20 Amp outlets vs 15 Amp outlets on 20A circuit

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DIYorBust

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There are many reasons for the sizing of components in a circuit. Through lots of engineering, testing, accidents and litigation standards have been established.

Most 15a outlets/receptacles are run for 30 days at 20 amps, along with many other tests before they get a ul listing. I also believe the nec permits 15a outlets to be installed on 20a circuits.
The code definitely permits 15A outlets on a 20A breaker. The question is, why is this okay? I think the anwer is that the 15A receptacles are fully designed for 20A, but exist only to reject a device with a nema 5-20p plug or similar from being plugged into a 15A circuit. Probably a review of UL specifications would confirm this.
 

Jadnashua

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Circuit breakers are designed to protect the wiring, not devices plugged into it. With a 15A receptacle, by design, you can't try to plug a single device into it that would exceed 15A, so the wiring is fine with being able to easily supply more in a 20A circuit. You could also look at it like it's fine to use 12g wire on a 15A circuit even though it's capable of handling more power...it can help keep the voltage drop low on a longer run.

Now, why they allow a 50A dryer/stove plug to be used with a 40A breaker doesn't seem as reasonable, but normally, those things are dedicated circuits to a specific device and hopefully, the wiring is sized correctly for that device.
 
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The code definitely permits 15A outlets on a 20A breaker. The question is, why is this okay? I think the anwer is that the 15A receptacles are fully designed for 20A, but exist only to reject a device with a nema 5-20p plug or similar from being plugged into a 15A circuit. Probably a review of UL specifications would confirm this.

The UL codes don't go into why things are a specific way, they just detail what it needs to be and how it's to be tested. If someone is on the committee (it's a combination of UL personnel and people from industry) they would likely know the "why".

I suspect the NEC code and the UL standard was updated/created to permit this because it was done frequently in the field. One train class I took given by UL stated that on single phase AC devices that they shall operate and fail safely if line and neutral are swapped, because they are frequently swapped when someone incorrectly wires an outlet. I don't recall if this applies to all devices, the training class was on medical electrical devices.

Also most if not all standard have a safety margin incorporated. ASME boiler and pressure vessel code does. Pressure vessels are tested to higher pressures than they are rated for.
 
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