NEC 210.21

Am I reading Article 210.21 (B) (1) properly?
If an receptacle is the only one on the circuit, the branch circuit rating
may be less than the receptacle? For example, according to this article, it would be allowable to put a NEMA 14-50R on a circuit with a 30A double pole breaker and wired with 10-3 wire? Or to place a NEMA 5-20R on a circuit with a 15A single pole breaker and wired with 14-2 wire?
I'm surprised with this - I always figured that the receptacle rating must match the circuit, with the exception of multiple 5-15R on 20A circuits and 14-50R on a 40A circuit when feeding a range.
Matthew
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The point is to protect the receptacle by insuring that it can safely carry as much current as the rest of the circuit, determined by the rating of the OCPD. If you put a 15A receptacle on a 50A circuit, it could burn up under overload conditions without ever opening the circuit. If you put a 50A receptacle on a 15A circuit and you plug in too big an appliance the OCPD opens. Of course, if you do that then you haven't sized the branch circuit for the connected load (see 210.23)
Ben Miller
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Ben Miller wrote:

But if there are 2 outlets on the 15A circuit, one of them can't be 50A (or anything but a 15A receptacle). I don't understand the logic for this section except for relatively obscure configurations like twist-lock. Replacing a 15A single outlet for an air conditioner with a 20A one (15 ckt) doesn't make sense to me.
A code proposal to require equal rating was rejected with the comment: "The recommendation would be overly restrictive in the case of some larger receptacle applications that do not directly correspond to the standard ratings of overcurrent protective devices. The panel notes that a 20-ampere receptacle installed on a 15-ampere branch cirrcuit is protected within its rating."
A duplex 20A receptacle on a 15A branch ckt is also protected within it's rating but is a code violation.
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| A duplex 20A receptacle on a 15A branch ckt is also protected within | it's rating but is a code violation.
However, the reverse (20A protection on a circuit of #12 or larger wire with 2 or more 15A configuation outlets) is OK. That's they way I plan to go in my kitchen ... mostly NEMA 5-15R on the required 20A circuit. Additionally, there will be 2 or 3 dedicated 5-20R and 6-20R on their own circuits at 20A. The idea is heavier load appliances plug into the single outlet circuits, and smaller appliances into the others. Here are some possible arrangements:
http://phil.ipal.org/usenet/aee/2006-10-28/ks-1.html http://phil.ipal.org/usenet/aee/2006-10-28/ks-2.html http://phil.ipal.org/usenet/aee/2006-10-28/ks-3.html http://phil.ipal.org/usenet/aee/2006-10-28/ks-4.html
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But to take the example of a 50A receptacle on a 15A circuit. What if there isn't a defined load? A receptacle implies a degree of portability, anything could come along. It seems like the way it's currently written, it leaves out a degree of flexibility.
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wrote:

MOST of the NEC is that way if you think about it, and it boils down to their stated purposes of establishing MINIMUM requirements for the sake of safety. There is a high probability that a job installed to code will be safe, there is no guarantee whatsoever that it will actually work!
For example, take the wire sizes referenced in 310.16. Install a 15 ampere circuit using 14 AWG, but make the circuit 1000 feet long. Will it be safe? Undoubtedly. Will it work? Probably not.
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Perhaps....
With only one receptacle on the circuit, they are somewhat considering it a dedicated circuit, not a generic 'branch' circuit. Then the OCPD rating may be limited by the specific load, and be lower than the other components in the circuit (wiring, receptacle).
Sort of a waste to use a large receptacle and wiring, then limit its use by a smaller OCPD. Unless the OCPD needs to be smaller for some reason *other* than protection of the wiring and receptacle (perhap a dedicated motor load??)
Just a 'shot in the dark'...
daestrom
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To clarify, it is also protecting the load equipment, as that is where the overcurrent condition would occur. Power cords and other circuits could create a potential fire hazard if only sized to carry 15A and a fault oxccurs that doesn't open the OCPD.
Ben Miller
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Benjamin D. Miller, PE
B. MILLER ENGINEERING
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