UV Lights fed from RTU power circuits

[TXMaster]

Hey, fellas. I have a situation where a customer wants to add UV-C lights inside some existing 3ph 480V RTUs. The units are fed through their base, so the power comes up directly into the unit through the curb. The lights would be minimal load (less than 1 amp at 277V). This is negligible compared to the RTU circuit and could be easily done with a fused wire tapped from one phase in the RTU.

However, the unit does not account for this tiny current on its label (Minimum Circuit Ampacity or Branch Circuit Selection Current). Technically, this little change would invalidate the engineered label. It is not practical to tap the 3ph RTU circuit before it hits the unit due to power going through the base into the unit- the circuit is not accessible on the roof until it enters the RTU.

I do not see anything in Section 440 that explicitly addresses this situation to allow/disallow tapping the circuit within the unit to add the UV-C. I understand there could be warranty concerns if what I add fails, like a burned connection or something, but I am wondering about a code violation.

Does anyone see a NEC violation by tapping the RTU circuit within the unit?

 

[DBill64]

Adding the lights and tapping power inside the unit would likely invalidate the UL listing (or whichever NRTL listing it has). If the listing is invalidated, then the entire install is no longer code compliant.

I see 3 routes here:

• The manufacturer may offer a model with UV-C lights pre-installed. (EXPENSIVE and unlikely)
• The manufacturer may offer a retrofit kit for adding UV-C lights. (Less expensive, but also unlikely)
• Add stand-alone UV-C lights in the downstream ducting of the RTUs as to not actually modify the RTUs themselves. (I would look into doing this option)

 

[jim dungar]

If the listing is invalidated, then the entire install is no longer code compliant.

Only the AHJ can make the call concerning a field modification of a Listed device. The Listing agency can help, but it is up to the AHJ.

Even something as simple as adding field punched conduit entries technically would invalidate a Listing. Only the AHJ can decide if the installation meets code.

 

[LarryFine]

The lights would be minimal load (less than 1 amp at 277V).

I'd be surprised if there is a neutral up there.

 

[TXMaster]

I am leaning towards turning down the opportunity. Running separate circuits, which I already know is the right thing to do, will be cost prohibitive in this facility. Thanks for everyone's input- I was hoping someone would have a magic bullet that I know doesn't exist!

I'd be surprised if there is a neutral up there.

Great point, Larry. Another thing I overlooked trying to be optimistic... I mean, I could add 2 fused wires and probably find a transformer to create a neutral, but that just adds one more thing to the listed machine that doesn't belong there and isn't blessed by the manufacturer. Then the issue of GEC connection for bonding and just so... many... problems. Sometimes, it's just not worth it.

 

[herding_cats]

You would add a UV-C bank downstream, seperate circuit. Or "upstream" on the return. I was part of the Tech-UV team in 2020-2021 designing UV-C banks in ducting under a dept of defense grant with Nitride Solutions of Wichita, KS. Putting UV into RTU isn't safe. The oxidiation is too severe and it damages plastic, filters, pretty much everything. The "BK Calculator" was established to create a watt/airflow guide to make sure you get the correct efficacy. I can tell you right now that 1 A of UV-C lighting will not have any efficacy at all. If you can relay the tonnage of the RTU or air handler I can help you size it. The best UV-C chamber is a round duct lined with white teflon, and a single (or multiple) bulb(s) down the middle of the duct. White teflon has the highest reflectivity for UV. Titanium dioxide compounds are good too for reflectivity.

You will find some of our contributions here on exposure time:

Portable Ultraviolet-C Chambers for Inactivation of SARS-CoV-2 - PMC

The goal of this project was to create and optimize the performance of portable chambers for reliable ultraviolet (UV) disinfection of personal protective equipment (PPE) and enable its safe reuse. During unforeseen times of high demand for PPE, ...

pmc.ncbi.nlm.nih.gov

There has to be the correct duration of UV exposure rated to the FPM (velocity) of CFM (volume). PM me for more info I could help you design it for your customer.