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2 hypothetical situations:

Situation 1: A 5-20R wall outlet is extended using an extension cord. The extension cord has a 5-20P plug, but only has a 5-15R outlet.

Situation 2: A 5-15R wall outlet is extended using an extension cord. The extension cord has a 5-15P plug, but only has a 5-20R outlet.

In situation 1, we know that whatever plugs into the extension cord will be safe, because it can only take less than what is available from the wall. We have extra amperage that won't be used.

In situation 2, user could plug in a 5-15P device, and that would be ok, since they would fit and are compatible. But what happens if user plugs in a 5-20P device and that device thinks it can draw 20 amps?

In the 2nd situation, would the device pulling more amps than is available damage the extension cord? Would the device itself be damaged? Or both? Or would nothing happen and the device would not work since it is not getting what it is expecting?

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    \$\begingroup\$ Something either burns out, melts, catches fires, or the voltage drops which prevents the current from getting any higher. Whichever is the weakest link. \$\endgroup\$
    – DKNguyen
    Commented Dec 17, 2021 at 3:25
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    \$\begingroup\$ I don't think situation 2 should happen. That extension cord should not exist. In practice, in the US, it is common to use a 20 A breaker for 15 A outlets. So in situation 2 it is very possible that nothing will happen. It will work fine. It is also possible that the wire will get hot, and it is also possible that a breaker will trip. This is in addition to the possibilities mentioned by DKNguyen. \$\endgroup\$
    – user57037
    Commented Dec 17, 2021 at 3:34
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    \$\begingroup\$ Also realize that this is not a matter of "amps being available". Even a 15 A socket will be able to deliver more than 100 A. The trouble is, the socket and wiring is not designed for that so it will melt. So the 15 A limit is a safety limit, to ensure all goes well, nothing melts or catches fire. \$\endgroup\$ Commented Dec 17, 2021 at 9:11
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    \$\begingroup\$ @MCG P = Plug, R = Receptacle (i.e. a socket). \$\endgroup\$
    – Graham Nye
    Commented Dec 17, 2021 at 14:55
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    \$\begingroup\$ @mkeith agreed, especially since I think very few people know what a 5-20 is and what the perpendicular neutral blade means. \$\endgroup\$
    – vir
    Commented Dec 17, 2021 at 18:04

2 Answers 2

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Any circuit with a NEMA 5-15R outlet will probably have a 15 amp breaker on it. As such, if you pull 20 amps through it, it'll work for a few moments until the breaker trips. The voltage may sag a bit.

If the circuit actually has a 20 amp breaker, and is properly wired for 20 amps (I'm pretty sure, but not certain, that this is allowed by the standards), then you'll just be pulling 20 amps from the 5-15R socket. It'll work just fine, because nothing in the socket is inherently limited to 15 amps, that's just the regulatory standards.

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When you use lower current extension cords , you are adding resistance which causes voltage to drop with more load like dimming of lights. Excess load also makes the wire/plug gets too warm in 10 minutes and may cause AC units to struggle starting up and trip the breaker. If any part of the plugs or cable gets hot, this is a hazard.

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