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Potential Code Issue in New(ish) Home (Could Affect NEMA 14-50 Install)

Discussion in 'North America' started by schneiderjohn, Sep 15, 2014.

  1. schneiderjohn

    schneiderjohn Member

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    #1 schneiderjohn, Sep 15, 2014
    Last edited: Sep 15, 2014
    Hello All,

    (Again, please excuse me if this is the wrong subforum. Still getting used to posting here, although I've been lurking for a while.)

    A little background: my home is relatively new (finished January 2013) and was built with two 225A-max panels in the garage and an outdoor combination metering panel on the exterior. Each of the interior panels are fed from the outdoor combination metering panel with a 150A (75C) breaker, each. The exterior and interior panels are connected via 2/0 AL XHHW-2 90C degree wire.

    While doing my research on installing the NEMA 14-50 receptacle and the HPWC, I started to become familiar with the NEC, especially in relation to wire gauges, ampacity, temperature, materials, etc. According to my understanding of the "NEC Table 310.15(B)(16)" (http://www.barr-thorp.com/wp-content/uploads/2011/04/Digest-176-NEC-Tables.pdf), my builder's electrician should have used the 75C rating of the wiring (not 90C), as the breaker and panels are only rated for 60/75 degree. This would put the 2/0 AL XHHW-2 wiring at a 135A rating, not 150A, which the breaker is rated at. So, the builder should have used 1/0 or larger CU wire or 3/0 AL wire, correct? Not 2/0 AL? Is this a code violation?

    I appreciate any available insight.

    Imgur album of the suspected issue: Potential Code Issue - Imgur
     
  2. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    I suspect your electrician relied upon article 310.15(B)(7), which permits a higher ampacity for feeder conductors that serve as the main power feeder to each "individual dwelling unit". I've seen multiple interpretations of this section from various inspectors. One inspector would say that's a code violation, because there isn't a single "main power feeder" and it doesn't supply "all loads that are part or associated with the dwelling unit". Another inspector I've heard from in the past says that the spirit of the rule applies, which is that the main power feeder will rarely have a load that approaches its rating except in a dead short situation, and so multiple panels serving all loads of the dwelling unit can use the ratings under 310.15(B)(7).

    It's likely the latter interpretation was used. If you have concerns, you can call your local inspector (city or county) and ask them, but I don't think you have to worry.

    - - - Updated - - -

    As to the question in your picture, breakers aren't directional, and so they're using that single panel basically as 2 discrete buses, rather than what you would normally see - a main that then feeds the lugs on the bottom with breakers to branch feeders.
     
  3. schneiderjohn

    schneiderjohn Member

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    Thank you for the insight FlasherZ! I had read that part of the code but assumed that the (B)(16) would act as a "superset", if you will, and augment (B)(7). Meaning, follow (B)(7), so long as it also complies with (B)(16) and, thus, comply with the tightest standard.

    Truth be told, as my home is within it's 2-yr electrical warranty period, I am hoping that my builder/electrician will replace the 2/0 AL line with 2/0 CU, as I have concerns with each stab in this interior panel already outfitted with 230 (max) Amps. Adding a 50A NEMA 14-50, let alone a HPWC, may cause a serious issue. I suppose I should call the inspector and find out their thoughts, as I don't have much of a leg to stand on, not being a certified electrician.

    As per the quote in the picture, I've found that the MC0816B1350RLTM exterior combo panel I have came equipped in this configuration, although it looked backwards to me, at the time:
    VEN3-8925986816030_large.jpg

     
  4. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Don't worry about the 2/0 AL, you'll be ok with it.
     
  5. jbabb

    jbabb Member

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    Another option that you have is to install the breaker for the 14-50 or HPWC in the outdoor combo panel. There is room for 2 double pole breakers there. The lower 150 amp breaker acts as a main for these breakers as well as the panel fed by the lugs at the bottom. This way, the added current from charging the Tesla is not going through the 2/0 AL feeder.
     
  6. tga

    tga Active Member

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    FlasherZ knows the NEC better than me, but if I am reading 310.15(B)(7) correctly, the 3/0 CU from the meter is good for 225A (See caption on 3rd picture of album, which says 3/0). Subtract the 150A already allocated, and that leaves 75A, so I don't think he could use a breaker larger than 75A (60A charge rate).
     
  7. jbabb

    jbabb Member

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    The 3/0 CU is protected to 150A by the main breaker since the current of any sub breakers plus the inside panel (fed by the bottom lugs) all flow through that main breaker. In theory, you could have a larger main as long as the 2/0 AL to the inside panel was then fed by a 150A sub breaker in order to protect that feeder wire. It is unlikely that he will actually trip the main, but the wire is protected.
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    The 3/0 CU is internal to the unit he's using, which is a combination meter - disconnect panel. The downstream feeders to the interior panels are 2/0 AL. In this particular case, the top half of the disconnect panel is using 3/0 CU from one set of lugs at the meter base, and the bottom half of the disconnect panel is using 3/0 CU from another set of lugs. No subtraction necessary.

    - - - Updated - - -

    In this particular case, it's not a "main" breaker - the disconnect is wired so that the top breaker (which would normally be the "main" feeding the bottom half of the panel with some branch circuits) is a service disconnect for panel #1 inside the home, and the bottom bus is a separate service disconnect and feeder to panel #2. See post #3 above.
     
  9. jbabb

    jbabb Member

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    Yes, the top breaker is a service disconnect for panel #1 inside the home, but the bottom 150A breaker is actually a "main" for the extra breakers in the bottom half (room for 2 double pole breakers there), plus the feed thru lugs to panel #2 inside the home. The wires connected to those feed thru lugs have to handle the current rating of that 150A breaker.

    I have a similar setup at my house with 400A service (meter rated 320A average, 400A peak), with the dual lugs at the meter, then two 200A panels inside the house. The main reason this setup used 150A breakers instead of 200A was so they could get by with smaller feeder wires, since the panels inside the house can handle 225A.
     
  10. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Yes, I realize that you could add additional breakers, but there are no other breakers in the bottom of the panel and as a result, it's just a service disconnect through to the lugs for panel #2.

    Many times these panels are set up so that the top section is the main, and then that feeds through to the lugs at the bottom like the old split-bus panels (FPE, etc.).
     
  11. tga

    tga Active Member

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    Nevermind, after looking at the pictures again, I retract this boneheaded statement.

    I thought it was the top breaker wired "backwards", not the bottom one.

    I thought the bottom breaker was wired meter->3/0 CU->lugs/bus bar->150A breaker->2/0 AL->house, which would leave the bus bars unfused (hence the assumed 75A limit).

    After looking at the picture again, it's obvious it's wired meter->3/0 CU->150A breaker->lugs/bus bar->2/0 AL->house, so the bus bars are protected.
     
  12. schneiderjohn

    schneiderjohn Member

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    Thank you for all the insight everyone! I think I will indeed branch off of the exterior panel with a 50A breaker there. Makes me much more comfortable than potentially overloading the interior panel. This way, I can go:

    Wired meter -> 3/0 CU -> 150A breaker (potentially upgrade to a 200A breaker, with the help of an electrician) -> lugs/bus bar -> 50A breaker -> #4 CU -> NEMA 14-50.
     

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