An exception for NEC Table 310.16, is it possible? Comment please

Discussion in 'Electrical Forum discussion & Blog' started by electrotuko, Dec 16, 2008.

  1. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    My load is a wall oven consuming 37A and requires using a 50A power line.

    Is it acceptable by the code to use 8 AWG THHN copper conductors which are rated 90 degrees centigrade? Keeping in mind that I am using no connectors except a circuit breaker connection and a breaker connection is rated at 75 degrees. It is a residential installation with the max ambient temperature 40 degrees centigrade (104F), dry location, wires going in the basement, through the walls only, and no attic activities.

    I did some calculations by using NEC Table 310.16. The result for the three “Current Carrying Conductors in Raceway or Cable or bundled†at the other parameters above is showing AWG 8 wires for the main conductors and AWG 10 for the GND.

    I am trying to relay on an exception that may apply if I am using no connectors except a circuit breaker connection (Circuit is NOT a Branch Circuit that supplies multiple receptacles) and my breaker connection is rated at 75 degrees.
  2. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    999
    Location:
    NY State, USA
    What is the kW rating of the range?

    Can I assume it is around 8,800 watts from your calculation? If the range is under 12kW then a 40A circuit wired with #8 is all that is required by code.
  3. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    Speedy Petey, thanks for this encouraging answer I was looking for a while!

    From the oven’s manual:

    "Electrical rating 8.9 kW, total amps 37.
    Double ovens require a separate, grounded 4-wire 240VAC, 50 amp service with its own circuit breaker."

    Does it mean that the 40A circuit (per your recommendation) will be OK?

    Also from the Table 310.16 calculation (I was using a software tool on electrician2 dot com) specifies OCPD for 50A.
    OK OK, hence I will use 50A circuit breaker. As a receptacle – 50A rated, 3-Pole, 4-Wire Grounding Surface Receptacle manufactured by “Pass and Seymour”, catalog # 3854.
    That is it, so simple!

    And I would guess that I can legally place three #8 and one #10 for ground in 1/2" FMC conduit, right?
    Last edited: Dec 16, 2008
  4. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    999
    Location:
    NY State, USA
    Well, you could have used a 40A circuit, but the instructions just took that a way from you. If the instructions say install a 50A circuit that's what you have to do.

    We do not use 310.16 when sizing kitchen cooking appliance circuits.
  5. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    OK, so, is the circuit I have described above, and was going to use, is considered as a 50A circuit?
    I really hate to ask this, what Table #### would you use when sizing kitchen cooking appliance circuits?
    Last edited: Dec 16, 2008
  6. codeone

    codeone Code Enforcement

    Messages:
    160
    Location:
    North Carolina
    My question why do you want to install in pipe?
  7. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA

    Manufacturer requires a 50A circuit then you will need to run a 50A circuit as that will trump the NEC sizing based on calculations.

    How are you going to run THHN?
    Plan on running conduit?
    Is this a home?
    Plan on buying some 6/3 copper.
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,562
    Location:
    North Carolina
    Both the breaker and receptacle is rated at 75 degrees so the conductor will be sized from the 75 degree column. If you are installing a 50 amp circuit using THHN copper conductors then #8 is good enough.
    If using Flexible Metal Conduit (FMC) it would require a ¾ inch raceway.

    If allowed in your area NM or SE-R would be a lot less expensive than FMC. If using SE-R aluminum then for a 50 amp circuit a #4 would be required but #6 copper NM cable would be required.

    If I was going to use a raceway of any kind from the overcurrent enclosure and the appliance then I would install a lock-out at the breaker and hard wire the appliance. I would not install a raceway between the overcurrent enclosure and a box for a receptacle unless it was a local or state ordinance.

    In order to be cost effective there would be no need for the expense of the receptacle, face plate, box and cord when for another foot of raceway and four more feet of wire I can save about 45 minutes of labor,
  9. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    wires

    The neutral wire only carries the current for the control panel, and maybe for the burners at the lowest setting, so it probably does not need to be #8, unless the specifications call for it.
  10. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,562
    Location:
    North Carolina
    There is no allowance to reduce the neutral of the branch circuit so it will need to be sized the same as the ungrounded conductors.
    There is an allowance to reduce the size of the neutral for feeders supplying more than one range or the service conductors supplying the range.
  11. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    Thanks so much to all of you helping to clear those things out.

    I will be using three 8 AWG THHN copper 90C rated conductors and one 10 AWG for GND for the double oven installation.

    Jwelectric, your answer was the most encouraging.
    The price is not an issue in this case. The reason I wanted to use a Flexible Metal Conduit (FMC) is that I would be able to use a separate THHN 90C rated wires and make a flexible cable. There are a few narrow areas in the path of cable ( two separated crawl spaces) where the more rigid and thick 6 AWG based, factory made, cable would be difficult to install. And it is almost impossible to find a cable (correct me if I am wrong) made with the new higher temp insulation material wires rated for 90C. If the factory made cable is rated for 60C, it would require 6 AWG conductors, hence making cable more thick and rigid.

    I was planning to use a ½†FMC. The all conductors easily fit in to it. The cable length is 40’.
    I have read that this size is OK for three 8 AWG conductors. I have one more, not current carrying GND conductor. It would not affect the heat transfer to the outside the conduit. And the “three 8 AWG†requirement was probably generated in the older days when the wires insulation material was old technologies, thicker and the more wires would not fit. The THHN has relatively thin isolation; all four wires fit very easily leaving even more room.

    Can I use ½’ FMC and secured to the joist by the metal fixture specially designed for stapling FMC conduits?
  12. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,562
    Location:
    North Carolina
    In order to be compliant the flex will need to be 3/4 inch. Yes you can secure the flex in any manner your heart desires. You can take your shoe strings and tie it if that pleases you although it wouldn't look very pretty.
  13. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA
    As stated above, you will need 3/4" FMC in order to comply.

    Since you are using THHN copper from beginning to end (I assume) you can use the 8awg for the current carrying conductors and 10awg for your EGC.

    However, please realize that you are allowed to do this because the 75 deg column rates 8awg for 50A. You are not using the 90 deg column unless all of your terminals are rated for 90 deg which I am positive they are not.

    This is just for future information so you don't use the 90deg column except when you are derating.
  14. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    Thanks a lot for your encouraging advises.
    I am finishing up my project using 1/2" FMC (do not have a luxury to use larger since it would not fit) and 8AWG and 10 AWG wires.
    I would guess that a small minor deviation from the code will not be a big deal, I am talking here about the 1/2" FMC instead of your recomended 3/4".
  15. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA
    Required, not recommended. But since you are doing your own work and will not be getting inspected you can do whatever is convenient.

    Compliance is not a luxury.
  16. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    Throw the instructions in the trash.:D

    To a certain degree it is. No one always follows the law. We all have our own things that we determine are safe, although non compliant. The laws have to be written to insure a safe margin of error and they must be written as to take away all decisions made by installers. I understand that.

    Would three 8's and a 10 in 1/2" flex pose a safety hazzard? Hmmm...I'd vote no. I wouldn't do it and I wouldn't recommend doing it but only because it's a violation.


    Speaking of non compliant installations...... don't install the flex first and the conductors later. I believe this is a violation too but I would never even consider pulling wire thru installed flex. That would just be stupid and would do way more harm to the conductors than slipping them in first.

    Of course, these are my personal opinions and in no way reflect the blah blah blah.........
    Last edited by a moderator: Jan 4, 2009
  17. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    When it comes to wiring I always follow code
    Driving in car I may not follow the speed limit
    But going 5-10mph faster usually can't cause a fire ;)
  18. electrotuko

    electrotuko New Member

    Messages:
    47
    Location:
    Chicago, IL
    Of course I have installed (pulled thru the FMC) wires first. It was very easy to do keeping FMC straight, plus I have experience from the past to do that.

    The project is finished. I have already installed this home made “partially code compliant†cable and made all connections. It runs through two crawl spaces, no attic. Cable is always opened, not covered by insulation except when it runs inside the wall, vertically for approx. 8 feet. It will have enough ventilation. Cable does not have sharp bending radiuses. I believe everything is done to have a reliable and safe installation.

    The THHN wires have relatively thin but strong insulation allowing easily fitting all wires inside ½†FMC.
    Maybe ¾’ size requirement was generated in the old days when wires insulation material was old technology thicker and wires inside the FMC would be too tight for the proper heat ventilation.

    Again, the reason for a smaller size FMC is that my electrical box does not allow attaching ¾†fittings (is not any free left, only for ½â€) and it would be difficult to make a hole bigger.

    This cable was installed together with two other ones (3/8†FMC factory made 2x12 AWG and 2x14 AWG) for the remote hood blower and for the convect microwave.
    All three were running trough the same installation path. It was not easy to comply with the code and use ¾†FMC for the oven.

    I am apologizing for make so much buzz in this professional forum.
Similar Threads: exception Table
Forum Title Date
Electrical Forum discussion & Blog hook up of table saw Jan 12, 2013
Electrical Forum discussion & Blog Help eliminating glare on worktable from overhead fixtures Dec 15, 2012
Electrical Forum discussion & Blog Acceptable way(s) to stack/bundle multiple NM cables Jul 25, 2012
Electrical Forum discussion & Blog Generac Portable Generator Nov 2, 2011
Electrical Forum discussion & Blog Is this portable generator hook-up OK? Oct 20, 2011

Share This Page