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Wire in Conduit on a Roof

Discussion in 'Electrical Codes' started by jar546, Nov 9, 2018.

  1. jar546

    jar546 *****istrator

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    Riddle me this:

    You have 3 WR GFCI duplex receptacles on a single 20A circuit that serves mechanical equipment on a roof.

    The conduit is on stands approximately 8" above the roofline. The area has a design temp of 91F as verified by the 2% ASHRAE design temp for June-July & August.

    What size wire would be code compliant for this installation based on NEC 310.15(B)(3)(c)? No other factors such as conduit fill is applicable for this. Enjoy!

    IMG_1320.JPG
     
  2. jar546

    jar546 *****istrator

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    And the answer is:
    No one on this forum does commercial electrical inspections.

    HOWEVER, this can easily be found on the flat part of a residential SFR with a flat roof so maybe this oughta be looked at.
     
  3. Paul Sweet

    Paul Sweet Sawhorse

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    I'll take a stab at it (I just needed a weekend day when I had a little free time)
    3 receptacles = 3 x 180 / 120 = 4.5A
    91 + 60 = 151 deg. ambient
    Try #12 - for 75 deg. ambient conductors ampacity = 25 x 0.38 = 9.5A
    #12 is adequate.
    It would be prudent to use 90 deg. conductors to allow for additional future loads.
     
  4. ICE

    ICE Sawhorse

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    Odds are great that a 90° wire has been used. 90° #12 is rated at 30 amp. 8" above a roof in a conduit has a temperature adder of 30° which results in 121°. The multiplier for 90° wire at 121° is .89. 30 x .89 = 26.7 amps

    We have a different take on the temperatures to use. Conduit exposed to direct sunlight has been determined to be 113°. That results in 143° with a conduit 8" above a roof and the multiplier is .71. 30 x .71 = 21.3 amps
     
    #4 ICE, Nov 17, 2018
    Last edited: Nov 17, 2018
  5. RJJ

    RJJ Platinum Member

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    Are the connections on the receptacles rated for 90 degree wire?
     
  6. ICE

    ICE Sawhorse

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    Not that I am aware of.
    The ampacity is limited to the 60° and 75° columns.....but if the wire is 90° then you can derate from there.
     
  7. RJJ

    RJJ Platinum Member

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    for testing most are 60 and real life 75. The device I thought would need to be listed for 90. Not sure of the derate?
     
  8. jar546

    jar546 *****istrator

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    The 90 deg column is for derating only unless you have component that are labeled 90 deg.

    Don't forget that under 100A you must use the 60 deg column, unless the terminals you are connecting to are labeled for 75.
     
  9. RJJ

    RJJ Platinum Member

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    Not sure I understand the derating. The 60 & 75 issue I know.
     
  10. ICE

    ICE Sawhorse

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    Well let me give it a shot. There are two reasons to derate the ampacity of a conductor. One is ambient temperature and the other is the proximity to other current carrying conductors. If the temperature exceeds 104° or there are more than three current carrying conductors in a raceway, the ampacity drops.

    Start with table 310.15(B)(16). Find the wire type and gauge to determine the allowed ampacity. It can be 60°,75°, 90° depending on the insulation type. For example THWN is found in the 75° column while THWN-2 is found in the 90° column.

    Next is the temperature. Table 310.15(B)(2)(b) gives the percentage of the ampacity that is permitted at a given temperature. For example, at 125° a wire found in the 90° column of table 310.15(B)(16) is allowed 84% of the ampacity that table 310.15(B)(16) identified.

    Here is an example: 12 awg THHN wire is in an attic with an ambient temperature of 113°. Using the tables: 30 amps x .95 = 28.5 amps.

    There is another consideration for raceways and cables exposed to sunlight on a roof. Table 310.15(B)(3)(c) provides a temperature adder depending on the height above the roof.

    Moving on we come to the issue of more than three current carrying conductors in a raceway. That takes you to table 310.15(B)(3)(a). Wires in proximity to expanding and collapsing magnetic fields have a current generated within them. That current is small and worthless but it will generate unwanted heat. So as a result, if there are more than three the is a percentage of the ampacity given in table 310.15(B)(16) that is allowed based on the number of conductors.

    Here is an example: There are 18 THHN #12 awg wires in a conduit. Table 310.15(B)(3)(a) allows 50% of the ampacity given at table 310.15(B)(16) 30 amp x .5 = 15 amp.
    Note that neutral conductors that carry the unbalanced current of multiwire circuits are not counted as a current carrying conductor.

    Jeff's example didn't specify a wire type so I'll use the most common 12 awg which is THWN-2. That wire is found in the 90° column and has an ampacity of 30 amps. The raceway is 8" above the roof and table 310.15(B)(3)(c) requires the addition of 30° in temperature to the ambient 91° in the example. At 121° the value found in table 310.15(B)(2)(b) is 85% . 30 amp x .85 = 25.5 amp.

    Writing this down made it seem like a lot of information but the reality is that it is easy peasy.
     
    #10 ICE, Nov 18, 2018
    Last edited: Nov 18, 2018
  11. RJJ

    RJJ Platinum Member

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    ICE: I understand your breakdown. I guess the term derate has me not fully understanding. I get all the other factors you explained and the examples. May be a senior moment! LOL! My first thought was the photo looks like Florida!? The 91 degree temp for an average day ok, but on a roof it is much hotter. I may be just over thinking the OP.
     
  12. ICE

    ICE Sawhorse

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    I did it for a few inspectors that I work with. This is the topic of an upcoming training class. Seems odd that they can stretch this into two hours. I will print this and hand it to electricians that get the correction to derate. So often they have never been given that correction and they assume it to be a conduit fill correction. So when I do a follow-up inspection they went from a 1" EMT to 1.25" EMT. Now I have a fight on my hands because they didn't understand.

    Recently I spent ten minutes in conversation trying to educate a contractor. He couldn't get past the conduit not being too full. He was loud, argumentative and interrupted me constantly. After a while he shrugged and said, "Can I use the 90° column". That's when I knew that he knew but kept resisting in a hope that I would give up or perhaps not know the code as well as I let on. I was done with him.

    This will help some but most of them don't have a code book so not much helps them.....well unless I do the math and give them all of the options......I don't like doing that because when that turns out wrong they claim that they did exactly what I told them to do......or they find out that #10 wire would have worked but they installed another raceway instead. It's just not my job man.

    We ask to see a contractors license before we issue a permit......we should ask to see their code book too. No code book = no permit. Come back when you are making a genuine effort to be an electrician.
     
    #12 ICE, Nov 18, 2018
    Last edited: Nov 18, 2018
  13. rogerpa

    rogerpa Silver Member

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    Spot on. This should apply to "builders" as well.
     
  14. RJJ

    RJJ Platinum Member

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    Well in PA you can get the license on line with a Visa card. ICE I totally understand your second or last statement. I agree they could have used a 10 wire given all the correction factors. Looking back over this the term derate I just never use or heard of.
     

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