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I have a device (a popcorn popper) that has an L14-20P plug.
I would like to use it with my stove's 14-50R receptacle.
Since the device is rated for 20 Amp, and the receptacle for 50, this should be ok, or am I overlooking something? And if it is ok, would the following be the correct wiring:

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Your wiring does match up the ground, neutral, and phases, so it would supply power, and since it is a 50 amp circuit it has more than enough juice for a 20 amp appliance, but it isn't really safe. The appliance and its cord etc. are designed to be safe with a breaker that trips at 20 amps, not 50.

  • Ok, that makes sense. Thanks for the warning, and good thing I asked... ;) – Frescard Oct 7 '15 at 16:23
  • How is plugging a 20A appliance into a 50A socket any different from plugging a 1A radio into a 20A socket? The panel C/B or fuse is intended to protect the branch circuit, not the utilization equipment. – ThreePhaseEel Oct 7 '15 at 23:11
  • Right, if they building's electrical is up to code, the branch circuit wires are sized for 50 amps, and the receptacle is rated for 50 amps, to match the 50 amp circuit breaker. But the appliance's internals and cord are sized / designed / rated based on a 20 amp breaker. So if there's a fault in the device and it's drawing more current than it should, the fire will start in the cord or appliance rather than in the receptacle or the wall. Appliances / devices made for 15A circuits have certain minimum size cords etc., even though their draw normally may be far less than 15 amps. – batsplatsterson Oct 8 '15 at 21:02
  • Consider this -- an appliance that uses tinsel cord (say a clock), which is rated for 0.5A as per Table 400.5(A)(1) and footnote c to that table, still plugs into a 15A outlet. Even my laptop's AC adapter (90W) uses an 18AWG SPT-2 cord that can handle at most 10A -- atop that, it has an IEC C5 connector on the other end, which is rated for 2.5A! – ThreePhaseEel Oct 9 '15 at 2:29
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Your wiring is almost correct -- it transposes the two hot legs, which is more-or-less harmless, fortunately, so you don't need to throw your adapter out if you have already built it. The good news is that the other answer is pretty categorically wrong:

240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).

240.5 Protection of Flexible Cords, Flexible Cables, and Fixture Wires. Flexible cord and flexible cable, including tinsel cord and extension cords, and fixture wires shall be protected against overcurrent by either 240.5(A) or (B).

(B) Branch-Circuit Overcurrent Device. Flexible cord shall be protected, where supplied by a branch circuit, in accordance with one of the methods described in 240.5(B)(1), (B)(3), or (B)(4). Fixture wire shall be protected, where supplied by a branch circuit, in accordance with 240.5(B)(2). (1) Supply Cord of Listed Appliance or Luminaire. Where flexible cord or tinsel cord is approved for and used with a specific listed appliance or luminaire, it shall be considered to be protected when applied within the appliance or luminaire listing requirements. For the purposes of this section, a luminaire may be either portable or permanent.

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