continuing topic on "15A outlets on 20A circuit"

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  #1  
Old 06-01-04, 02:22 PM
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Exclamation continuing topic on "15A outlets on 20A circuit"

I'm confused, because even though 15 amp outlets do not accept 20A plugs, preventing more than 15 amps to be drawn from the receptacle....what if you have two appliances on a duplex receptacle that add up to over 15 amps, but indivually are less than 15 amps each? Or a bunch of christmas lights that add up to over 15 amps? Will the outlet accept it? I mean is it worth it to have 15A outlets everywhere you have a 20A circuit knowing that you could blow those outlets when you plug two 9 amp appliances into those receptacles?

And what is this "feed-through" term that I hear encompassing GFCI's?
 
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  #2  
Old 06-01-04, 03:28 PM
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The NEC permits 15A receptacles only on 15A circuits.

On 20A circuits, you may use either 15A or 20A receptacles.

On 30A circuits, you can only use 30A receptacles. And so on.

Because '15A' receptacles are permitted on 20A circuits, they have to be tested for operation on 20A circuits.

The only thing the '15A' designation indicates is that you are prevented from plugging in loads rated for more than a certain amount. It does not indicate the overall current handling capacity of the receptacle. This (more or less, if you use 'backstab' receptacles, much less ) is pretty much set by NEC requirements.

'Feedthrough' is the use of a receptacle to carry current through the terminals on the back. If you look at a receptacle, it will generally have _two_ hot and neutral terminals. You can supply the receptacle with one hot and neutral terminal, and then use the other terminals to supply the rest of the circuit. There are cases where this is prohibited, and there are reasons not to do it, but it is possible. In particular, GFCIs have two different kinds of terminals, the 'line' terminals and the 'load' terminals, with a hot and neutral on each. You _must_ supply power to the hot and neutral 'line' terminals. The 'load' terminals are protected by the GFCI, so that you can use a single GFCI receptacle to supply multiple additional receptacles. This is called 'feedthrough'.

-Jon
 
  #3  
Old 06-01-04, 04:58 PM
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Smile Re:

Thanks, that answers a lot of questions. But, what is the current carrying capacity of 15A and 20A receptacles? In case it goes over 15A, it wont blow if it resides on a 20A circuit w/12 AWG?

Also, what is the difference between the rate of the load, and the current capacity? Because isn't plugging two 9 amp appliances into a duplex receptacle adding up to 18 amp load?
 

Last edited by electric1; 06-01-04 at 05:13 PM.
  #4  
Old 06-02-04, 07:03 AM
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I consider your confusion an important point. No matter what the NEC does there will always be a way to create an unsafe situation. Having a basic understanding of electrical loads goes a long way when it comes to keeping things safe.

The one thing that you might not have thought about is the amount of time it takes to cause a problem. It is true that you could load 18 Amps on a 15 Amp duplex receptacle but the receptacle could handle that load for a LONG time before it overheated enough to cause a problem. The NEC knows that it is unlikely that you will have two devices pulling that much current for a long enough time to cause a problem. Anything that is going to run for three hours or more should be derated anyway.

The practical reason for a 20 Amp circuit with 15 Amp outlets is to avoid nuisance tripping. Say you plug in a vacuum cleaner for 20 minutes on a circuit that already has a big home theater on it. 15 Amp circuit breaker goes pop. 20 Amp is fine and the duration of the load was so short there is no way you are going to cause a fire.
 
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Old 06-02-04, 07:24 AM
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The only difference between a 15 amp receptacle outlet and a 20 amp receptacle outlet is the connection for the neutral prong of the inserted plug. The connection for the hot and ground prongs are identical between the receptacles.

In other words, even if you pulled 20 amps through the 15 amp receptacle you would not damage the receptacle.
 
  #6  
Old 06-02-04, 01:31 PM
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Since this has strayed from the realm of 'how do I do this and what is code' to 'why does this work?', I will throw in my rambling essay

Another point to keep in mind: the current rating of a conductor is not some sort of wall. Current ratings are nominal values used to pretty much insure good safety. These current ratings include considerable safety factors, and in addition are calculated presuming worst conditions. If you go above the 'rated current' of a conductor, things will probably work just fine; similarly there are situations in which being below the 'rated current' of a conductor will still show problems. The NEC permitted current is a _legal_ cut-off intended to keep circuits on the safe side of the physics.

The thing that damages conductors is not current , but is instead overheating. When current flows through a wire, it generates heat. The thicker the wire, the lower the resistance, and the less heat generated. This has to be balanced against the heat being carried away from the wire. The better the heat is carried away from the wire, the lower the temperature for a given current flow, and the higher the current that can be safely carried. The rules for conductor ampacity in the NEC presume somewhat poor heat flow from the wire. In other words, if you run 35A through a 12ga wire, it will _probably_ work just fine. Not a good idea, and _not_ to code, but the physics says that it will probably work.

In addition, it takes time for the heat production to translate into excessive temperature rise. This means that you can run excessive current through a wire safely, if this is for short enough time. Circuit breaker design specifically includes a 'time/current trip characteristic' which permits excessive currents to flow for small amounts of time, to permit motor starting and the like. That 20A breaker will let 40A through for a substantial amount of time before tripping, and in doing so will protect the wire properly. The NEC permits conductors to be 'overloaded' for short periods of time in specific defined ways, eg. for motor starting.

Finally, the amp rating of a receptacle has much more to do with factors other than the current handling capacity of the conductors. The actual conductors in the receptacle are much thicker than the copper wire feeding them, and won't generate much heat. Far more heat will be generated in the pressure junction between the receptacle conductors and the plug blades than in the conductors themselves. The conductors are much larger than the wires connected, and have to be larger because they also perform the function of springs. As these springs get weaker over time, the pressure will be reduced and the heating at the junction between the receptacle and the plug blades will increase. I would bet that a standard 15A duplex receptacle, in good mechanical shape, with good heavy duty cord caps plugged in, could safely carry 20A per receptacle (total 40A through two paths) for an extended period. Take this same 15A duplex receptacle, stick oversized pins in it and damage the springs, and then connect a loose fitting plug, and 5A might be enough to make it overheat.

-Jon
 
  #7  
Old 06-02-04, 02:02 PM
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What if you plug in a bunch of christmas lights that add up to say 17 amps in the 15A outlet on the 20A circuit and leave them on all night, every night....the circuit breaker won't pop....will the 15A outlet overheat then?
 
  #8  
Old 06-02-04, 02:32 PM
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Yes, it is possible to defeat your electrical safety system if you try hard enough. But the fire will probably start in the Christmas light wires before it starts at the outlet.

The electrical codes also cannot prevent you from putting a 100-watt bulb in a 60-watt fixture, which is a much more dangerous thing to do and starts more fires than overheated receptacles. They also can't stop you from putting your finger in a light socket, or crossing the street without looking both ways, or many other things that will kill you.
 
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Old 06-02-04, 02:48 PM
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So, a 20A receptacle has an equal chance of overheating at the extreme conditions you mentioned as a 15A receptacle?(and i emphasize the extreme conditions). It basically just has the extra slot in it.
 
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Old 06-02-04, 02:56 PM
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I believe you are asking questions that only someone from a receptacle manufacturer, or from U.L. testing labs, could answer without pure speculation. It's an interesting theoretical discussion, but firm answers are hard to come by. Also, keep in mind that risk level is a continuous spectrum. Very small differences in the installation (such as the kind of electrical box, the insulation in the wall, the air flow in the room, the placement of furniture, etc. affect the likelihood of problems. As winnie said, the rules have a reasonable amount of margin in them that allow them to be used in a variety of installations. This is necessary so that we don't need to have U.L. come into our homes and conduct a field test every time we want to install an outlet.

Although I'm as curious as you, we may both have to settle for trusting that the NEC committee and U.L. know their job. And we have to trust ourselves to use our electrical system in a reasonable manner.
 
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Old 06-02-04, 03:41 PM
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To add to the Christmas light example, most of the newer Christmas lights have a built in fuse, to prevent you from stringing along too many sets. They also warn you about not exceeding more than just a few strings. Now of course a warning won't stop some people, but blowing the little fuses in the light strings will.
 
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Old 06-02-04, 04:21 PM
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Talking

Thanks, you guys have been a lot of help.
 
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Old 06-02-04, 04:24 PM
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Originally Posted by electric1
So, a 20A receptacle has an equal chance of overheating at the extreme conditions you mentioned as a 15A receptacle?(and i emphasize the extreme conditions). It basically just has the extra slot in it.
Actually you bring up a key point. The chances of an outlet failing under any normal condition is very slim (except for the dreaded backstab ones). What does fail and cause fires every day is overheated extension cords. The NEC has very little control over how extension cords get used.
 
  #14  
Old 06-03-04, 07:31 AM
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Listed duplex receptacles will pass twenty amperes through their terminals continuously without failure. That includes the fifteen ampere patern (NEMA 5-15R) receptacles.
--
Tom Horne
 
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