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  1. #1
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    Jul 2009
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    Default Determining if line is 15A or 20A?

    I am replacing an outdoor receptacle. I was wondering if there is a way to determine whether the line can handle 20A using a multimeter?

    Are there any other ways to determine this?

  2. #2
    Join Date
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    Default Re: Determining if line is 15A or 20A?

    Quote Originally Posted by j0hne View Post
    I am replacing an outdoor receptacle. I was wondering if there is a way to determine whether the line can handle 20A using a multimeter?

    Are there any other ways to determine this?
    No , a multimeter wouldn't provide you any information.
    In order for you to determine if the *line* can handle 15 or 20 amps is by the physical gauge of wire. The jacket covering will be marked something like 14 / NM90 or 12/ NM90 --- where 14 = 14 gauge copper which is correct for a circuit protected by a 15 amp breaker --- 12 = 12 gauge copper would be correctly protected by a 20 amp breaker.

    Exception would be 12 gauge copper wire can be correctly rated to be used as branch wiring on a circuit protected by a 15 amp breaker ---- 14 gauge copper cannot be the branch wiring on a circuit protected by a 20 amp breaker.


    Aluminium wiring is 12 gauge for 15 amp and 10 gauge for 20 amp.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
    Join Date
    Jul 2009
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    Default Re: Determining if line is 15A or 20A?

    Thanks for the quick reply.

    Is there something I can look for on the breaker itself that would give me this information?

  4. #4
    Join Date
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    Default Re: Determining if line is 15A or 20A?

    If you can't find any writing on the wires, look at the thickness of the copper wire itself. A 12 gauge wire is beefier than a 14 gauge wire.

    Breakers are visibly marked "15", "20" or any other size.

  5. #5
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    Default Re: Determining if line is 15A or 20A?

    Quote Originally Posted by j0hne View Post
    Thanks for the quick reply.

    Is there something I can look for on the breaker itself that would give me this information?
    The breaker should be marked 15 or 20.
    That is if the correct breaker and wire gauge are correctly matched then that should be an indicator of the ampacity of the circuit.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
    Join Date
    Jan 2008
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    2,188

    Default Re: Determining if line is 15A or 20A?

    You usually can't 'upgrade' to a higher amp breaker like this as the original wiring was sized for the breaker and circiut installed when built. However when copper was cheaper some electricians used 12ga. throughout as a way to keep the logistics and inventory easier to handle. You may or may not be so lucky and all you can do is check the wire size at the outlet and the breaker it goes to. If you aren't sure of what you're doing, get an electrician to do it so you don't learn how good you fire insurance coverage is the hard way!

  7. #7
    Join Date
    May 2008
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    1,643

    Default Re: Determining if line is 15A or 20A?

    A word of caution: if the wiring is 12 gauge copper and the circuit breaker is 15A, make sure that ALL of the wire in the circuit is 12 gauge (no 14 gauge) before replacing the breaker with a 20A.

    A breaker must be sized no larger than the ampacity of the smallest installed wire in the circuit, with consideration given to the distance the wire runs. If the endpoint of the circuit is very far away from the breaker panel, a 12 gauge wire may be derated to 15A. Drawing in excess of 15A on a very long run of 14 gauge wire can result in voltage drop which could damage tools and appliances.
    The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.

  8. #8
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
    Posts
    726

    Default Re: Determining if line is 15A or 20A?

    A quick way to tell what size wire you have is to try to strip it with a wire stripper..if it's too snug try the next larger slot..then read off the tool.

    There are power quality analyzers like the Ideal 11-165 (if memory serves me) that will among other tasks, measure the resistance of the hot, neutral and ground back to the main panel just by plugging into a receptacle.

    It will also measure voltage drop so you can tell where in a circuit a receptacle is located. The further it is from the panel the greater the drop.

    Great tool for about $300.

    Good Luck from Columbiana, Alabama
    Maurice Turgeon, http://thesemi-retiredelectrician.com

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