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I want to install an exterior receptacle outside of my walk-out basement. There is a 20A circuit which ends in a spot exactly where I would want to go through the wall to install the mounting block/exterior receptacle (i.e. I could easily tap into the circuit right there). There are 15A receptacles on this 20A circuit inside the house which I believe is fine. If I tap into this circuit outside am I ok to use a 20A weather/tamper resistant receptacle? Thanks in advance.

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    That's exactly what you should use. What's your specific concern?
    – isherwood
    Jun 15 at 13:57
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Yes you can use a 20 amp weather rated receptacle if the circuit is GFCI protected. If the circuit is not GFCI protected it will need to be a GFCI receptacle and the cover needs to be an inuse type or heavy duty type.

It is code legal to put 15 amp receptacles on a 20 amp circuit and 20 amp receptacles. See table 210.21.B.3

You can not put a 20 on a 15 amp circuit though and GFCI protection is required outside and normally a inuse or heavy duty cover if in a wet location (a covered porch may allow a standard cover if not a wet location)

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There is a ton of confusion on this, because the general rule is: Receptacle sizes must exactly match breaker size. But there are actually quite a few exceptions. The relevant exception here is probably the most common:

  • A 20A circuit may have any number of 15A and/or 20A receptacles.

The only exception is that a 20A circuit with only one receptacle must have a 20A receptacle.

This is allowed because, among other reasons, the guts of a 15A receptacle are designed to handle 20A passing through - and in fact could have 20A usage simply from the 2 receptacles in a standard duplex receptacle - e.g., a 12A heater on one receptacle and an 8A tool or appliance on the other receptacle. Really only the the prong configuration is different.

But you can't put 20A receptacles on a 15A circuit. If you did that and someone plugged in a 20A device (or 16A continuous) then you would overload the wiring just from one device functioning properly, which would be a bad thing.

15A receptacles are far more common in residential construction, simply because 15A devices are far more common in residential usage. Even the devices where that 33% increase in available power (from 1,800 W to 2,400 W short term, or 1,440 W to 1,920 W continuous usage for 120V (nominal) 15A and 20A circuits) would be really helpful, like portable heaters, are almost 100% sold as 15A devices (12A continuous) because that guarantees compatibility with older homes that have few, if any, 20A receptacles installed. I have seen some larger Uninterruptible Power Supplies (aka battery backups) that require a 20A circuit and occasionally some power tools. But standard vacuum cleaners, computers, TVs, portable heaters, kitchen appliances, hair dryers, lights, etc. - all 15A NEMA 5-15 connectors.

Using 15A receptacles "everywhere" makes things easier for electricians - just stock one item on the truck instead of two. But the 20A receptacles are readily available at both big box stores and electrical supply houses.

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    No, the receptacle in this case doesn’t have to match.
    – Ed Beal
    Jun 15 at 14:13

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