How did I not start an electrical fire?
January 6, 2010 6:44 AM   Subscribe

How did I not burn down my house? I realized I had run close to 5000 watts on a 15-amp line, where 1500 watts would be considered the most to put on the line.

I have an old (1920's) house, and a couple of years ago replace the fuses with a 100-amp circuit board. The wiring is spidery (each line may services multiple rooms), and it wasn't until I started to analyze the electrical use that that I realized the amps that were going through the lines.

The 15-amp breaker on one line tripped a couple of times. In retrospect, it probably had two space heaters (1500 watts each) on it and then the coffee pot, and maybe a television... The tolerances I've read about say maximum 1800 watts for 3 hours, but obviously we went way over that.

I also remember seeing, many places, multiple strips coming out of the wall socket. Assuming most people don't pay much attention, how is it there aren't more fires? I know engineers build in safety tolerances, but there is a big difference between 1800 watts for 3 hours and 3000+ watts for 8 hours. Are there 'true' engineering tolerances known only to those not ignorant enough to push them??
posted by dragonsi55 to Home & Garden (9 answers total) 2 users marked this as a favorite
You got lucky.

Engineering tolerances are frequently very high. Safety factors of 2 to 5 beyond the nominal use case are common. The key word there is "nominal". Situations frequently go out of specification (building code): wires are bent at too sharp a radius, circuit breakers don't flip fast enough or aren't sensitive enough, insulation is stressed and brittle from age and too much overheating, and so on. This is why safety factors exist.

Your house didn't burn down because of excess capacity built into the electrical codes, probably a good initial installation job and because the insulation had not degraded enough to cause a "thermal excursion" as my firefighter friends say. Knob and tube wiring is pretty good stuff if you don't mess with it. All of these thing needed to be true: you got lucky.

You should plan to rewire that part of the as soon as you can afford to. Repeated high-draw stresses on house wiring makes fires much more likely. Try to use those circuits at well below the 15A limit until then.
posted by bonehead at 6:56 AM on January 6, 2010

What you think happened didn't happen. If the current is routed through a working 15 amp breaker, then that breaker tripped whenever the load went significantly over 15 amps (1800 watts). It's the breakers and fuses that prevent fires.

Appliances don't necessarily draw the amount of current they're rated for, and even if they do some of the time, they don't do it all of the time.

The size of the breaker is related to the gauge of the wiring it's connected to. The codes which determine which size breaker can be connected to which size wire have become more conservative over the years. Yes, most wiring can carry considerably more power than the code allows before anything bursts into flame.
posted by jon1270 at 7:02 AM on January 6, 2010 [1 favorite]

also, sometimes it takes a long time for things to develop problems. for instance, if you routinely overload a 14 ga wire (nominal max amps at appropriate temperatures is 15 amps, actual amperage for some purposes about 20 amps), it may run hot.

the problem comes that over time, you have been baking that insulation. so it flakes off and becomes brittle and then the slightest nudge can cause the insulation to fall apart in dust.

most kitchen light fixtures that take incandescent bulbs have insulation like that -- people tend to user overwattage light bulbs in them and the light is often on for very long periods of time.
posted by rmd1023 at 7:02 AM on January 6, 2010

Thanks so far. The lighting... The wiring in the attic uses knobs, but spiders out in a way that is no longer used. I'm using CFL's but trying to switch to LED's- so far the best I've found are actually stage/DJ lights, but they all need grounded outlets.
posted by dragonsi55 at 7:09 AM on January 6, 2010

Replace the knob and tube at your earliest convenience. There is no benefit to keeping it and there are many many reasons to get rid of it.
posted by electroboy at 7:44 AM on January 6, 2010

Just need to second jon1270 -- did you run the space heaters, the coffee pot, and the TV simultaneously, all at their max power draw? Probably not.

I'm having a hard time understanding your description of the breaker box setup, but the only way to pull that much current would be to take a single branch circuit and accidentally connect it to multiple fuses in parallel.
posted by range at 8:07 AM on January 6, 2010

Replace the knob and tube at your earliest convenience. There is no benefit to keeping it

The benefit is he doesn't spend thousands of dollars replacing it. If he is worried, he could replace his breakers with AFIs (arc fault interrupters). They wouldn't prevent a fire due to a wire overheating, but they should prevent cracking insulation from starting a fire.
posted by malp at 8:36 AM on January 6, 2010

Eh, AFIs aren't exactly cheap, so if you're doing any appreciable number of circuits it's going to cost you. DIYing new wiring isn't really that expensive or difficult, and old knob and tube really can be dangerous, particularly if a previous owner decided to insulate the attic and covered up the wiring. Knob and tube needs air circulation to cool it off, and putting anything over it is a fire hazard. Further, if you have knob and tube, chances are that none of your outlets/appliances are grounded, and almost certainly not properly grounded.
posted by electroboy at 9:00 AM on January 6, 2010

Most insurance companies (in Canada at least) will not insure a house that has knob and tube. If your insurance company finds out what you've got be prepared to replace the wiring or lose your home insurance.
posted by GuyZero at 10:54 AM on January 6, 2010

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