Why are there 15- and 20-amp receptacles?

[David Nebenzahl]

Actually not a dumb question, as I trust you'll see.

Reading through the thread above about using 15-amp outlets on a 20-amp kitchen circuit, and even following the discussion to an external forum (Mike Holt's) raises this question in my mind: why do we have outlets in both ampacity ratings?

From what I gather from the Holt forum, which included much back-and-forth about UL ratings (and an actual email response from UL itself), both 15- and 20-amp outlets are capable of handling 20 amps, both through the receptacle blades and through the pass-through between the sets of screw terminals. So it's not an issue of overloading the device itself. A 15-amp outlet will work fine on a 20-amp circuit with a

20-amp load.

So I can only conclude that the real reason one would use a 15-amp instead of a 20-amp outlet on a 15-amp circuit would be prevent 20-amp loads (with their special plugs) from being plugged in, right? On a

20-amp circuit, no reason to care about this.

Is this correct? It seems to come down almost to a visual thing: if you don't see the T-slots, don't bother trying to plug in a 20-amp device.

[Of course this only applies to North America and places with similar-type electricity: dunno how this all plays out in the UK, for instance.]

[dpb]

David Nebenzahl wrote: ...

...

Should one not say "obviously"????

--

[David Nebenzahl]

On 11/11/2010 1:49 PM David Nebenzahl spake thus:

Forgot to add this:

The upshot being:

o It's OK to put 15-amp receptacles on a 20-amp circuit o It's *not* OK to put 20-amp receptacles on a 15-amp circuit

Right?

[RBM]

That is correct regarding multiple outlets on a branch circuit

[trader4]

The exception being a SINGLE 20 amp receptacle is apparently allowed on a 15 amp circuit. You can find a thread here from a few weeks back where we went back and forth on that at length. The code says for a single receptacle, the receptacle must have a rating at least equal to that of the circuit.

Does it make any sense? I'd say no and I sure would not wire a receptacle that way.

[David Nebenzahl]

On 11/11/2010 3:58 PM [e-mail] spake thus:

Riiight, the exception to the rule. Ugh.

I'm with you there.

[Larry Fishel]

Are you asking what is reasonable practice, or what would be safe?

If everything else is to code on a 15 amp circuit, it should have a 15 amp fuse/breaker and everything downstream should be able to handle at least that much. So, even if you plug a 50 amp load into the outlet, it SHOULD pop the breaker/fuse before anything really bad happens. The question is, how much faith do you have that everything other than the outlet is correct and in good shape.

Not that you can't start a fire with 15 amps...

[Tom Horne]

If you keep reading from there you will find that the maximum receptacle size on each circuit is laid out in Table 210-21(b)(3).

Table 210-21(b)(3). Receptacle Ratings for Various Size Circuits

Circuit Rating (Amperes) Receptacle Rating (Amperes)

15 Not over 15 20 15 or 20 30 30 40 40 or 50 50 50

-- Tom Horne

[bud--]

210.21-B-1 allows that. It is a general permission, not specific to a 20 on a 15.

But a 20A receptacle on a 15A circuit is a violation of 406.3-A and can be tagged. The rule for receptacles would be expected to be in the article on receptacles, which is 406.

There remains a hole - you can install a single (not a duplex) 20A non-grounding receptacle on a 15A circuit that has no ground if it is the only receptacle on the circuit. Rather limited application. And you would have to find a single 20A receptacle without a ground.

[bud--]

210.21-B-3 is "two or more receptacles".

210.21-B-1 is a *single* receptacle on the circuit, and allows a single

20A (or 40A...) receptacle on a 15A circuit. [It also disallows a single 15A receptacle on a 20A circuit.]

When I first read this a few years ago it was hard to believe. IMHO

210.21-B-1 should be rewritten. In response to a proposal to do that the code panel said that they don't have a problem with a single 20A receptacle on a 15A circuit.

But it is a violation of 406.3-A.

[Jules Richardson]

The plugs are always fused in the UK, and of course domestic AC is always

240V, so there's largely no need to mess around with different outlet styles (there are some different plug and outlet types for very high draws, but those aren't generally seen in home situations, and things like electric stoves are normally hard-wired into a wall plate). Plug fuses there are usually 13A maximum, and if I remember right the maximum current draw on the ring circuit is around 30A.

I rather miss UK electrical installs - it's nice having 240V everywhere, and the components are generally to a much higher standard than in the US. The downside is that UK plugs are pretty huge, which creates issues for storage requirements (particularly for things like chargers for portable devices)

cheers

Jules

[David Nebenzahl]

On 11/12/2010 8:10 AM Jules Richardson spake thus:

So how does that work? What form factor are the fuses--glass tubes? cartridges? Easy to see if they're blown and replace them?

[Jules Richardson]

They're ceramic bodies I think - they're not transparent, so you can't obviously see if they've blown without checking them with a meter. They're about 1" in length and 1.4" in diameter (maybe exactly, but I'm going from memory).

cheers

Jules

[David Nebenzahl]

On 11/12/2010 11:09 AM Jules Richardson spake thus:

So the plugs must be huge. Yuck. Glad I live on this side of the pond (at least regarding electrical plugs).

[Robert Green]

If you keep reading from there you will find that the maximum receptacle size on each circuit is laid out in Table 210-21(b)(3).

Table 210-21(b)(3). Receptacle Ratings for Various Size Circuits

Circuit Rating (Amperes) Receptacle Rating (Amperes)

15 Not over 15 20 15 or 20 30 30 40 40 or 50 50 50

__________________________________________________________

Thanks for clearing that up, somewhat. (-:

It makes a lot more sense to allow a 15A receptacle on a 20A circuit but not vice versa. If you plug in a 15A device into a 20A circuit, so what? Not true of the reverse and from what I can tell, the NEC frowns on connecting loads larger than the capacity of the circuit. It would also explain why

15A receptacles probably do pass-thru 20A with no problems via the screw terminals, but couldn't pass 20A through the plug connector blades, which I assume are made of thicker metal in the 20A receptacles. I actually just bought a mix of both so I'm going to go see how they are marked and report back.

Of course, all of what I just wrote isn't valid if it's true you can use a

50A receptacle on a 40A circuit. Is that line correct? Why would they allow you to use a 50A receptacle on a 40A circuit if they don't allow that combo at lower amperages?

-- Bobby G.

[Tony Miklos]

Back here in the US, I have an old Variac with a slightly large plug. It holds two 1/4" glass fuses that pop out and are easy to check and change. Pretty neat plug actually.

[The Daring Dufas]

A lot of test equipment I've worked on has a socket, switch and fuse like this:

The circuit breaker in the electrical panel protects the building wiring not the equipment. The fuse in the power connector protects the piece of equipment.

TDD

[Wayne Whitney]

Because there are no 40A receptacles.

Wayne

[Robert Green]

Did there used to be and the mention in the NEC is a grandfathering sort of thing? What's the point of mentioning in the NEC something that doesn't exist.

-- Bobby G.

[David Nebenzahl]

On 11/12/2010 3:36 PM The Daring Dufas spake thus:

Heh; don't they say that in some electronic equipment a $5 transistor is used to protect a 10-cent fuse?