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Does this mean that junction boxes have to be 12in above the deck even though my datum plane is below the deck (not as in picture)?

My dock's deck is about 4 feet above the high-tide water line at it's lowest point and thus the datum line is half way up the dock supports. Can't junctions be under the deck?

2 Answers 2

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Can't junctions be under the deck?

No, they can't.

You have to satisfy both conditions:

  • 2' above highest high tide

and

  • 12" above deck
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  • and conduit has to be run 12" above the deck as well?
    – Alex
    Commented Jan 2, 2019 at 20:28
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NEC 555.13.2.1 wiring methods and installation allows the wiring to be on the underside for both floating and fixed.

(2) Portable Power Cables. Extra-hard usage portable power cables rated not less than 75°c (167°F), 600 volts; listed for both wet and sunlight resistance; having an outter jacket resistant to temperature extremes, oil, gasoline, ozone, abrasion, acids, and chemicals shall be permitted as follows:

    (1) As permanent wiring on the underside of piers (floating or fixed)

    (2) Where flexibility is necessary as on piers composed of floating sections

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    Does "wiring" include junction boxes, or just cables and conduit? Can you quote the section in your answer? There seems to be disagreement here and that would help clarify.
    – isherwood
    Commented Jan 2, 2019 at 17:04
  • I did, if you read that section the wiring can be below the deck I have done this and and it had to be approved by the army core of engineers and the port authority it is allowed but must be above the datum in the case(s) I did it it was 4' above as this was needed to provide shore power for fishing vessels. Like so many other sections of code there is an exception to many rules. So it is not an absolute.
    – Ed Beal
    Commented Jan 2, 2019 at 20:08
  • I meant the text of the section. We don't all have a code book or online resource ready at hand. :)
    – isherwood
    Commented Jan 2, 2019 at 20:22
  • Extra hard usage portable power cables rated not less than 75 c 600v listed for both wet and sunlight resistance having an putter jacket resistant to tempature extremes,oil, gas, ozone,abrasion acids and chemicals shall be permitted as follows 1, as perminant wiring on the underside of piers floating or fixed. I remember The boxes underneath had to be spliced with submersible / listed kits as I had to go dumpster diving to get the bags the kits came in because the port authority had not seen that brand. But I did not look up that section.
    – Ed Beal
    Commented Jan 2, 2019 at 20:31
  • "portable power cables", these cables wouldn't be portable. I've got the NEC 2017 here on an eBook and can't find anything about laying conduit/junctions under decks. its very odd considering I've never seen a deck with conduit running atop it.
    – Alex
    Commented Jan 2, 2019 at 20:37

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