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OK, the one answer I got was way off target, and another commenter couldn't figure out what I was asking, so I'm restating the question...

Are 15A 3-prong grounded outlets actually rated to 20A, allowing them to be used on 20A circuits? And if so, why, when the outlet can never be asked to supply more than 15A by anything that would be plugged into it?

More info:

This question had a comment that 15A outlets were actually rated to 20A. However, by design, there is nothing that will plug into a 15A outlet that will draw more than 15A. An appliance that draws more than 15 amps would have a "T-blade" plug that wouldn't go into a standard 15A outlet.

In addition, outlets are always wired in parallel, meaning that a 15A outlet would never have to transmit the amperage draw of a "downstream" outlet; you never "daisy-chain" outlets by connecting hot to neutral, for a number of reasons.

So, there is AFAIK no situation in which a 15A receptacle would ever have to handle more than 15A, even if installed on a 12AWG, 20A-breaker circuit.

If all the above is true, why would a 15A receptacle even have to be rated to 20A?

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What are you trying to achieve with this question? Are you asking if it's a good idea to use 15A receptaces on a circuit with a 20A breaker? –  dbracey Mar 7 '12 at 20:16
    
That's part of it. Mostly I'm curious; if the bit about rating 15A outlets to 20A is true, it doesn't make sense, even if you DO put them on a 20A line. –  KeithS Mar 7 '12 at 20:20
    
I gave you this link on your previous question. Can you let me know what questions you still have after reading it? diy.stackexchange.com/questions/12115/… –  Kellenjb Mar 7 '12 at 20:35
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@KeithS a receptacle rated at 15A is only rated for 15A. However, there is a safety tolerance built in. It's like a weight limit on a swing, if the swing is rated at 300lbs would you expect it to break as soon as a 301lbs man sits on it? –  Tester101 Mar 7 '12 at 23:04
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"there is nothing that will plug into a 15A outlet that will draw more than 15A" Not true! Plug in a power bar, or one of those cheap plastic things that plugs into one outlet and gives three more outlets. Then plug 20A worth of kit into that. –  user12228 Mar 25 '13 at 2:17

4 Answers 4

up vote 11 down vote accepted

A 15A outlet is rated for 15A, with a 20A pass through. That is to say that the 20A circuit is in continuance throughout the circuit, however the receptacle itself (the contacts) are rated for 15A continuosly. Any single appliance with a 15A plug will not normally draw more than 80% of 15A, or 12A. The total circuit draw (multiple appliances - same circuit) can be 20A before the breaker trips.

A 20A breaker used with 12awg wire can feed multiple 15A outlets, one example is the kitchen. The reasoning is so that today's demanding appliances, which draw more current, can be used with a 20A breaker without the worry of nuisance tripping. If more than a total of 20A were to be drawn from the circuit, the breaker will trip.

It should be noted that any circuit that is intended to be 20A must use a 20A recepticle.

If the appliance were using 15A, it will be safe with the 20A breaker ( @ 80% = 16A). If it were to short, it will trip the 20A breaker just as it would a 15A breaker. A 15A receptacle can take 20A for a short time with no problem. The receptacle is overrated, otherwise it would blow up upon a short. A short circuit in actuality can be hundreds of amps in a very short duration. The breaker and receptacle are rated as "Time overcurrent" meaning it can take a lot of current for extremely short durations, and will trip on lesser currents that occur for a longer time.

An example one can relate to (refer to chart): Joe plugs in two electric heaters in his family room. Everything works fine until 20-100 seconds later the breaker trips! Joe overloaded his 20A circuit, by drawing 40A! The breaker will allow this overload for a short time. If the overload were bigger, say 60A the breaker would trip faster from 10-35 seconds. If the trip was due to a direct short, the breaker will trip Immediately. Breakers actually have a "Load characteristic curve" that you can tell when it will trip in time vs current. enter image description here

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OK. So, a follow-up is, is multiple 15A receptacles on a 20A circuit really safe? If a device on one of those 15A receptacles gets a short, the receptacle will be overloaded pretty severely before the 20A breaker trips. –  KeithS Mar 7 '12 at 20:48
    
@KeithS-please see my addition. –  SteveR Mar 7 '12 at 21:00
    
Good to know. Thanks. –  KeithS Mar 7 '12 at 21:19
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When you say "It should be noted that any circuit that is intended to be 20A must use a 20A recepticle" -- what do you mean "be 20A"? Do you mean any circuit that is intended to serve an appliance drawing 20A must use a 20A receptacle? –  Shimon Rura Mar 7 '12 at 22:39
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@Shirmon-yes, exactly. An appliance rated for 20A must have a 20A receptacle. Also any single outlet 20A circuit must be a 20A receptacle. –  SteveR Mar 7 '12 at 23:21

Most receptacles are duplex and allow you to plug two appliances in. If each appliance consumes 10 A, the receptacle and the circuit would be supplying a total of 20 A.

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The rating and actual capability of all elements in a domestic power circuit is designed and regulated so that an over-current condition is detected and acted upon by the correct element in the circuit. Put another way: exceeding the current on the circuit should only ever trip the breaker. The plug and socket are going to be capable of more, and the wiring in the wall of much more (often twice as much). But you still want the breaker to trip before melting the socket.

If the breaker is rated for 20A, then you want everything else to be capable of passing more current. That means wiring, plugs, sockets and appliance wiring. The idea of a 15A plug is that devices are not supposed to draw more than 15A when sold with a 15A plug. This is where the regulation comes in.

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Devices may not draw that much current when behaving as intended, but what about fault conditions? What assurances are there that e.g. a motorized device which is supposed to draw 12 amps continuously won't have a bearing failure that causes it to draw 20? –  supercat Jan 27 at 23:51
    
In that case, you still want everything other than the breaker to be able to pass more than 20A the breaker will trip at. –  staticsan Jan 28 at 2:59
    
From the little I've seen of industrial equipment wiring, circuit protection at a given level of current is always supposed to be upstream of anything that can't handle that much. If a faulted appliance might draw more than 15 amps, why would it be considered safe to have a socket assembly that could only be able to handle 15 protected by a 20 amp fuse? –  supercat Jan 28 at 15:57
    
You're missing the point. A device sold with a 15 Amp plug is not supposed to draw more than 15 amps. That's the rules. But the plug itself will almost certainly survive 20 amps with no damage because the rules say you put a 20 amp fuse on a circuit with 15 amp sockets. In fact, I know the cable will survive 30 amps undamaged (I've seen it). –  staticsan Jan 28 at 23:45
    
My old microwave, which was plugged into a hex tap which I believe was UL approved, failed in such fashion as to draw enough current to visibly damage to the hex tap, but did not trip the 20A breaker for the circuit. I don't know how much current it drew for how long, but the 20A breaker would not seem to have been adequate protection for the hex tap. –  supercat Jan 28 at 23:48

I think the best way for a layman to understand this common question is to look at it from the appliance perspective - that is, what's being plugged INTO the receptacle? ANY appliance that draws more than 15A but less than 20A by code MUST have a 20A rated plug on it. You can't insert at 20A plug into a 15A receptacle because a 20A plug has one sideways prong. So ANY item that you CAN plug into a 15A receptacle BY DESIGN will draw at most 15A (usually at most 80% of 15A) and is safe under all normal circumstances to plug in to the 15A receptacle. That is why 15A receptacles are allowed on 20A convenience branch circuits which are intended to have multiple receptacles on a single circuit. It should be noted that a 20A APPLIANCE circuit MUST have a 20A receptacle, because it is designed to provide the full 20A to a SINGLE appliance.

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"It should be noted that a 20A APPLIANCE circuit MUST have a 20A receptacle, because it is designed to provide the full 20A to a SINGLE appliance."....In the US at least this is NOT true. No where is it written (other than local amendments) that any receptacle on a SABC must provide a full 20A to a SINGLE appliance. Are you saying this is a local written amendment in your area? –  Speedy Petey Sep 17 at 23:13
    
A (legal) appliance that actually requires up to 20A is going to have a 20A plug on it unless it has been illegally modified, right? That plug can only physically be plugged into a 20A receptacle. Code does not permit putting a 20A receptacle on a 15A circuit. Therefore, we're kind of talking semantics here. An actual 20A appliance in fact MUST have a 20A receptacle, because you can't plug it in otherwise, and the receptacle itself IS designed to supply a full 20A to the appliance. What code says about the design of a Small Appliance Branch Circuit is sort of an extra topic, isn't it? :-) –  Craig Nov 23 at 1:52
    
If you put multiple 20A receptacles on a 20A circuit and plug in enough load to exceed 20A, then of course the breaker will trip. So if you know ahead of time that you'll have multiple 20A appliances, you probably want to run a dedicated 20A circuit to each receptacle. Or could pull #10 wire and use a 30A breaker. No individual appliance will draw more than it needs (30A breaker won't "push" more power to the appliance), and unless they are continuous loads you could put multiple 20A appliances on that 30A circuit (breakers protect the wire, not humans, to prevent fire, not electrocution). –  Craig Nov 23 at 2:00

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