Derating the Over Current Device Question for BB

Originally posted on: January 10th, 2012, 10:39 PST

http://forum.solar-electric.com/showthread.php?14290-wireing-questions&highlight=20%25+buss

Example:

•200 Amp panel * 20% = 40 amp maximum breaker(s) for solar
But, you can also derate the main breaker to give you some more headroom:

•200 amp panel - 175 amp main breaker + (200 amp panel * 20%) = 25 amps + 40 amps = 65 amps of solar
Some inspectors will allow you to put multiple solar circuits in the main panel (solar breakers must be installed at opposite end of bus bar from main breaker). Others will require a dedicated sub panel for all the solar breakers, then one solar breaker in the main panel.

Could you point me to the code article that allows the 25 amp difference between the OCD and the Load Center Buss to be added to the 20% of the Load Center Buss?

I ran into this 18 month ago and the inspector of that jurisdiction allowed me to derate the OCD to pick up enough to cover the 2 amps I needed to pass inspection. Now I'm in a different jurisdiction and the inspector wants the code article and I can't find it again.

TIA

Comments

  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Derating the Over Current Device Question for BB

    http://www.sea.siemens.com/us/internet-dms/btlv/Residential/Residential/docs_LoadCenters/RPFL-LCWPV-1209.pdf

    There is a reference to the code
  • SwordsmanSwordsman Registered Users Posts: 6
    Re: Derating the Over Current Device Question for BB

    Very helpful. Thank you.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: Derating the Over Current Device Question for BB

    Here is a link to a note from John Wiles about the "controversy" some inspectors may have regarding NEC code section 690.64(B)(2):

    http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load%20Side%20Connections.pdf

    From the article:
    "An AHJ may certainly look at a specific installation consisting of a specific set of supply
    breakers, loads, and locations of the same and evaluate the ampacity requirements of the
    conductors or busbar."
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Derating the Over Current Device Question for BB

    I have to believe that any code inspector is going to limit the installation to the current active code requirements. Since the modification were not included in the 2011 revision, the 2008 code is still in effect. They are not putting themselves at risk to deviate from the code and may be even more restrictive than the current NEC.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: Derating the Over Current Device Question for BB
    Here is a link to a note from John Wiles about the "controversy" some inspectors may have regarding NEC code section 690.64(B)(2):

    http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load Side Connections.pdf

    So, the point of contention is the concern for the current rating of the Panel Board, not just the bussbar. The Panel Board must be able to handle "...the heating associated with
    the thermal trip elements in the common thermal/magnetic molded case circuit breakers."


    My favorite solution is:
    "Another example (proposed for the 2011 NEC but not accepted) is to allow a conductor
    fed from supply breakers at each end, to have an ampacity of the greater breaker rating,
    not the sum of the breakers, when the conductor is marked, “Multiple Power Sources—
    Do Not Tap” every ten feet where the conductor is accessible."


    This would be like using the 10' or 20' tap rule. NEC 240.21 (B)(1)-(2) In fact I don't think any code says it cannot be applied. When I asked JW about this a few years ago he just said the tap rules were not intended to be applied to a power source.

    Alex Aragon
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Derating the Over Current Device Question for BB
    So, the point of contention is the concern for the current rating of the Panel Board, not just the bussbar. The Panel Board must be able to handle "...the heating associated with
    the thermal trip elements in the common thermal/magnetic molded case circuit breakers."


    My favorite solution is:
    "Another example (proposed for the 2011 NEC but not accepted) is to allow a conductor
    fed from supply breakers at each end, to have an ampacity of the greater breaker rating,
    not the sum of the breakers, when the conductor is marked, “Multiple Power Sources—
    Do Not Tap” every ten feet where the conductor is accessible."


    This would be like using the 10' or 20' tap rule. NEC 240.21 (B)(1)-(2) In fact I don't think any code says it cannot be applied. When I asked JW about this a few years ago he just said the tap rules were not intended to be applied to a power source.

    Alex Aragon

    45C is a concern here as ambient can reach these kinds of temps and radiation heating on a external panel can go much above. I have had breakers with about 50% load trip mid to late morning because of this, my panel is east facing. I have started to consider shading the panel and my inverters via landscaping.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: Derating the Over Current Device Question for BB
    solar_dave wrote: »
    45C is a concern here as ambient can reach these kinds of temps and radiation heating on a external panel can go much above. I have had breakers with about 50% load trip mid to late morning because of this, my panel is east facing. I have started to consider shading the panel and my inverters via landscaping.


    A real nitpick:

    The UL Marking Application Guide for Moulded Case Circuit Breakers
    http://www.ul.com/global/documents/offerings/perspectives/regulators/electrical/newsletters/MoldedCaseCircuitBreakersMG.pdf

    "20. 60/75°C Wire — All circuit breakers rated 125 A or less are marked for use with 60° C,
    60/75°C or 75°C only wire. This marking indicates the proper wire size for termination in
    accordance with Table 310.15(B)(16) of the NEC . It is acceptable to use wire with a higher
    insulation rating if the ampacity is based on the wire temperature rating marked on the breaker. For
    breakers rated more than 125 A, the proper wire temperature rating is 75°C and it is optional for the
    breaker to bear this marking."

    I nominate this as one of the most ignored and misunderstood code issues. It basically says that you may not use the 90C column in NEC Table 310.16 for wire ampacity when it is connected to "Moulded Case Circuit Breakers". By this rule you may not use #6 THHN for the output of a 60 amp charge controller when it is connected to a continuous duty 60 amp circuit breaker.

    Alex
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