Best Practice when AC/DC transformers (wall worts) are needed above a drop ceiling.

[GxPGuy]

Greetings,
I'm an instrumentation technician that has been tasked with installing a differential pressure monitoring system in a commercial environment. Pressure measurement is made using 4-20mA loop powered transmitters. Each transmitter will be powered/connected to a custom communication device that is powered using a wall wort style transformer (I was voluntold to use this device) that has a micro USB connector on the end. So essentially, the device is powered by a usb charger. It was requested that we locate the transmitters in clusters, in boxes above the drop ceiling. This means I will have as many as 12 wall transformers to contend with above a drop ceiling at each transmitter box. Electricians will handle any AC requirements, and I will certainly discuss with them but while searching for solutions/recommendations/code requirements online, I came across this site and figured I would ask. What are the best practices/code requirements/safety considerations when dealing with these types of transformers above a drop ceiling in a commercial environment?

Let me add that that whole thing feels "dirty" which is why I'm asking the experts....hoping to find a solution that I can feel ok about.

Thank you for your time!
Bob

 

[suemarkp]

Do you have to use a wall warts, or would a receptacle type USB outlet work to power it? If so, they even make them with no 120v receptacles if you only want USB. I'd probably go with the combo 120v and USB type though so the 120v is available if needed.

There was a thread about what can be above a drop ceiling. I can't remember if wall warts were indicated as OK. They probably meet the CL2 limits, but not sure they are marked as such.

Is this ceiling an air handling space, or is the return air ducted?

 

[hbiss]

There was a thread about what can be above a drop ceiling. I can't remember if wall warts were indicated as OK.

You can have receptacles above a drop ceiling but you can't plug anything into them. They can be used for things like tools or a drop light but nothing permanent.

If you locate the receptacles in a steel hinged cover box with the wall warts plugged into them inside the box, that would be OK. BUT the USB cords coming out of the box will have to be listed CL2 cables, CL2P if it is an air handling space. Not sure if you will be able to find anything like that. You don't say what is on the other (communication device) end of the cable. Since this is simply 5VDC, you might be better off with some other type of wall wart that has a more friendly output (like screws) which would allow the use of an off-the-shelf CL2/CL2P cable. Or even supplies that can handle more than one communications device.

-Hal

 

[GxPGuy]

Thank you for the fast responses! Above the ceiling is not part of the air handling system so...one less thing. I like the idea of a screw terminal transformer if the cables need to leave the metal enclosure. The USB side of the transformer is connecting to a small single board computer that will convert the 4-20mA signal from the pressure transmitters to ethernet. I'm thinking these devices can reside in the same metal box as the transformers. The wires that will penetrate the box would be AC, Ethernet, and the control cable for the 4-20 loop. I'm also searching for a USB charger "hub" that can handle powering multiple devices.

 

[Flicker Index]

How much power input does each end device need?
Basic powered USB hub like that will still provide power with just power input.
Back in the old days, there were two classes of USB devices. Those that need up to 100mA, like keyboards and mouse would power with just the signal cable, but the power input was used to power larger devices like speaker.

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www.amazon.com

Caveat though. While individual ports maybe current limited for safety (i.e. a short isn't going to flood all the available current of the power source of the hub, they're not galvanically isolated, so they're not individually "floating" as it is with using individual AC adapters where the secondary of each are in their own island. So, ground loop situation is possible.

 

[Todd0x1]

Thank you for the fast responses! Above the ceiling is not part of the air handling system so...one less thing. I like the idea of a screw terminal transformer if the cables need to leave the metal enclosure. The USB side of the transformer is connecting to a small single board computer that will convert the 4-20mA signal from the pressure transmitters to ethernet. I'm thinking these devices can reside in the same metal box as the transformers. The wires that will penetrate the box would be AC, Ethernet, and the control cable for the 4-20 loop. I'm also searching for a USB charger "hub" that can handle powering multiple devices.

Which single board computer? Many of those have power options besides the USB connector.

edit: These are nifty, unfortunately they don't make a 5v one.

JSU Series Power Supply Junction Box On Triad Magnetics

Browse JSU Series Power Supply Junction Box in the Triad Magnetics catalog including Item #,Input Voltage (AC),Output Voltage (DC),Output Current (DC),2007 EISA Efficiency

catalog.triadmagnetics.com

edit again: heres a little din rail DC-DC converter that outputs 3 amps of 5vdc. So for example you could install that triad thing wherever there's power, run CL2 wire from there to your enclosure, and put the DC-DC converter in your enclosure to power your 5v stuff.

DDR-15G-5

Shop for the Mean Well DDR-15G-5 DC/DC converter from TRC. We offer unbeatable discounts and ship same day. Buy DDR-15G-5 with confidence and get fast shipping!

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[GoldDigger]

As long as the devices all use the original basic USB power specification, you are pretty free to use a wide variety of power-only cable configurations to power the USB jack. But more and more devices, and in particular those which need higher power delivery, either for charging or for simultaneous power and charging rely on a negotiation protocol that increases both the supplied voltage and the maximum current. The handshake hardware can be present even in a tiny wall wart, and just providing a constant 5V on the right wires from a common supply cannot provide the same functionality. With newer standards, the high power mode does not require a direct brand match between device and power supply, and this may lead you to believe that just a simple low power USB setup is in use.

 

[Todd0x1]

As long as the devices all use the original basic USB power specification, you are pretty free to use a wide variety of power-only cable configurations to power the USB jack. But more and more devices, and in particular those which need higher power delivery, either for charging or for simultaneous power and charging rely on a negotiation protocol that increases both the supplied voltage and the maximum current. The handshake hardware can be present even in a tiny wall wart, and just providing a constant 5V on the right wires from a common supply cannot provide the same functionality. With newer standards, the high power mode does not require a direct brand match between device and power supply, and this may lead you to believe that just a simple low power USB setup is in use.

Good points, thanks for bringing this up. Curious to find out what OP's SBC is. Many of them that take a USB power input still have a straight 5V input somewhere else.