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I'm finishing a section of my basement, which currently only has one bulb for bare light. In planning the electrical arrangement of things, how do I estimate the load of light fixtures or electrical outlets so I know if I need heavier gauge wire, or additional circuits?

Yes, I could read the labels of the light fixtures and appliances I plan to install/use. But that doesn't allow for potential fixture changes or room re-utilization in the future. I'm thinking there must be a rule of thumb for "lights fixtures are this many watts, outlets are that many watts." I've just no idea how many.

EDIT - Follow up question

I'm seeing a lot of commentary which seems to imply 15 amp circuits are the norm (including other questions on diy.se). I know there are a few 15 amp circuits in my house, but most of them are 20. Why would you prefer one size circuit over the other? When would you use a 20 amp circuit?

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My curiosity is whether you count, for example, a standard "Edison" light bulb socket as using the wattage you intend to use (25 watt compact fluorescent bulb) or the maximum wattage that could be installed, i.e. 120W incandescent bulb? –  JYelton Dec 12 '11 at 20:03
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you can have up to 1440 watts per 15 amp circuit. The math: (120V * 15A) * 80% = 1440 watts. 80% is a safety margin, so the circuit is not at 100% load. It's a good idea to have lights and receptacles on separate circuits, so the lights don't dim every time you vacuum the rug. –  Tester101 Dec 12 '11 at 20:33
    
@Tester101 Yes, but how many watts do you expect a light fixture to consume? 60? 100? 120? How many watts is an outlet expected to consume? You can overload a 15 amp circuit with a single space heater, yet there are typically multiple outlets on one circuit. –  Scivitri Dec 12 '11 at 21:16
    
Receptacles are unknowns, you'll hope that any single device that can be plugged into a 15 amp circuit will consume less than 1800 watts. If somebody plugs in too much stuff, the breaker trips (hopefully). I can't see a good way to plan how many receptacles a room should have, other than following the receptacle placement suggestions from NEC. As for lights, how many do you plan on installing in the room? you can have 24 60watt, 14 100 watt, or 12 120 watt lights. It's beter to decide what you want, and then figure out how many circuits you need. –  Tester101 Dec 12 '11 at 21:39
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15A circuits are used because it's cheaper (less copper = lower cost), and the cable required is thinner and more pliable. 20A circuits are usually only used when a 15A circuit is too small, and/or code requires it. –  Tester101 Dec 13 '11 at 19:26

2 Answers 2

up vote 9 down vote accepted

Minimum receptacle spacing.

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.

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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.

Calculating loads

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.

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An important thing to remember is that the receptacle spacing requirement exists to reduce the use of extension cords which are a major fire hazard. For any given room the number of receptacles will not change the load. More receptacles mean less extension cords. I almost alway put in more than code requires. –  Craig Dec 13 '11 at 19:53
    
The lighting calculations of VA/ft² is very interesting, it could be in its own Q&A thread... –  JYelton Dec 13 '11 at 23:10
    
Okay, it was more math than I expected, but this was exactly what I needed. I like tying square footage to power for lighting; that makes changing light fixtures safe. (For the curious, that's 20 60w bulbs in a 20x20 room. Lots of light.) I'm curious where 180VA per receptacle comes from, and if it's something which may increase in the future. But it seems a good number to work with. Thanks! –  Scivitri Dec 13 '11 at 23:34
    
I found this: "The number of receptacles permitted on a branch circuit for commercial occupancies is based on 180 VA per receptacle [220-3(c)(6)]." But that is for commercial not residential. There is no maximum for load per receptacle for residential and at least one page I found said that the 3VA/sq. ft. includes the load for "general purpose" receptacles (e.g. not receptacles for a specific purpose like a washing machine or sump pump). –  Craig Dec 14 '11 at 7:48
    
@Craig You are correct. As far as I know, there is no limit on the number of receptacles on a circuit in a residential dwelling. However, common sense says the more receptacles that are on the circuit, the more likely it is the circuit will be overloaded. 10 is just a reference point, I'm not saying if you have 11 the world will end. –  Tester101 Dec 14 '11 at 12:51

The rule of thumb is, a standard 15-amp circuit should have no more than 12 things plugged into it. That's 12 light bulbs (some count fixtures) and plug outlets. This is very conservative, but ensures you won't throw a breaker in any normal circumstance.

For a more exact specification, a 15-amp circuit at the U.S.-standard 120V will provide 1800W of power (the equation is very simple; V * A = W, so 120VAC x 15A = 1800W). You can normally exceed 1800W for "transient" periods (peaks, less than 1sec above the limit), as a non-GFCI/AFCI breaker is usually "slow-trip". So, to determine your wattage requirements, simply add up the wattage draws of all the things you'll be plugging into that circuit.

You see how the "12 things" rule can be very conservative for certain types of circuits: 12 60W incandescent light bulbs is only 720W, less than half the max output. Even 100W bulbs would only total up to 1200W. And if you've been a green homeowner ans swapped out as many incandescents as possible for CFLs, those 100W bulbs are actually only drawing 26W each, for a draw of a whopping 312W.

However, a circuit almost never has just light bulbs. Consider the circuit into which you plug your home theater. The average plasma TV draws about 300W, with some 60" monsters drawing up to 600W. Then add a 300W home theater audio system, Blu-Ray player (200W), gaming consoles (150-200W each), DVR (50W), desktop computer (depends on the tower but a decent gaming rig will have at least 500W) and even a modest home theater could be drawing over half of the available power of the circuit from a single 2-plug wall outlet. THEN, add light fixtures that run on the same circuit (still perfectly legal in many situations, though the kitchen and bedrooms should have dedicated circuits for outlets and/or "built-in" appliances), and when you plug that 9-amp vacuum cleaner (1080W JUST FOR THAT) into the next outlet, turn it on and trip the breaker, you should no longer be wondering why.

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"12 things" is a nice, simple guideline. When counting wall outlet plates (which are typically 2-plug) does that count as 1 or 2? –  Scivitri Dec 13 '11 at 3:17
    
My calculations say 10 receptacles per 15A circuit, and 13 for 20A. 15A * 120V = 1800 volt-amperes, 1800 volt-amperes / 180 volt-amperes = 10. 20A * 120V = 2400VA, 2400VA / 180VA = 13.3. –  Tester101 Dec 13 '11 at 13:25
    
@Scivitri - It's less about how many outlets are available, as how many you plan to use at the same time. It's called "demand load" in code, and there are various factors based on load types (consumer electronics, major appliances, etc) that you can use to reduce the estimated capacity a branch circuit will need from the total "plugged-in" load you would need in theory if EVERYTHING were plugged in. –  KeithS Dec 16 '11 at 16:23

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