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Thread: Public input for 705.12(B)(2)(b)

  1. #11
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    Quote Originally Posted by amluto View Post
    Isn't the whole NEC more or less this compromise? That doesn't necessarily mean that a more lenient rule here couldn't still achieve the CYA goal.
    I'd say less a compromise than it is more... and for that part that is more, the compromise is usually on the conservative side. The point here is that the compromise has already been made when you compare to 705.12(B)(2)(3)(a) to (b)...i.e. 120% vs. 200%. You will be asking for more leniency than 120% provides. I'm willing to bet that your amended version, even if it presents an excellent theoretical substantiation, the CMP will demand that you provide substantiation through tests made in cooperation with one or more NRTL.

    I would like the stipulation to allow 200% as long as sum of load OCPD's does not exceed the bus rating. Just look at any service with two to six disconnecting means and understand that the sum of the service OCPDs can be greater than the service conductor ampacity as long as the service conductor ampacity is not less than required for the load.
    Last edited by Smart $; 12-27-16 at 02:25 AM.
    I will have achieved my life's goal if I die with a smile on my face.

  2. #12
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    Quote Originally Posted by Smart $ View Post
    I'd say less a compromise than it is more... and for that part that is more, the compromise is usually on the conservative side. The point here is that the compromise has already been made when you compare to 705.12(B)(2)(3)(a) to (b)...i.e. 120% vs. 200%. You will be asking for more leniency than 120% provides. I'm willing to bet that your amended version, even if it presents an excellent theoretical substantiation, the CMP will demand that you provide substantiation through tests made in cooperation with one or more NRTL.
    I neither have the capability to make that happen nor the funding to pay an NRTL to do it for me. (Nor do I know exactly what would be tested. Would they set up a 400A panel as I described, max it out, point a thermal camera at it, and make sure that everything is safe until breakers start tripping?) I could submit to the code making process anyway and see what happens, though.

    I could, just maybe, arrange for a test like that at a very much nationally-non-recognized lab, and it might not even cost that much. Ebay has the right kind of panelboards for ~$200 and used breakers aren't all that expensive. The tricky part would be finding or borrowing a power supply that can generate a circulate a few hundred amps of 60Hz AC.

    Quote Originally Posted by Smart $ View Post
    I would like the stipulation to allow 200% as long as sum of load OCPD's does not exceed the bus rating. Just look at any service with two to six disconnecting means and understand that the sum of the service OCPDs can be greater than the service conductor ampacity as long as the service conductor ampacity is not less than required for the load.
    I don't know the history, but I assumed that the 120% maximum was about limiting the potential nasty effects of harmonics. Suppose your solar inverter generates an output current that is far from being a sine wave. Then I could imagine a load (especially something like a conventional dimmed lighting load) that draws a higher-than-expected fraction of its current from the inverter. If you plug the top of the panel into the service, put the inverter at the bottom, have a lot of dimmed lighting loads connected near the top and have a lot of linear loads connected near the bottom, I could imagine harmonic currents in the middle of the panel that are a decent fraction of the sum of the two supply currents. With the 120% rule, you'd have to have a very abusive load indeed to cause a problem, but with 200%, there could be less margin for error.

    For what it's worth, I bet you can damage a normal panel with just one supply with abusive enough loads. Just plug a few big capacitor banks in at one end of the busbar some inductors with roughly equal (but opposite) reactance at the other end of the panel. You'll get reactive power flowing through the busbar and there's no practical limit that I can see on how much reactive current could flow relative to the size of the main OCPD. Of course, you could probably only do this intentionally, and the NEC probably has something to say about capacitor banks, too.

    As for services, I don't know what the NESC says about it, so I don't really know what keeps people from exceeding calculated load an overheating a service conductor. My house has an overhead service drop that I'm quite confident is not adequately protected by the single OCPD at the service entrance. I think it is, however, indirectly protected by the distribution transformer on the pole -- if I believe the stenciled numbers on that transformer, it can't supply enough current to damage the insulation on my service drop. I hope that either the NESC or the power company's common sense requires that something protect at least the utility-owned conductors.

  3. #13
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    Quote Originally Posted by amluto View Post
    ...
    That would address the safety concern but not the usefulness concern. ...
    The code scope is limited to the former. The latter is outside the scope of the NEC and is a design issue, not a code issue.
    Don, Illinois
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  4. #14
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    As a general rule POCO does not try to protect secondary (service) conductors. If they burn up, it is outside.

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  5. #15
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    Quote Originally Posted by GoldDigger View Post
    As a general rule POCO does not try to protect secondary (service) conductors. If they burn up, it is outside.
    True, but service entrance conductors, though also outside or considered such for the most part, fall under NEC jurisdiction. I'm fairly certain there are many multi-disconnect services out there that the sum of the OCPDs is greater than 120% of the service entrance conductor's ampacity.
    I will have achieved my life's goal if I die with a smile on my face.

  6. #16
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    Amluto, I like your proposal in principal. I would simplify it by just adding the allowance of a feed through breaker in a panel as an exception.

  7. #17
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    Quote Originally Posted by Smart $ View Post
    True, but service entrance conductors, though also outside or considered such for the most part, fall under NEC jurisdiction. I'm fairly certain there are many multi-disconnect services out there that the sum of the OCPDs is greater than 120% of the service entrance conductor's ampacity.
    And, because most feeder or branch circuits are not loaded to breaker capacity, even in commercial use, I do not see any inherent problem in the sum being greater than the conductor ampacity. NEC does not allow that when there is a single breaker, and a POCO may well throw in a 100% or 120% requirement even though the service disconnect is after the service point. But not necessarily with the expectation that it will somehow protect their conductors or transformer secondaries.

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