Will a 15 amp circuit support the lighting I plan to install?

Question

I am in the middle of a basement finishing project and have a question.

I have three circuits for my basement, 2 15 amp and 1 20 amp. I'd like to use one 15 amp for all lights, one 15 amp for eight outlets in the finished part, and one 20 amp for outlets in the shop. For the lights, I came up with the following based on research.

One 15 amp circuit can support 1800 watts, after reading a little, electricians plan for 80% load, so 1440 watts. The new energy efficient lights out there are only 14 watts so I could have up to 102 lights? Way more than I'd need down there.

Are there any issues with this plan? Any code concerns? Thanks.

Answer 1

It looks like you have spent some time and research working on this, well done. We calculate general purpose outlet for dwelling units at 180 W/VA or 1.5A per duplex receptacle. If you have an NEC handbook illustration 220.4 is a great example.

The lighting is a bit of guess work and since we already know the outlets it would be safe to use 1.5 W/VA per square foot of living area to determine the load. Understand that the NEC isn't interested in what you are installing, it's interested in what may be installed by whomever at a later date.

That being said. I don't really see a problem with your thinking, but I do have one question. Are any of these receptacles being used for a specific duty such as a refrigerator or microwave or such. If so they must be calculated at the nameplate rating rather than the general purpose 180W.

My final thought is why not just run all 20 amp circuits and that would insure you for expansion or if you missed something. We call it Value Added Resource. For minimal cost it may pay off in the end.

Answer 2

In dwellings, National Electrical Code says to use 3 volt-amperes per square foot, to calculate load requirements for general lighting and general-use receptacles. So a 15 ampere circuit can cover 600 sq.ft. of general lighting and general-use receptacles.

600 sq.ft. x 3 VA = 1800 VA
1800 VA / 120 volts = 15 amperes

Since this is a dwelling unit basement, you can use the same 3 VA per sq.ft. to calculate the requirements for the receptacles in the "shop" area. However, since you're likely to use larger loads in this area, you might want to do the calculations using 180 VA per receptacle instead. For a 20 ampere circuit, this would allow 13 receptacle outlets.

180 VA x 13 = 2340 VA
2340 VA / 120 volts = 19.5 amperes

So depending on the square footage of the basement, and the number of receptacles required in the "shop". You may be fine proceeding as planned. Obviously, if there's any "special" equipment in the shop (240V loads, large equipment, etc.), you'll have to install appropriate circuits for them.

Depending on the type of shop, and what equipment you'll be using, you might not be happy with a single 20 ampere circuit. For example, if you're going to have a wood shop with a table saw and dust collection, you're likely going to want more circuits.

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The finished area of the basement would require AFCI protection for the circuits. Depending on whether the shop is considered finished or not, those receptacles will require GFCI or possibly AFCI protection.

Answer 3

A mistake that many diy's make with electrical work is installing more circuits and capacity than is needed. So far you haven't mentioned any reason why you couldn't put the entire basement (including the shop) on 1 15 amp circuit. Don't use 12 gauge wire unless you have a good reason to do so. You'll have a much better chance of a neat and safe install with 14.

A few of the things that you should be thinking about: Are the need for GFCI's, AFCI's and TR receptacles. What kind of boxes are you going to use for the rec's and sw's? The circuit layout and not putting more than 2 NM cables per box. How are you going to run the wire on and through the wall and ceiling framing?