Wet Location in Concrete Slab

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DPF6615

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Burlington, MA, USA
I am in design on a small (16' internal diameter) telescope observatory dome. There will be control cabling (USB, RS232 serial, etc.) from the telescope in the center of the dome to a PC workstation on the inside perimeter of the dome. The client is looking for a system to avoid the control cabling being run loose on the ground from the telescope to the workstation to reduce the tripping hazard. The client proposed solution is to provide trench ducts with removable covers from the telescope to the workstation.

The floor is going to be a 5" thick concrete slab sitting on top of 6" of gravel backfill. Wouldn't this make the trenches wet locations per Article 100 (2017) "Installations underground or in concrete slabs or masonry in direct contact with the earth" and subsequently require the conductors inside the raceway to be wet location listed? The control cabling that comes with the telescope is not wet location listed and I doubt any future control cabling the client would provide would be wet location listed, so I am reticent to support the trench concept. The dome will not have HVAC (don't ask me why), so I am concerned about the potential for moisture buildup in the rest of the space.

Our initial proposal was to use overfloor raceway (https://www.legrand.us/wiremold/rac...oor-raceway/ofr-series-overfloor-raceway.aspx), but the client shot that down over concerns about that equipment holding up in a non-conditioned space.
 
Location, Wet. Installations underground or in concrete slabs
or masonry in direct contact with the earth; in locations subject
to saturation with water or other liquids, such as vehicle washing
areas; and in unprotected locations exposed to weather.
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There is no concrete around the control cables so how can they be "in" the slab?
 
DPF6615 said:
The dome will not have HVAC (don't ask me why), so I am concerned about the potential for moisture buildup in the rest of the space
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Observatories seldom are "climate controlled", preferring the telescope(s) to be the same temperature as the outside air. Some of the large ones will close the dome during the day to maintain whatever temp. they had from overnight, then open around dusk so the instruments can adjust to the outside air. (Mountain top observatories don't usually have that much of a humidity problem.)
 
I imagine a pair of 2" PVC conduits would be preferable to a trench alone.

Humidity could be controlled with forced-air ventilation of some kind.

petersonra said:
There is no concrete around the control cables so how can they be "in" the slab?
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The same way conductors in a conduit mounted outdoors are still outdoors.
 
While observing, you want a minimum of air movement (or any kind of movement) in the dome. That is one of the reasons to have the inside temperature and the outside temperature the same.

Also, FYI, the observatories I worked at had a separate foundation for the telescope, with a gap between it and the floor slab.
 
Barbqranch said:
While observing, you want a minimum of air movement (or any kind of movement) in the dome. That is one of the reasons to have the inside temperature and the outside temperature the same.

Also, FYI, the observatories I worked at had a separate foundation for the telescope, with a gap between it and the floor slab.
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Thanks for the clarification on the environmental conditions. I'll double check with structural, but I believe they are setting up the telescope foundation to be structurally isolated from the building foundation.
 
electrofelon said:
If there is a vapor retarder installed under the slab (which there should be), then it doesn't seem to meet the definition of a wet location.
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[MENTION=72722]electrofelon[/MENTION]: Have you come across AHJ's/inspectors that agree with the explanation of a vapor retarder/barrier eliminating the direct contact between the slab and earth, subsequently making the slab not a wet location?
 
LarryFine said:
The same way conductors in a conduit mounted outdoors are still outdoors.
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There is a huge difference between being "outdoors" and located "in" a slab. I don't see how you define being in a slab if it is not in the slab. if it is not within the concrete it can't really be "in" the slab.
 
zbang said:
Observatories seldom are "climate controlled", preferring the telescope(s) to be the same temperature as the outside air. Some of the large ones will close the dome during the day to maintain whatever temp. they had from overnight, then open around dusk so the instruments can adjust to the outside air. (Mountain top observatories don't usually have that much of a humidity problem.)
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Thanks [MENTION=85409]zbang[/MENTION] for the clarification. I just thought it was the client trying to save a few bucks ;)
 
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