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RE: [TowerTalk] Non-corrosive RTV ; NEMA boxes

To: <>
Subject: RE: [TowerTalk] Non-corrosive RTV ; NEMA boxes
From: "Keith Dutson" <>
Date: Wed, 12 Jan 2005 16:29:24 -0600
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Thanks for the info on Dow 748 sealant.  I found a supplier and ordered
three 12cc syringes.  I actually have two of these relay boxes and they will
be sealed on separate dates since one is in my shack and the other is
presently at 152 feet.  Both boxes are Hoffman NEMA 4X.  Before the relay
was installed, there were no holes in the box.  Now there are 9 holes to
accommodate the hardware and cabling.  I plan to remove all hardware and
then reinsert after filling holes and gaps with this sealant.  Finally, four
small holes will be drilled in the bottom, one in each corner, for drainage.
I am thinking about stuffing these holes with plugs made from a cellulose

Keith NM5G

-----Original Message-----
[] On Behalf Of Kimo Chun
Sent: Wednesday, January 12, 2005 12:04 AM
Subject: [TowerTalk] Non-corrosive RTV ; NEMA boxes

FWIW, Dow Corning makes 748 Non-corrosive RTV sealant (White). 100% Silicone
Rubber; Safe for food contact; Non-corrosive; Less objectionable odor; -67F
to +350F (-55C to 177C).

Applications: Sealing corrosive-sensitive electrical and electronic
equipment (electrical connections, wire entries, power and control cable
connections, etc.); Bonding and sealing food processing equipments; Formed
in-place gaskets for sensitive substrates.
I have the 3 U.S. Fluid Ounce ( 90 mL) tubes.

In general, NEMA-3R and 4X are the types of boxes you ought to use. However,
the 3R may only retain its "rain-tight" rating when it is "upright". There's
also no guarantee in a hurricane with horizontal wind driven rain, hi.

My personal rule, which I carry on to work whenever possible, is to never
penetrate the roof (top of the box) unless it's the last resort. Walls can
be penetrated with appropriate weather-tight connectors but even better if
they're under an eave of a roof (or lip of a box - that's how the 3R boxes
are when they are "upright"). Best to go from below whenever possible. It is
also very true that it is difficult to make a box watertight and dry (except
for specialized underwater boxes) especially here in the tropics (and as
soon as you start making holes in it). Don't even try. Use drain holes. When
mounting operating radio equipment (repeaters) permanently outdoors (at
least here in Hawaii) I put the entire equipment cabinet in a larger wall
mounted cabinet (3R or 4X) and use forced air ventilation to keep a stream
of outdoor air circulating through to keep moisture build-up to a minimum.
Of course, filters and deflectors are used. Compensation for your varied
climate conditions must also be taken into account. We're lucky in that

If you must mount things internally and through a wall, consider putting a
little RTV on the screw before you drive it through the hole.
If you can find (and afford) specialized boxes some come with studs welded
to the back of the box and have a floating plate / chassis that sits on
these studs. This will allow you to mount some hardware without any holes in
the outer box (not counting ingress and egress of cables, etc.) These are
common on naval (brass) shipboard boxes but are very expensive. There are
commercial boxes as well. Unfortunately, electrical boxes with any special
qualities are expensive.

Good luck.

Kimo KH7U

See:  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.

TowerTalk mailing list


See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

TowerTalk mailing list

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