI protected at this point - either with GFCI breakers or with a GFCI as the "first thing in line" as it comes off the breaker.
Tester 101's answer to my question about lighting circuits did come up with a figure of 3 volt-amperes per square foot based on use as a dwelling for minimum power required. But minimums can be exceeded.