Modifying an NEMA 4 box. How to ensure it stays watertight? 4

[KGisME]

TL/DR: Gasket tape (EPDM or Silicone?), outdoor caulk, or RTV gasketmaker for sealing a big hole in a rooftop NEMA 4 enclosure? And why?

Hi I figured EEs would have the most experience with this problem, so I'm posting it here rather than in the ME section. I’m doing the mechanical engineering for a one-time internal project that involves several NEMA 4 enclosures that will be installed on the roof of a 4-story engineering office building. Honestly they could probably be NEMA 3R, but they want these to last as long as the building they’re being mounted on the roof of. For reasons I don’t understand or have control of, they already bought prefab NEMA 4 enclosures and removable backplates from Hammond even though the rest of our designs are still in progress. I’m told they meant to get NEMA 4x boxes for longevity, but for some reason the boxes we bought are just plain NEMA 4 (painted carbon steel) Hammond enclosures. Not sure how that happened, but it happened before I started working on this project. I can’t see a reason they’d even want to wash down these enclosures. There's no source of oil or unusual industrial contaminants, just normal outdoor dust / pollen, etc. There are also no significant amounts of highly corrosive chemicals where these are to be mounted. The closest thing is rock salt they put on the parking lot during occasional winter storms, but again that’s 4 stories below. 

Here’s where the fun begins… there are going to be about 30 total connectors, Heyco cordgrips, and RF adapter bulkheads on the side of this enclosure. The enclosure is big, 60”h x 36”w x 16”d, made from painted 14ga carbon steel. The RF adapter bulkheads have an oddball hole shape and size (apparently called “Double D”) that we can’t find a Greenlee or similar punch to make the hole, at least not in the correct size. So we decided to do something we were already considering, which is to consolidate all the connectors to the lower half of one side of this box, and make a custom removable mounting plate for all the connectors on our waterjet. Then we cut a big (33"h x 13"w)rectangular hole in lower half of the side of the Hammond box where the connector mounting plate will go. Ugh, I hate to cut a big hole in the side of an expensive prefab NEMA 4 box, but it's our best option at this point.

The removable connector panel will be rectangular, made from unfinished 6061-T6 aluminum sheet, either 11 ga (.090in) or 14 ga (.063in), about 15"w x 35"h and I'm putting flanges on all sides of it for rigidity. I could make it from stainless if someone thinks its necessary, but I'd have to order it. Not a huge deal, as we have tons of other stuff to order for this project, but we have the aluminum in stock here already.

I have the removable panel on the outside of the box now because I thought it would be less likely for water to leak in that way. If anyone thinks it's better on the inside, I'm all ears.

So, my question is what's the best way to seal this hole where my removable connector panel will be? Keep in mind this is on a rooftop, and while we're not at a very high altitude nor tropical latitude, it will not have any shade at any time, and the roof surface is white. So it will be getting blasted with UV.

Gasket tape? If so, how do I ensure it doesn't leak at the 4 corners where the tape meets? And EPDM or silicone? The cost difference between the 2 isn't a big concern.

Or regular outdoor caulk?

Or some type of Formed-in-place (RTV?) gasket maker stuff? I'm not sure how that handles UV though. I've looked for data on it's UV resistant properties, and none of the documentation I can find seems to mention UV resistance. Seems like it's all made for under-the-hood automotive applications. I know Hammond and others have a FIP gasket on their doors, which would of course be ideal, but I can't find any of that style FIP gasket-maker in a small package for a one-time use like this.

I found bonded sealing washers on McMaster. I guess I need those on every single screw/bolt hole I put through this? There a lot more than 30 holes, because a bunch of these connectors are 4-hole flange-mount style.

And would it be preferable to put the screws that attach the panel to the Hammond box around the perimeter on the inside or outside of whatever water sealing material I use?

 

[TugboatEng]

If you want it to last a long time, NEMA 3R has the advantage of not relying entirely on gaskets for ingress protection. Gaskets deteriorate over time and are subject to misinstallation. NEMA 3R uses hoods and overlapping flanges to protect the internals. It is not truly water tight so anything that gets in can drain out which is a positive. NEMA 3RX is available if you need corrosion resistance as well.

Silicone and EPDM gasketing are both highly UV and weather resistant. Most other commonly available materials do not perform well.

If the connectors carry a NEMA 4 rating they should have all of the hardware to install them to that rating in the box. No additional sealants or washers should be necessary.

Again, going back to the NEMA 3R standard if you mount your connectors on a downwards facing angle they will be inherently protected from weather even if not sealed correctly or misinstalled.

 

[waross]

Either install a drain fitting or drill a 1/8" hole at the lowest point in the box.
Differential pressure cycling will draw moisture past almost any seal.
Drill a hole to drain the inevitable moisture.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Adding a small length of downwards facing tubing to the vent hole will almost eliminate any moisture entrance even if directly sprayed.

 

[KGisME]

Tugboat, I'll consider NEMA 3RX in the future (if I can find them, most manufacturers I've seen offer either 3R, 4 or 4X, but I've not seen many 3RX options in catalogs). But again I'm stuck using the NEMA 4 boxes they already bought for this project. Some of the connectors do have an IP67 or IP68 rating. But that reminds me, I need to find gaskets for the D38999s still.

waross and tugboat, I do have these Hammond NEMA 4X vents

https://www.hammfg.com/electrical/products/climate/sdvn4

on my box to allow some natural convective airflow through the box. And I have a heater on a hygro-thermostat to prevent condensation. Do you think I still need a drain hole even if we never are going to wash these down?

 

[TugboatEng]

Those should work if you can keep the box tight which can be done. We have to use sealed boxes for the marine environment and don't usually have too much trouble. It's usually trivial details like nitrile o-rings under bolt heads that fail due to weather exposure which leads to water ingress.

With that said, absent direct spray, a hole in the bottom of the box can be very beneficial if you do get a leak at some point during the lifetime of the enclosure.

 

[che12345]

I am thinking as an alternative solution, whether is it practical.
1. Cut the rectangular hole say 5mm all round smaller than the mounting-plate. Place the mounting-plate outside and weld all round the edges. Attention: the weld will damage the painting around it. Send the box to any switchboard fabricator workshop. They can clean up the damage paint and re-spray a suitable say [power paint] that is suitable for the site environment and UV resistance as well.
2. It is advisable to maintain the panel heater which should to be located at the lower part and with a drain hole with fitting opening facing down.
Che Kuan Yau (Singapore)

 

[KGisME]

My main question was really about gasketing around the perimeter of the big hole though. Any tips to keep the seams at the corners watertight? Is it necessary to get crazy with something like notching a pocket into the side of one piece of gasket so the end of the adjacent piece fits into it (kind of like a dovetail joint? but without the angles)

Just use silicone foam gasket if possible to keep it from shrinking? Any ideas on what width or thickness (that's available off the shelf) would be best?

Here's what I can find:

<-this is from the MSC catalog

https://www.mcmaster.com/products/g...osure-rating~nema-4x/enclosure-rating~nema-4/

https://www.hammfg.com/electrical/products/accessories/1481dj

Sending it out to a welding shop is really not an option. Again, we've already purchased the boxes, and the removable panel will be unfinished aluminum, or possibly unfinished stainless steel. Obviously you can't weld 2 different metals together.

As I said in my last reply, I have a heater in these with a hygrothermostat to prevent condensation, as well as passive NEMA 4 vents.

 

[waross]

Condensation can and will occur at any temperature.
Standard anti-condensation protection is a small (100₩, 250₩) 240 Volt heater permanently connected to 120 Volts.
That will protect at any temperature and has the fewest failure points.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[KGisME]

As stated before, I already have a heater in here (400w) on a hygrothermostat.

Why would you intentionally connect a 240V heater to 120V? You can go ahead and do that if you want, but I prefer to use electrical equipment correctly as designed.

 

[davidbeach]

240V heating elements connected at 120V with no thermostat or other controls is a very common approach in outdoor electrical equipment. I produces 1/4 the heat output that it would at 240 and stay warm without getting overly hot. Lasts forever because there’s no thermal stress on it. Keeps the interior enough above ambient to be noncondensing. If the enclosure is too large use more rather than hotter.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[TugboatEng]

Silicone rubber doesn't age under normal outdoor exposure. As long as it is closed cell it should work very well to prevent water entry. Foam doesn't require a lot of clamping force so reinforcement of the sheet metal should not be required. If the part does not need to be removable, silicone sealants work well and don't require any compression at all. You may consider placing sleeves or washers between the plate and enclosure. This creates a gap for the silicone to fill. This reduces stress on the silicone as the enclosure flexes.

You can also bond the plate using acrylic epoxy. No screws required.

 

[bacon4life]

Applying 120 volts to a 240 volt strip heater is the standard way for electric utilities to use strip heaters for anticondensation protection. It is also common to see one element always on and a second element on a hygrostat/thermostat.

In addition to reducing the likelihood of the heater failing from thermal stress, reduced voltage operation also reduces the risk overheating other items. Such items include fixed items like wiring, unavoidable junk like dust/insects/rodents, as well as it reduces the consequence of inadvertent contact by a person working in the enclosure.

 

[FacEngrPE]

Welding on flat surfaces often results is warping that will make sealing more difficult. As you want the sealing faces flat avoid welding.

I have used silicone caulk rated for outdoors in applications like this. once applied the joint should be made up tight to squeeze the caulk to a thin layer. the caulk will seal the joint ans the screws as long as the screws are all tightened before the caulk cures.

You can get this caulk in a paintable version if that is needed.

 

[TugboatEng]

I disagree with this. Squeezing the caulk to a thin layer puts stress on the caulk if there is any movement of the joint. Because this is thin sheet metal expect movement.

 

[waross]

Have you considered a glancing plate for on!y the double D connectors?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Technically, if you need to put something on the outside of a Type 4 enclosure, the device must bear its own Type 4 listing and you must follow the manufacturer’s instructions in installing it.

If you have a non-conforming device that you want to mount, there is no option for doing that and maintaining the Type 4 listing. The only possible solution is to put it behind a listed Type 4 window or access door. Most of the enclosure manufacturers have those available now.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

Tug - your experience with sealant compounds is different than mine but not all sealing compounds are created equal. I grant if sealing with welding all around, then there is no requirement for any sealer. Controlling warping while welding sheet metal can be challenging,

IEEE Std 45-2002 IEEE Recommended Practice for Electrical Installations on Shipboard recommends that cable entries into enclosures be on the bottom, which if followed makes the roll of the sealant less important.

Per jraf's point regarding preserving the NEMA 4 listing, Hammond sells gasketed "Gland Plates" up to 76" x 40" for their enclosures.