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I could imagine the various receptacle shapes being a tool to protect you from making a mistake about what you plug in.

Consider a 15A breaker with 14ga wiring to NEMA 5-15R receptacles. The circuit goes to 3 different rooms. In each room I plug in a device that draws 6A, and I overload the circuit and trip the breaker. So, a 15A recep didn't protect me from anything.

If, on the other hand, each circuit had exactly one receptacle, then you could look at the plug configuration and know how much load you can put on that circuit. You might daisy chain a bunch of power strips on that recep and overload it that way (or not, if you're just charging a bunch of Kindles) but you have the information you need to decide if that's going to work for you.

I get why there are different receptacle shapes by voltage. But this is about amp rating of circuits, wiring, outlets, and devices. I'm interested in the codes that govern what's behind the walls & built in to houses, not best practices when plugging in. Assume ignorant/lazy users.

Why do receptacle shapes vary by amperage? What's the purpose?

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Regarding "a 15A recep didn't protect me from anything" -- US receptacles (with a few exceptions) don't include any sort of over-current device. So a 15A receptacle rating means that that receptacle will feed 15A of current without overheating, catching on fire, or otherwise malfunctioning. The rating means you must use a device that breaks the circuit if over 15A is drawn, because above that current the receptacle may fail. Same idea as with wiring, which must be thick enough to carry the circuit's maximum current without overheating. –  Shimon Rura Apr 25 '11 at 1:33
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I'm not really sure exactly what your question is in reference to the multiple room examples you give. Without referencing the exact NEC sections, it basically allows 15amp circuits in living spaces and bedrooms, a minimum of two 20amp circuits in kitchens with home runs (single purpose circuit) for certain Higher draw appliances such as refrigerators, dryers, water heaters, etc. It has always been working practice to split up some lighting from receptacles and also use multiple circuits to separate higher use rooms such as a living room from bedrooms. There are limits to the number of receptacles on a branch circuit, but I don't have the reference at hand.

As far as the different shapes of receptacles, 15amp, 20 amp, 30amp etc. This is quite obvious in the fact that the male plug on appliances is slaved to it's draw of amps. These different shapes assure that a machine that draws 17amps cannot be plugged into a 15amp feeder. Granted, one could put several 6 amp devises on one 15amp and overload it, but that is what the circuit breaker is for. Under normal and historical uses in residential structures, it would be rare to easily overload a circuit with typical household items, such as lamps, stereos, fans etc. if someone is going to use multiple high draw appliances, then common sense has to kick in and design the circuit plan to accommodate the anticipated load. When i design a house plan, I always use a minimum of a 20 circuit panel(most of the time a 40) and separate branch circuits to each room, basement and garage, as well as a few for lighting or outdoor usage.

I don't know if this really answers your question, but that's all I got for ya!

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To add on a bit, one of the functions of the circuit breaker is actually to protect the wire. The wire can connect to several receptacles, but regardless, a 14ga wire is still only rated to 15A, hence why the breaker is 15A (note though, for very long runs, there is a voltage drop, and the current rating is effectively lower. You usually don't run into this in a house, but it's worth mentioning). –  gregmac Apr 25 '11 at 1:14
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Also, we don't run individual circuits with individual breakers to each receptacle because of the cost: it would use a significant amount more more, plus many more breakers, and the majority of the time, it's not needed because most devices in a house don't pull that much power. The service to most houses now is 100 or 200A (200 being most common), and so even if you have 30 receptacles, that is 30*15A + 40A (dryer) + 40A (stove) = 530A.. so you can't run that much at once anyways, even if you wasted the extra money on the wire, breakers and labour. –  gregmac Apr 25 '11 at 1:18
    
You've hit it on the head--the different shapes only protect against plugging an appliance into an underpowered circuit. They don't protect in the other direction (plugging too many appliances into a circuit). –  Alex Feinman Apr 25 '11 at 15:12
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In the days before electronic circuit breakers, overloading a circuit meant a trip to the hardware store to get a replacement fuse. So, by preventing you from plugging in an appliance that could blow your fuse, having different shaped receptacles could save you some time, money and inconvenience.

Since you mention ignorant/lazy users, remember that a blown fuse was sometimes replaced with a coin. If someone did just that on a circuit rated for 15A and then plugged in a 20A load on it, you would get more heat dissipation in the wiring than you should, which could end up causing a fire.

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Getting my house rewired with circuit breakers ~8 years ago was the greatest improvement I've done, even if I hired someone else to do it. Now I just need to get around to re-wiring the kitchen and basement, so the laundry won't trip if too many lights are on upstairs, and I'd be able to use the toaster and microwave at the same time! ... I was probably blowing a fuse once a week when I first moved in. –  Joe Apr 25 '11 at 20:10
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To prevent plugging in a load to a circuit that definitively can't support that load. That's not to say that (for various reasons) the proper plugged circuit CAN support the load, but it's certain that the improper one can not.

Note: In the US at least, the 20 amp plug is seldom used, at least on the load side of things. I have an air compressor that is rated for 20 amps (and boy howdy does it need it), but the plug is the same one for 15 amp circuits.

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It is actually quite rare to find 20 amp rated outlets used anywhere, which is why 20 amp devices often come with 15 amp style cords. Also the internals of newer 15 amp outlets, are usually exactly the same as the internals of 20 amp outlets. –  Brad Gilbert Jun 25 '11 at 1:57
    
20A devices are generally rare in residental. However, a device that draws more than 15A MUST use a T-blade plug. 15A outlets are NOT rated for 20A. They are rated for 15A, and can TOLERATE 20A flowing through the terminals behind the plate, allowing for installation in a 20A circuit (in that situation it still can provide only a maximum of 15A to a device plugged into that actual receptacle; but, the entire circuit can tolerate a higher "demand load"). –  KeithS Apr 5 '12 at 19:25
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I think the rating is for the receptacle itself. The different plug shapes allow the manufacturer to make lower cost receptacles for lower amp loads.

Imagine if every receptacle was built to carry 30 amps. The receptacles would all cost more and would normally be under utilized.

If you acidentally installed a 15 amp recepticle on a 30 amp circuit, you could not overload the receptacle by pluggin in a 30 amp load. The plug would not fit.

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I think you're correct - that 15A receps might fail under a 30A load. However, 15A receps are rated at 20A, and the code explicitly allows them to be protected by a 20A breaker. You can put more receps on a 20A circuit than on a 15A circuit. It's a special case, though. –  Jay Bazuzi Jun 23 '11 at 4:53
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The breaker, and the plugs, did indeed protect you. From an electrical fire caused by overloading and overheating of one or more components of said circuit.

Electrical codes exist and are enforced primarily to prevent situations in which an electrical system can pose an imminent hazard to human life or safety, and further, most safety codes are in place for fire safety, with the next largest subset of codes being for shock safety. All other considerations, including convenience, are secondary, and usually "convenience" code requirements, like maximum spacing between wall receptacles, exist for cases where someone faced with an inconvenient electrical situation will do something inherently dangerous to solve their problem.

Case in point: for convenience, to avoid the homeowner running extension cords around the room, a room will have many outlets on one circuit, which if all used at the same time would be unsafe. This is acceptable because normally, you don't plug in something to every plug on a circuit and turn them all on (notable exceptions may include kitchen outlets and outlets powering your home theater). It's called "demand load"; different rooms in the home have different expectations for the percentage of available outlets used at one time, and that's used to determine amperage requirements of the branch circuits supplying those outlets.

Additionally, the receptacle design and the circuit breakers provide redundant levels of fire safety. You cannot buy anything off the shelf that plugs into a 15A socket and draws more than 80% of that (12A; a safe "continuous" load). If it draws more than 12A it requires a T-blade plug for a 20A outlet, and cannot draw more than 16A. The design of the plug protects the receptacle first, and the circuit second; you can't plug in any single device that would exceed the rating of the plug. The breaker then acts to protect the entire circuit; if circuit load exceeds circuit rating for any reason (short-circuit, too many devices, and/or illegal modification), the breaker trips.

You consider plugging three 6A devices into a circuit rated at 15A, and tripping the breaker, a "nuisance". What has happened is that the engineer who designed your home's electrical system didn't plan on you ever needing 18A total draw on that circuit, based on the type of space it was intended to be, and the resulting demand load calculation given the number of outlets a space that size is required to have. You have three options:

  • Find a receptacle on a different circuit that one of the devices can be plugged into, spreading the load between breakers.
  • Pull a new 15A circuit on a new breaker to serve some of the outlets currently on the overloaded circuit, again spreading the load between breakers.
  • Beef up the existing circuit to 20A by pulling a new run of 12/2 Romex in place of ALL existing 14/2 on the circuit (including between "daisy-chained" outlets), increasing the maximum load of the circuit to meet the new expected demand load.
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