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Thread: 100A sub panel wire size

  1. #1
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    100A sub panel wire size

    I had a subpanel installed and it failed for several things one item is that the electriction ran 3c 2awg alumin wire and the inspeactor says that is not good for 100A. The electrition came back and had to run a new wire and he is insisting the 2awg alumin wire is fine for 100Amps. Where can I look this up, my subpanel and main have about 8' of wire connectiong them.

    Should he of run #1 wire? Can I preove teh #2 is ok or should I just change the breaker to a 80 amp breaker for the inspector.
    -=Jason=-

  2. #2
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    Feb 2008
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    Eddington, ME
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    I am not an electrician. Just going by what my inspector said to use in my 100amp sub panel (I didn't need an inspection either). For 100amps my inspector said to use #3 copper or #1 aluminium. He said some areas allow #2 aluminium if you use chart 310.15(B)(6) in the NEC (not all areas allow this). It sounds if you drop to 80 or 75 you can use whats installed. Do you need the full 100amps?

    BTW I did my with 2/0 aluminium SER. It was only slightly more than the other wire and my run was 85' so I was also worried about voltage drop. The BORG didn't carry anything in between. So I went with the big stuff. So now if I needed to for some reason. I could bump up the amps without changing the feeder.
    Last edited by Ben Cadotte; 03-07-2008 at 4:56 PM.

  3. #3
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    Quote Originally Posted by jason lambert View Post
    I had a subpanel installed and it failed for several things one item is that the electriction ran 3c 2awg alumin wire and the inspeactor says that is not good for 100A. The electrition came back and had to run a new wire and he is insisting the 2awg alumin wire is fine for 100Amps. Where can I look this up, my subpanel and main have about 8' of wire connectiong them.

    Should he of run #1 wire? Can I preove the #2 is ok or should I just change the breaker to a 80 amp breaker for the inspector.
    You hired the license, and paid the premium, to install a code legal 100 amp sub panel. You contracted for a 100 amp sub panel, not an 80 amp panel.

    There are processes that the license can go through to appeal the inspectors ruling. In the meantime though you are without an inspected panel.

    Can you prove that the wire is adequate? Possibly. It still may not overturn the inspectors position though.
    Can you install an 80 amp breaker to pass the inspection, and then install a 100 amp breaker after you pass the inspection? Yes, and it will be a violation of your permit on record.

    What you are experiencing is a growing trend in electrical installation/inspection. To attempt to protect future users from DIY's. Inspectors are increasingly adopting a more conservative approach to the inspection criteria in anticipation of future un-inspected modifications that will be made to electrical installations, by unlicensed individuals, ie. the homeowner. I'm assuning that this subpanel is in your residence. If this is not correct, it helps to more understand the inspectors position.

    In your case, you hired and paid a licensed electrician for a legally installed and inspected subpanel. It is up to the license to have the permit signed off for a 100 amp subpanel. They should have been aware of the prevailing inspection criteria that would have to be met.

    I know this is a harsh and unhelpful answer, but this issue is the responsibility of the electrician.
    Don't settle for an 80 amp installation, if you contracted for a 100 amp.

    PS

    You stated that this was one of several things that failed. What were some of the others?
    Last edited by Mike Cutler; 03-07-2008 at 5:54 PM. Reason: insert more correct terminology

  4. #4
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    I would have to agree with what Mike said. The licensed electrician you hired should have known what would or would not pass inspection. And, even if he didn't, it would still seem to be his responsibility to bring the installation up to code, according to the inspector.

  5. #5
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    Western Kentucky
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    I agree with Mike and Mark, the electrician should have known that the wire was too small. In the NEC 310.16 it shows allowable ampacities of conductors, they give diffrent temperature ratings but the trick is that most breakers are designed for use at 75 degrees C, and that is the chart that must be used for wire sizing. Its a issue that we see all the time. but a licenced electrician should have known to use #2 awg wire for feeding a 100 amp panel. I would recommend getting him to replace it with the correct size to make your panel legal. This is something that you will have to live with from now on out and it should be done right the first time.

    Glen

  6. #6
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    Mar 2007
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    Berks/Montgomery Co. Pa
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    #2 alum. wire should be sufficient for 100 amp sevice. But it depends on how long your run is. I remember my electricial (neighbor) tellin me that you lose some amperage over a certain distance. I can't remeber the distance off of the top of my head.

    I ran #2 alum. wire for over 150 ft. to my shop and I have no problems.

  7. #7
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    Jan 2007
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    McDonough, GA (near Atlanta)
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    Subpanels

    Last year, I installed a 100 amp subpanel in my basement. All the licensed electricians I asked said that 2 gauge SER aluminum met the code requirements, but if you read the NEC, it doesn't seem to be sufficient. I still don't know what the right answer is.

  8. #8
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    Having known two people who had electrical fires due to aluminum wiring, I would go with copper. I know techniques have improved lately, but aluminum oxide isn't conductive and it causes overheating at the end points when it corrodes. Copper doesn't corrode as easily. Aluminum is fine for overhead feeder wires where a fire in air just results in interrupted service, but anywhere it runs in a wall I go with copper.

  9. #9
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    FYI, copper has a conductivity of 5.8e7 mhos/meter while aluminum sits at 3.5e7 mhos/meter. So copper conducts electricity about 2/3 better or put another way, aluminum has more resistance to current flow than copper so aluminum will drop more voltage across itself than copper.

    Maybe that sounds like a lot but it isn't. Calculate the amount of voltage drop in 100 m (328') of Cu and Al and you get on the order of microVolts (e-6)...no big deal.

    The only small catch with aluminum is oxidation and that just needs taken care off with some anti-oxidant placed at connection points.

    Also, when the wire heats up from current flow, the coefficient of thermal expansion for Cu is 17 while 23 for Al. Again, not a big deal if properly torqued.

    Just some fun little facts for a Friday.
    Crown Molding: cut, cope, cuss, caulk, chill....

    Did you know SMC is user supported? Please help.

  10. #10

    NEC Code

    The pertinent NEC reference is chart 310.15(B)(6). This is the somewhat confusing and controversial topic of "Reduced Size Service Conductors". This is only permitted for "individual dwelling units of one-family, two-family, and multifamily dwellings". Basically, in most areas of the country, 2 AWG AL conductors are acceptable to feed a 100A panel in an individual home through this exception to traditional wire sizing which would dictate a max of 85A (rounds to 90A) for that wire size. The thought is that private homes rarely operate a large percent of circuits at maximum capacity for extended periods of time. However, not all local codes recognize this current NEC policy.

  11. #11
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    Ok thanks the inspector already failed me for the 2awg not being big enough for 100Amp and ground not connected. I had the electrician come back after many phone calls and threats and he installed a whole new wire because the ground was not long enough since he cut it but still use 2awg and is insisting it is good enough for 100 amp.

    I think I am screwed I am going out to go get a 85amp breaker and stick it in, that should be good enough for my shop.
    -=Jason=-

  12. #12
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    Aug 2005
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    Jason,
    On a positive note, 85 amps seems more than sufficient for a home shop. I have 50 amp subpanel and run up to two machines (5 hp total) at once plus the 2 hp dust collector and I don't even have a hint of trouble.

    It may be easier on your blood pressure to just de-rate the panel. Unless there are several folks working in your shop at once and/or you have some huge commercial equipment that should be borderline three phase, I can't imagine you would have a problem.

    cheers, Jeff

  13. #13
    Quote Originally Posted by jason lambert View Post
    I had a subpanel installed and it failed for several things one item is that the electriction ran 3c 2awg alumin wire and the inspeactor says that is not good for 100A. The electrition came back and had to run a new wire and he is insisting the 2awg alumin wire is fine for 100Amps. Where can I look this up, my subpanel and main have about 8' of wire connectiong them.

    Should he of run #1 wire? Can I preove teh #2 is ok or should I just change the breaker to a 80 amp breaker for the inspector.
    Good advice here. Get what you paid for. Don't fudge it. If you didn't want it inspected, you could have whipped something up yourself, eh? The "correct" answer here is 'what the inspector and the electrician can come to an agreement on'. You should not be caught in the middle. Hope you didn't pay until you got sign off . .. that's always inspirational to the service provider.
    If liberty means anything at all, it means the right to tell people what they do not want to hear. -- George Orwell


  14. #14
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    Apr 2007
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    Indiana
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    When I installed the 100amp service in my shop 15 yrs ago, the inspector would not allow anything but copper wiring in the shop. Its a little more money but well worth it.

  15. #15
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    South Windsor, CT
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    First, the AHJ (Authority Having Jurisdiction) decides whether the installation meets the local code requirements. If there is a point you want to challenge, you can ask to see the code requirement that an inspector is basing a decision on.

    The topic of conductor ampacity requirements for “subpanels” has been the topic of spirited debate on the professional electricians forums. Section 310.15 has the rules for conductors from 0-2000 volts. There is a table, 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2 that defines an allowable reduction in ampacity for conductors that meet certain circumstances.

    The point of this reduction is that the load served by most Dwellings is rather varied in nature and won’t approach the real ampacity of the conductors. I interpret this allowance as applying to the main feeds to a dwelling, not the conductors used to supply a “subpanel.”

    What we call “subpanels” are not specifically addressed under the NEC, because they are considered to be Branch Circuit and Lighting Panelboards.

    The gist of the differences amongst the electricians et al is whether a “subpanel” qualifies for this smaller conductor size.

    This is defintely one of those cases in the NEC where it's not clear where the rules apply.

    If the AHJ in your area says that their interpretation of section 310.15 is that you have to use conductors rated at full ampacity for 100 amps, then the electrician would need to use either #3 copper THHN/THWN conductors IN CONDUIT, #1 NM cable with copper conductors or 1/0 aluminum SER.

    Rob
    Addy protocol: unlicensed, homeowner electrician
    Last edited by Rob Russell; 03-08-2008 at 4:41 AM.

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