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Thread: Convenience Receptacle in a tunnel.

  1. #1
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    Convenience Receptacle in a tunnel.

    I have a few question about a 120 volt 20 amp duplex receptacle circuit for a 620' tunnel.The customer is wanting one every 40'.
    Question1. When calculating the voltage drop for wire size, can I calculate at the 80% of the 20 amp breaker?

    Question2. Do I need to calculate to the last device? Or calculate the voltage drop for the first 300' and then the remainder 300'?

    Question3. Do I calculate at 3% or 5%?

    I was thinking the 3%, 20 amp at 620'. Given variables requires #1 wire.
    Question4. Is this correct?

    Given the result, we planned on #1 wire, set a jbox every 40', make a pigtail joint to #10, and whip into a single gang box for the receptacle.

    Question5. Is this over kill?

  2. #2
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    better plan on a point load of 20 A at the last recept imo 3%'drop

    0.03 x 120 = 2 x 620/1000 x 20 x R
    R = 0.145 Ohm, you need R<0.145 from NEC table, steel cond, Cu
    from table 1/0 = 0.13, #1 = 0.16
    imo you could get by with the #1 at ~ 3.3% drop

    imo pig tail can be #12
    The difference between genius and stupidity is that genius has its limits.

  3. #3
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    Quote Originally Posted by DSCOTTD View Post
    Question5. Is this over kill?
    The 3 and 5% stuff in the NEC is nothing more than a suggestion. The fact of the matter is voltage drop is load dependent and anything that works on 120V is made to tolerate voltage as low as 90V.

    What's being powered up at these recepts every 40'? Hammer drills? Cell phone chargers? Sump pumps?
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

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    Quote Originally Posted by ActionDave View Post
    The 3 and 5% stuff in the NEC is nothing more than a suggestion. The fact of the matter is voltage drop is load dependent and anything that works on 120V is made to tolerate voltage as low as 90V.

    What's being powered up at these recepts every 40'? Hammer drills? Cell phone chargers? Sump pumps?
    So basically you are saying if the receptacle may never get used, don't worry about it and pull #12.

  5. #5
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    Quote Originally Posted by Ingenieur View Post
    better plan on a point load of 20 A at the last recept imo 3%'drop

    0.03 x 120 = 2 x 620/1000 x 20 x R
    R = 0.145 Ohm, you need R<0.145 from NEC table, steel cond, Cu
    from table 1/0 = 0.13, #1 = 0.16
    imo you could get by with the #1 at ~ 3.3% drop

    imo pig tail can be #12
    Thank you for the reply.

  6. #6
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    I don't think that ActionDave is saying 'ignore voltage drop'. Rather he is saying that it is a design issue, not a code issue. You need your customer to define what you are using these receptacles for!

    If the only reason is to supply low power loads, then you don't need to size for even 16A.

    But say the customer expects to run some sort of largish motorized tool; even though this is nominally a 20A circuit, you may need to size it for 60-100A of starting current...but this transient requirement may be fine with a 10% drop,

    You simply can't know without talking to the customer about what they want to plug in.

    Depending on the desired loads, you can steer the customer to something that they can be happy with.

    -Jon

  7. #7
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    Quote Originally Posted by DSCOTTD View Post
    So basically you are saying if the receptacle may never get used, don't worry about it and pull #12.
    That's not what I said at all. You asked "Is this overkill?" and I said there is no way to know that unless there is some notion of what the load is going to be.

    I ran a 20A circuit about six hundred feet out to an electric gate opener. All that was being powered up was a trickle charger for a battery and a camera. I could have gotten away with 12AWG but went ahead and pulled 10AWG which was overkill for the loads being served.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

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