VFD installed inside roof top unit

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Ohms law

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Sioux Falls,SD
My foreman had me install a VFD in a RTU, inside the supply side of the unit. The engineer added the VFD to this unit to control fan speed.
I don't agree since it is not going inside the electrical cabinet section of the RTU. Can you do this? It does not seem right. This VFD would be installed in a plenum basically. Maybe I am over thinking it.
 
Ohms law said:
My foreman had me install a VFD in a RTU, inside the supply side of the unit. The engineer added the VFD to this unit to control fan speed.
I don't agree since it is not going inside the electrical cabinet section of the RTU. Can you do this? It does not seem right. This VFD would be installed in a plenum basically. Maybe I am over thinking it.
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It is either a plenum or it isn't.

There is no actual requirement that electrical gear of any kind be installed in an "electrical" box (with a few exceptions), and many VFDs are made to be wall mounted specifically so they can be hung where it is convenient.
 
I was just curious since the VFD enclosure is the same as other VFD's within the building, which are not rated for any type of outside use. I was thinking that it would be considered a damp location since it is a roof top unit pulling air in and exposing moisture to the equipment. I know the air is constantly moving, so maybe it is probably nothing to think about.
Thanks for the reply.
 
Worst case put it in an enclosure, especially if it is one of those semi open drives - they are not rated for direct wall mount indoors either, they are intended to be mounted in a control cabinet.
 
I opened one of the VFD enclosures to get more information and on the door it has a NEMA-1 rating.
So is the inside of the roof top unit where the supply air travels considered to be outdoors or is this RTU considered a structure in its self. Sorry if it feels like I am searching for a certain answer. I want to feel good about the installation. There have been many issues/change orders with the design on this particular job.
 
Is the VFD installed in the airstream that supplies the building?

Unless it rains inside of the RTU, NEMA 1 equipment could be acceptable.
 
I probably would have put it in it's own enclosure and then mounted it to the side of the RTU. In the plenum just sounds like poor design to me regardless of what the NEC says about it.
 
Ohms law said:
The VFD is located after the roof top unit filters. It is on the input (fresh air) side, which essentially supplies air to the building.
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I would have to say that is either a plenum or an air duct. Either way, being on the supply side, there are restrictions on the flame spread and products-of-combustion characteristics of everything inside it.
I would not put any field wiring there except as specifically called out in the NEC as permitted for short distances and to wire sensors inside the ducts. (Unless the wiring and devices had a specific plenum/air duct rating.)
 
GoldDigger said:
I would have to say that is either a plenum or an air duct. Either way, being on the supply side, there are restrictions on the flame spread and products-of-combustion characteristics of everything inside it.
I would not put any field wiring there except as specifically called out in the NEC as permitted for short distances and to wire sensors inside the ducts. (Unless the wiring and devices had a specific plenum/air duct rating.)
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I will have to wait and see what the inspector says. At least all supply and return with damper motors have sampling tubes to smoke detectors.
 
Cow said:
I probably would have put it in it's own enclosure and then mounted it to the side of the RTU. In the plenum just sounds like poor design to me regardless of what the NEC says about it.
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The VFD is inside a NEMA-1 enclosure mounted in the RTU. I like GoldDiggers most recent response, what he said is how I would like to describe my concerns.
Cow said:
I probably would have put it in it's own enclosure and then mounted it to the side of the RTU. In the plenum just sounds like poor design to me regardless of what the NEC says about it.
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Pierre C Belarge said:
Make a phone call to the RTU manufacturer, ask them if the VFD can be installed within the section you have mentioned. Whatever their answer is, ask for a written response via email so you can show the inspector or your foreman.
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why would their opinion on the code matter to an inspector more than someone else's opinion?

and why would any honest inspector take such an opinion as being especially authoritative?
 
petersonra said:
why would their opinion on the code matter to an inspector more than someone else's opinion?

and why would any honest inspector take such an opinion as being especially authoritative?
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Only way I see their opinion mattering is if the equipment in question is a listed accessory of the main unit intended to be installed in that location according to the listing. Otherwise field modifications that are not covered by listing instructions are going to be covered by NEC in most places.
 
It sounds like the enclosure is installed in an area that may block some airflow into the unit. I can see the RTU manufacturer not liking that, and asking them to sign off on that in an email sounds doubtful to me.
 
I could be missing something but to me this is a clear cut violation of 300.22(B)

300.22 Wiring in Ducts Not Used for Air Handling,
Fabricated Ducts for Environmental Air, and Other
Spaces for Environmental Air (Plenums). The provisions
of this section shall apply to the installation and uses of
electrical wiring and equipment in ducts used for dust,
loose stock, or vapor removal; ducts specifically fabricated
for environmental air; and other spaces used for environmental
air (plenums).

(B) Ducts Specifically Fabricated for Environmental Air.
Only wiring methods consisting of Type MI cable, Type
MC cable employing a smooth or corrugated impervious
metal sheath without an overall nonmetallic covering, electrical
metallic tubing, flexible metallic tubing, intermediate
metal conduit, or rigid metal conduit without an overall
nonmetallic covering shall be installed in ducts specifically
fabricated to transport environmental air. Flexible metal
conduit shall be permitted, in lengths not to exceed 1.2 m
(4 ft), to connect physically adjustable equipment and devices
permitted to be in these fabricated ducts. The connectors
used with flexible metal conduit shall effectively close
any openings in the connection. Equipment and devices
shall be permitted within such ducts only if necessary for
the direct action upon, or sensing of, the contained air.
Where equipment or devices are installed and illumination
is necessary to facilitate maintenance and repair, enclosed
gasketed-type luminaires shall be permitted.
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iwire said:
I could be missing something but to me this is a clear cut violation of 300.22(B)
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It is pushing what you highlighted with red, but at same time if it is a listed accessory installed in equipment listed to accept the accessory and in compliance with listing instructions I don't think NEC can say much about it - but also seems likely this is not the case for the OP.
 
kwired said:
...If it is a listed accessory installed in equipment listed to accept the accessory and in compliance with listing instructions I don't think NEC can say much about it...
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I would agree, but that doesn't sound like the case.

A NEMA 4X VFD is not prohibitively more expensive than a NEMA 1: I would've mounted this thing on the outside of the RTU in a heartbeat.
 
kwired said:
but at same time if it is a listed accessory installed in equipment listed to accept the accessory and in compliance with listing instructions I don't think NEC can say much about it - but also seems likely this is not the case for the OP.
Click to expand...

So in other words if it was entirely different it might be OK? :p
 
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