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Thread: NEMA 3 verses NEMA 4

  1. #1
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    NEMA 3 verses NEMA 4

    With regard to exterior electrical installations I would like to get some feedback on the service life of NEMA 3 verses 4. A little background, I have produced several designs over the past several years and when it comes to the NEMA rating I err on the conservative side buy specifying 4 in all exterior applications. Many have argued that NEMA 3 is acceptable, however I specify 4 to last. Unfortunately, once the design is complete I move on and rarely get a chance to observe the field and how things held up. I have also had several discussions with contractors and others over the years on the topic. I understand all conditions are not created equal such as a disconnect being installed beneath an overhang verses in direct contact with weather.
    For those of you that spend a good deal of time in the field modifying/replacing systems etc. is 4 worth the added cost? Furthermore, has anyone experienced a failure of a NEMA 3 due to its construction?

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    I use 3R, sometimes but what is different about a straight 3 and a 4? Only difference in T110.28 is that a type 4 is hosedown resistant.

    A lot of what I otherwise use has multiple ratings - 3,4,12,13 or if corrosion resistant 3,4, 4x, 12,13
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  3. #3
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    NEMA 3R requires a corrosion resistance test consisting of a salt water spray for xx hours, then allowing to dry and repeating xx number of times etc. etc. Most mild steel enclosures that have been treated for rust prevention in some way, then painted with an epoxy powder coat will last a good number of years. But unfortunately, the 3R standard can be met in a myriad of "cheap" ways as well, leading to a general dislike of them in some sectors. So it comes down to quality, which generally relates to price. NEMA 3R also means outdoor use in wind driven rain, but does NOT require sealing against water entry, only that water not be allowed to build up to contact any live parts. In fact 3R requires that if any water gets in or condenses, it must be able to drain out, so it needs a "weep hole" of some sort.

    NEMA 4 is about the wash down spec (100 GPM at 60 PSI from a 1" nozzle aimed from any direction, a.k.a. a "fire hose test"). But the basic corrosion test for NEMA 4 is identical to NEMA 3R. Where the corrosion issue gets addressed is in NEMA 4X, and that usually means stainless steel, aluminum, fiberglass or plastic with door gaskets. So if you want corrosion resistance, you need to specify 4X, not just 4,

    That said, the price/performance ratio between N3R and N4X is very large, so if you don't need the wash down aspect, you can probably replace a 3R enclosure every 5-10 years 1 or 2 times before you get to the cost of the SS 4X in the same piece of equipment. There's something to be said for keeping it simple.

    PS: NEMA 3 is not rerally used much in electrical equipment. "Outdoor use, resistant to FALLING rain", whereas 3R adds "wind driven" rain and sleet, plus a requirement that the enclosure have a "drip lip" on top so that if the door is opened in the rain, the guts are somewhat protected. So most mfrs actually use 3R since it covers 3 and 3R.
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  4. #4
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    If corrosion resistance is your concern, maybe specify non-metallic boxes. A water intrusion rating is generally neither here nor there when it comes to corrosion.

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