March 23rd, 2009


Who knew?

Role Playing Games are good prep for being an electrician. Seriously.

Yesterday afternoon I was noodling on the details of running two 15 Amp circuits into the office -- one for the computers and peripherals, one for a window air conditioner.

The office is the farthest point in the house from the main electrical panel, and the point the the office where I want to put the outlets is the far wall of that room. So, distance comes into play. About 2/3 of the run would be through the basement, with the last 1/3 passing up an interior wall and across the ceiling of the garage before popping up into the office. The part in the basement can be wired with what we all think of as household wire -- that white, flat-ish, roughly 1/2 inch wide stuff called ROMEX (like Xerox, a trade name that got turned into a generic term). For the 1/3 of the run going through the garage I'll have to run individual wires through conduit. Enter the tables.

Having some experience with this, I started with the assumption that I would need to use 12ga wire (you would normally use 14ga wire for a short run on a 15A circuit, but this is not a short run). The first step is to find the native ampacity -- current-carrying capability -- of the wire. The stuff Home Despot sells is type THHN/THWN rated for 192°F (90C), with a rating of 40 Amps at 86°F ambient temperature.

Next comes derating. First we look on the temperature chart. Garages can get quite hot in the summer, so I went with the 105-114°F line. For the 192°F wire column that gives a derating figure of .87. I'm running two circuits, so that means four current-carrying conductors in the conduit (we can omit the ground wires in this part). Looking on the "more than three wires in a cable" table, for 4-6 conductors I have a second derating factor of .80. Figuring: 40A × .87 × .80 = 27.84A. Which means that, after taking into account high ambient temperature and the mutual heating effect of multiple wires in a conduit, I still have sufficient ampacity to carry a 15 Amp circuit. Encumberance anyone?

Now comes conduit sizing. From the sizing table, six 12ga wires (now we have to count the ground wires) requires a minimum 1/2" diameter conduit. Which is good, as 1/2 inch is lighter and less expensive than 3/4", the next size up.

Last is calculating voltage drop. Because of the long run (I figure ~140 feet), I need to be sure that the wire is large enough that the voltage drop isn't too much. Yet another table gives us the cross-sectional area of a 12ga wire in "circular mills" (I don't even know what this unit is, but it must be very small, as the value for 12ga wire is 6530). This goes into a formula: ΔV = (K × 2 × cable length × current)/6530. The variable "K" is a value for the resistance of the wire; 12 for copper at 50% load and up, 11 for less than 50% load. My anticipated loads for both circuits are 5-6 Amps. So, my formula looked like: (11 × 2 × 140 × 6)/6530 = 2.8 Volts. Given that power at the main panel meters at 125 V, dropping ~3 V at the office still leaves me with 122 V -- more than adequate for the equipment we'll be powering.

After all of the hours I've spent playing Call of Cthulhu, Vampire, Feng Shui, D&D and the Star Trek RPG (and others), this felt like a perfectly natural exercise in working through attributes and modifiers. Circuit creation as character creation. Who knew?

I'll probably tackle this project in a month or two.