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Thread: Neutral Wire Size-safety

  1. #1
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    Aug 2006
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    Red face Neutral Wire Size-safety

    When running an underground service entrance wire in SCHEDULE 40 PVC my calculations show that I must use size 250 since I'm going over 200 feet. In many locations I see a much smaller wire size for the neutral wire. The NEC code requires that there be less than 3 % drop from the meter base to the power panel...What is a SAFE WIRE SIZE for the NEUTRAL give a size of 250 for both phases of the hot side for 240 Volt....The 'theory' behind the smaller wire size for the netural of course assumes that there is load on both phases and therefore the overall current in the neutral is less...BUT.....I don't feel comfortable with this assumption since the load for each phase may be quite unbalanced at times....SO...BASIC QUESTION: What is a safe size for the neutral...should I just go with 250 for both hot and neutral ?? Certainly this is safe but maybe a little over kill...maybe something in between....I couldn't find any guidance in the NEC Code so can someone give me an answer...

    thanks
    wec

  2. #2
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    Feb 2003
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    The NEC has no voltage drop requires for general service, feeder, and branch circuits, but check your local building codes...

    Article 220 provides the calculation used to determine the size and rating of the grounded conductor. Its not an estimate, guess, or rule of thumb issue. The code details this clearly in 220.61
    Bryan P. Holland, MCP

  3. #3
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    Aug 2006
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    Red face Neutral Wire Size

    Bryan:

    Thank you for your response. My issue is really with the assumptions of the NEC code. I appreciate your finding of the calculation for the neutral wire size. What I have issue with is the assumptions. I think I'm just going to go with the same size as the calculated 'hot' wire size which certainly will be conservative.

    I find that I have issues with some of the assumptions of the NEC code in general. For example: When I use 15 amp recepticles, rated for 15 amp, I use 12 gauge NM-B wire in walls and 15 amp circuit breakers in the panel. The weak link of course is the connections to the recepticle and the recepticle itself. Since people like to use 'triple sockets' and these new 'six socket' extensions the 20 Amp breaker is just not conservative enough. Each 'appliance' is of course under the 15 amp rating BUT if someone plugs in a 'wall heater or two' the circuit recepticle is over heated and then you have a potential issue. The breaker will blow at the 20 amp +/- 20% but the recepticle will over heat beforehand due to its 15 amp rating.....seems to me...

    So, I appreciate the comment about it not being a guess, to me that's not the issue, its the fact that I don't believe the assumption in the calculation itself is always valid. Certainly if there is load for each phase the overall neutral current will be reduced but I don't think its wise to rely on this...so unless you see an issue with using the same wire size for the neutral as the two 'hot phases' I think I'll just do that......

    Again, thank you for your response !
    Kindest Regards
    wec

  4. #4
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    Aug 2006
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    Exclamation Can't find 220.61 in NEC book 2002

    I am aware of section 220.22 but I can't seem to locate 220.61....do I have the wrong code book ?? or is there another section that I can review....

    Thank you again for your response...I appreciate it very much !

    I'm a very conservative person and I want to insure that my installation is safe and very conservative.

    Kindest Regards

    wec

  5. #5
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    Quote Originally Posted by wec
    The breaker will blow at the 20 amp +/- 20% but the recepticle will over heat beforehand due to its 15 amp rating.....seems to me...
    Not likely, 15 amp duplex receptacle are manufactured with the knowledge they may be protected by a 20 amp OCPD.

    Certainly if there is load for each phase the overall neutral current will be reduced but I don't think its wise to rely on this...
    It is done all the time and it is safe.

    The neutral must be sized large enough for the maximum current that can be applied to it.

    In most cases that means a one or two size reduction, in other cases it may be reduced much more if the majority of loads are line to line.

  6. #6
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    Aug 2006
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    Question Section of NEC which gives formula for calculating Neutral Wire size

    One final question:

    I was referred to section 220.64 but I have no such section in my NEC 2002 code book.....is there another section that shows the calculation for the determination of the neutral wire size for a three wire split phase system
    PH1, PH2, Neutral 120/240...common service entrance to a residence ?

    I have section 220.41 then it jumps to section (article) 225.1

    I'd like to see what the NEC says I can reduce the wire size of the neutral to over such a distance. Our local code requires that we don't allow more than 3% voltage drop under full load and adjust the wire size accordingly independent of distance to insure less than 3% drop. With my anticipated distance of 200 feet I have gone to a size 250 instead of 4/0 for 200 feet, buried 30 inches below grade.

    Thank you everyone for answering my concerns and the education !

    Kind Regards to all

    wec

  7. #7
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    In the 2002 NEC you would want to look at 220.22.

    I suspect you could apply 220.22 to determine the maximum load on the neutral than apply your voltage drop calcs to come up with a reduced but but adequate neutral.

    But there is no issue other than money if you want to run full size.

    The large 3 phase commercial services we do are typically shown with full size neutrals even though a large majority of the loads are line to line.

  8. #8
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    What kind of research scienc are you primarly envolved with??


    Charlie

  9. #9
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    Aug 2006
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    Red face research

    My background is electrical engineering and biomedical engineering.
    At the present time my research work involves solar energy, lightning safety, biomedical engineering, and power conversion.

    Due to my biomedical engineering experience, I find myself being very conservative in my work. When dealing with AC power mains I find myself questioning some of the assumptions made by the NEC...such as the 15 amp rating on AC recepticles found in homes yet acceptable to utilize a 20 amp breaker. In my own home I utilized 12 AWG NM-B and placed 15 amp circuit breakers in those areas where I used '15 amp rated' plugs...in the kitchen area or appliance area I choose to install 20 AMP rated plugs and then and only then did I place 20 Amp circuit breakers in the panel. Throughout the house there is no 14-2 NM-B although it is acceptable for a 15 amp system.
    All wire is 12-2 NM-B or larger diameter...

    In biomedical engineering we typically are extremely conservative with our choices of components...for instance a Solid Tant Capacitor that has a 35 WVDC would never be used in an application where the voltage actually exceeded 15 volts. This conservative approach is based on statistics of failures in Solid Tant capacitors as a function of applied voltage versus rated voltage....

    So, in the domain here, I am asking you folks who are more qualified than me to insure that my assumptions are correct. The line of questioning here is for some work that will be done by a certified bonded electrician....I'm asking to insure that I meet or exceed NEC requirements. My electrician is out of town this weekend and I'm trying to procure the correct size triplex wire so that I can install it in the PVC schedule 40 prior to him returning to town ...this will allow him to work on the panel, meter and leave the 'grunt work' to me !

    Kindest Regards

    wec

  10. #10
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    Jul 2006
    Location
    Ohio
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    Quote Originally Posted by wec
    In biomedical engineering we typically are extremely conservative with our choices of components...for instance a Solid Tant Capacitor that has a 35 WVDC would never be used in an application where the voltage actually exceeded 15 volts. This conservative approach is based on statistics of failures in Solid Tant capacitors as a function of applied voltage versus rated voltage....wec
    It's a sign of the times. I was taught a long time ago that capacitors must be rated 2x the peak voltage expected in the circuit. And that resistor wattages should be 1.5x the calculated worst-case circuit wattage, with one component failure. I later found both of these rules imbedded within a Military Specification, with statistical failure support. Can't remember what the MS number was. Nowadays, I see circuits with caps rated for 1.2x peak voltage, and resistor wattages rated for 1.1x calculated circuit. Not what I would call a quality design, but then I'm not a millionaire either.

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