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Thread: location of main OCD when derated

  1. #1
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    location of main OCD when derated

    We stumbled across a little different scenario for the location of a de-rating of a main OCD. Scenario: new pv system installed on a manufactured home with a maximum current output of 47 ampere; Interactive inverter output circuit landing on the 200 ampere panel bus bar located in the manufactured home (110ampere calculated load); the manufactured home panel is fed from a 200 ampere service pedestal on the lot which includes 3 additional OCD for swimming pool, 30ampere/120volt RV outlet and a convenience outlet.

    The installing contractor is indicating a 175 ampere main OCD for the manufactured home panel is no longer available and wants to de-rate the main OCD in the pedestal to 175 ampere. From all I have read and researched in the NEC, the bus bar rating where the PV circuits are landing must meet the 120% rule to protect the bus bar. The PV circuits could be landed on the pedestal bus bar, however the contractor is wanting to avoid trenching.

    In reviewing 705.12(B)(2) the only other option I see available is (c) unless the panel OCD is reduced to 175 ampere; or does changing the main OCD in the pedestal meet the intent of the code?

    appreciate you comments!

  2. #2
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    All the panels between where the PV interconnects and the utility POCC have to meet the 705.12 requirements. If the home panel has a 200A main OCPD changing the OCPD in the pedestal will not satisfy the letter of the NEC. But maybe could be argued to be an alternative method.

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    I think what the installer is proposing is likely fine, although you've left out some details of how the panel in the home is connected to the one on the pedestal. (Does the home panel have a main breaker? Is the connection to the pedestal panel through a breaker or feed through lugs?)

    The main OCPD protecting a panelboard does not always have to be in that panelboard. If there are two in series then the smaller one will govern.

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    Thanks for the input.

    The panel in the home does have a main breaker, according to the contractor, main OCD's for this panel are no longer produced.

    Feeders serving the panel are lugged to the pedestal. An option I also proposed was a fused disconnect between the pedestal and the home panel. This would have protected the panel in the home without de-rating the pedestal.

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    Quote Originally Posted by PatCBO View Post
    Thanks for the input.

    The panel in the home does have a main breaker, according to the contractor, main OCD's for this panel are no longer produced.

    Feeders serving the panel are lugged to the pedestal. An option I also proposed was a fused disconnect between the pedestal and the home panel. This would have protected the panel in the home without de-rating the pedestal.

    That should work but who want’s fuses for their home’s main OCD?

    I’d want to make sure that the fuses were easily replaceable by the homeowner and would leave them some spares.

  6. #6
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    Quote Originally Posted by PatCBO View Post
    Thanks for the input.

    The panel in the home does have a main breaker, according to the contractor, main OCD's for this panel are no longer produced.
    Doesn't mean you can't get one. I've gotten lucky one or two times with new-old stock main breakers on eBay.

    serving the panel are lugged to the pedestal. An option I also proposed was a fused disconnect between the pedestal and the home panel. This would have protected the panel in the home without de-rating the pedestal.
    If the lugged feeders are opposite end of the pedestal busbar main breaker then the contractor's suggestion works. If they are not then you need to do something else. The fused disco works if that's practical, although probably more expensive.

  7. #7
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    JB is correct, and this is a no-brainer. The main breaker in the service panel protects the service panel bus, the feeders to the house panel, and the house panel bus, which is also protected by it's own main breaker. If the service main is derated to 175A, this is the value that is used in the 120% rule calculations for all down-stream components (unless of course there is a down-stream breaker that is even smaller). The 120% rule does not state that the breaker protecting the bus bar has to be located in the same panel as the bus bar, it just has to be protected by it. The contractor's proposed solution is a good one as long as the total load at the site does not exceed 175A.

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