I am adding an outlet above my kitchen cabinets so my wife can plug in her christmas decorations that are up there. If I take power from an existing kitchen outlet that is 12g (20A circuit) does my section need to be 12g also if it dead ends at an outlet?
It seems like a simple “yes” answer at first, but then I realized that the extension cord we’ve been using these past years is only rated 15A, and this is essentially the same thing but inside the wall, right?
This 654-sq.-ft. ADU combines vaulted ceilings, reclaimed materials, and efficient design, offering a flexible guest suite and home office above a new garage.
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Replies
Yes. 12gauge.
Unless you want to install a 15amp breaker and 'derate' the whole circuit.
Proper circuit is designed such that breaker will protect CIRCUIT wiring.
Protecting accessory cords from meltdown is up to you.
Thanks for the response.
Simple answer is that if the circuit you are tapping off of is fused at 20A you need to use #12 cables. There are also issues about what can be run off of a kitchen circuit but these are secondary to getting the wire size correct.
Get the first point wrong and you have a real, if rather small, possibility of a fire. Get the second point wrong and you get a grumpy home inspector, because it might be a violation of the rules, when you go to sell the house. If possible keep the firemen and the home inspector happy. If you have to choose only one. Keep the fireman happy. The rules depend on what the local jurisdiction says. Some places have no real rules. Most use some form of the NEC. Some have not updated the requirements for decades. NEC just writes rules. Local jurisdictions adopt and modify them as they see fit. Check with the local building departments as to what, if any, rules apply.
Extension cords and cables run in walls. Not the same thing. Wiring, cables, run in the walls are assumed to be surrounded by insulation and is not easily observed. If there is a problem, a short for example, it is not likely to be caught before it is time to call the fire department and seek refuge in the front yard. These problems can cook for years unknown to the HOs before exibiting any detectable symptoms. Often billowing smoke and leaping flames are the first clues.
The same is not assumed to be true for extension cords. These cords are designed to be temporary wiring only and under direct observation and supervision at all times when plugged in. Of course in real life this is not exactly true. Extension cords are used for months, run under carpets by the careless and far less supervised than they should be.
Too often extension cords are used when a circuit should be run and a receptacle added. Of course people will be people. Reluctant to take the time or spend the money to do it 'right'. Every year there are house fires when people run out of luck before they get around to correcting the situation. I have seen extension cords used for decades. Sometimes you can see the copper of both conductors where the insulation has broken down with time and wear. Scary. Especially when you find it by lifting a rug.
This largely explains the difference in the code between the fuse, or breaker, used to protect the conductors and the other rules that apply so much more stringently to house wiring than to extension, or other flexible, cords.
what if you plug something heavy in there one day that needs 20 amps and then it burns the 15 amp wire because the 20 amp breaker in the panel did not pop.
The best employee you can have but you wouldn't want him as a neighbor " He the shifty type"
You wouldn't use #14 house wiring on a 20A breaker. Bad form and against the rules. Not safe either. At least not in permanently installed house wiring. Undersized wiring like #14 on a 20A breaker is not the end of the world. Won't condone the practice or install it that way but no need to run away screaming. I have worked on dozens of houses, those that I can clearly remember, that had been overfused. Of the thousands, possibly hundreds of thousands, of houses like this out there a few burn every year. A risk. An unnecessary risk. But so is driving without a seatbelt.
Extension cords are under different rules due to the temporary, unconfined and supervised nature of their use.
Of course the #14 on 15A and #12 on 20A rule incorporates a substantial safety factors. These additional factors account for the enclosed nature, buried in insulation, and provisional nature of loads, a short commonly pulls many times the current rating of the breaker before it trips. Until the breaker trips the only limit on the actual current is the impedance of the transformer and the conductors. Something close to 100A, I'm running on memory here so don't use these numbers for design, on a 20A circuit is not uncommon. Until the breaker trips the cables have to carry the current without overheating to the point of causing a fire.
I have taken part in demonstrations where we ran well over 100A through a length of #12 romex, it held well over five minutes, without it shorting out. In fact the runs, as long as we kept the conductors apart, kept going long after the insulation has burned off. Point being that breakers, or fuses, actually protect insulation not conductors. The actual conductors are quite happy glowing red hot for quite some time. You wouldn't want this sort of behavior inside a wall. At least not unless you really like to meet firemen.
My reference book shows the fusing current (where the wire will melt) as 235 amps for #12.
I have seen the trip curves for SQ D breakers. You can download them from their web site.
I don't remember the details, but most residential breakers have 2 trip mechanism.
A long term thermal. It has been a while, but I think that it some like they will handle 10% overload continously and at 30% it will take 10's of minutes. But don't hold me to exact number.
And a magnetic fast break. Those are used for short circuit protection and for something like 10 times overcurrent they trip in a fraction of a second.
Wasn't aware of a second mechanism. I know there are "high magnetic" breakers we have to use to protect some electronic equipment. Not exactly sure what they mean. Spec calls for them so we get them, at a considerable additional cost. Quite dear they are. I guess one of these days I might look into the terminology and mechanism more deeply. Put it at the end of my to-do list. You know the list. The one where the last one third never sees daylight.
From what I understand most, excluding Federal Pacific and some older Zinsco models where the wiring acts as a fuse to protect the breakers, breakers fall into the category of "inverse time". I understand the idea being that the greater the current draw of the overload the faster they trip.
Not to hijack this thread, but I had an electrian working on my house changing the fuse box to circuit breakers (upgraded the service). He belived that fuses were better. He said that breakers are mechanical devices that sometimes failed and were much slower than fuses to react to overloads. Is that true, and if so why can't you get fused electrical panels?
True and not as important as it might seem.
Fuses are still used in critical areas where you have to be dead sure the power will be disconnected quickly enough to prevent damage to electronics. They use the term "current limiting" fuse. Sound like a dead obvious distinction as all fuses and circuit breakers when they blow or trip limit current. The designation is a term of art in electrical work. It means the fuse will blow and clear in less than one and a half cycles. 1/40th of a second. Fast enough to limit the massive current rise common to a short circuit.
Fuses have an advantage in that they are electrically simple and operate through a basic principle of physics. They have no moving parts, are difficult to tamper with and are relatively immune to their surrounding environment.
Breakers are mechanical devices. Parts wear, corrode and seize up. Lubrication fails and springs loose their tension. They are also far more sensitive to their environment. Vibration and shock can trip them. High temperatures makes them more sensitive while low temperatures make them less so. Most incorporate a compensation device, something else to go wrong, to correct for this but its capabilities are limited and fallible.
All that said modern breakers are much better than in the distant past, are pretty good at doing their job and have one big advantage. When one trips you, just about anyone even without training, can reset it once the problem is corrected. You tell your kid not to plug in their hair drier on the same circuit as the space heater.
Fuses, once they function, are used up. They must be replaced. Problems being that seldom are the right fuses on hand in sufficient numbers. A fuse blows. You replace it. It blows. Now you know it wasn't a fluke. You look around. Unplug the toaster. Replace fuse. It blows. Now your down three fuses and still haven't found the problem or restored power and you have no more fuses. Twenty miles to town. You have a larger fuse that you think you can force to fit ... Too often the story ends with a building on fire, a family out in the cold or someone injured.
Modern breakers made by any of the many reputable manufacturers are very good at what they do. What slight precision and absolute reliability deficit they may have compared to breakers is more made up by reusability and their ability to be reset without exposing the HO to line voltages.
Where millions of dollars of delicate equipment must be protected it is not uncommon to see both breakers and fuses. The breakers are good at overloads while the fuses provide a quick disconnect if a short happens. Best of both worlds. A belt and suspender approach. you could do this in a home but it is overkill.
One thing you can do to increase the reliability of your breakers is to, turning the main off first, is to turn all of your breakers off and back on a single time every year. This, I have been told, redistributes the grease around the pivots and keeps them from binding. This can be done with the panel cover on and without exposing yourself to line voltage.
I usually do this as part of a standard panel check that goes a long way to detecting and avoiding upcoming problems. Most electricians will do these checks for a nominal price. Cheap insurance.
Back in the 60's when power semiconductors where just comming out I work for a while on a prototype electronic drive system for DC motors on a steel rolling mill. Those motors would do high speed reverses.
At that time the SCR's where expensive and fragile. They kept comming out with faster fuses to use with them.
But it was always a question about whether the fuse where protecting the SCR's or the SCR's where protecting the fuses.
BTW, all of this "stuff" is related. The semiconductors are rated by I**2t (I squared time time). Basically just the heat generated by time. That is the something that causes fuses to open and it is also the same thing that causes the thermo trip in breakers, the inverse curve.
Another advantage of breakers is that you can do remoted tripping, remoted reseting, and adjustable trip points. If the power company used fuses ONLY then even minor traffic accidents, squires across lines, and the like would put large areas out of power for hours rather than a limited areas that often only see a blink of the lights.
Here is an interesting fuse story, but I have heard of similar problems with CB's.
Airplanes use(d) a ground based nav system called VOR, operates in 108-118 mHz band. They used to use vacum tubed equpipment and mechanically rotated antennas and took lots of maintance. Typcially there would be a visit once a week to check on the equipment and tune the transmitter.
Once site would go out about every couple of weeks. They would get out there and find nothing wrong, but the fuse blown. Replace it, check the curents, etc, etc and leave. This went on for a several months.
Then one day they happened to arrive for a scheduled check, just minutes after the fuse had blown.
They went to replace it and it was hot. Seems that the fuse holder was defective and making bad contact and the resistance was casuing extra heating that added to the self heating of the fuse.
Goes to show what an old journeyman told me 'Most high tech problems aren't high tech. Check the basics first.' is still true.
many VFD issues, unexplained tripping or blown fuses and the vast majority of harmonic problems are rooted in simple issues. Things like loose, corroded or damaged connections.
Worked on one harmonic problem that the engineers wanted to spend $20,000 correcting. After we found a set of bad neutral connections and replaced them the problem vanished. Consulting engineers still wanted to go with the major repair, you don't suppose this had anything to do with their making extra money, until the resident plant EE spoke up and told the plant manager that it was necessary.
"He said that breakers are mechanical devices that sometimes failed and were much slower than fuses to react to overloads. Is that true, and if so why can't you get fused electrical panels?"
Once had a serious meltdown and a close call for fire due to failed breakers at my parent's house. Fellow hooked up the pool pump for the season. Motor ran ok, but there was a short. Breakers did not trip. Fortunately, the fellow went inside one last time to place a phone call before he left, and smelled the smoke (no one was home).
Many wires had been run together in a bundle along the sill, and we had a big melted romex mess. Scorched wood, melted fiberglass,... Scary stuff.
Clearly, this is a rare event, but your perspective changes when it happens this close to home.
Particularly with older wiring (like knob & tube), I take some comfort from having it run through fuses. Often, this stuff was fused on both the hot and the neutral, which is interesting...
Anyways, corrosion was clearly the problem in this case. Water had been leaking in around the service cable entrance, and dribbling down into the basement panel.
Check yours!
THANKS! Great explanations.
12g w/20A outlets it is.
One more thing to think about, depending on the box size, when you splice in the new wire, you may over-fill the box.You get out of life what you put into it......minus taxes.
Marv