Technical Corner

For the article this month there is a scenario which occurs quite a bit in the electrical field. Instances such as mounting electrical equipment above an acoustical drop ceiling, specifically a bus duct.

Below, courtesy of the NEC 2017 handbook you will see an illustration of a bus duct mounted above a suspended ceiling with lift out panels. Under section 368.10 (B) Behind access panels-permits mounting above a ceiling as long as there is access to it. By reading definitions under Article 100 of the 2017 NEC you will find the verbiage:

Accessible: admitting close approach; not guarded by locked doors, elevation, or other effective means.

Readily Accessible: capable of being reached quickly for operation, renewal, or inspections without requiring the use of tools or portable ladders.

Obviously, this scenario would require a portable ladder to go up and remove the ceiling grid so I wouldn’t think this is readily accessible. However, the code section seems to allow just an “Accessible” situation, which ultimately leads to our answer that a busduct can be mounted above a suspended ceiling.

Another step further, if we pick on this illustration some more, you will see ceiling supports which drop down in front of the busduct.

Do you think there needs to be frontal clearance from the face of the actual bus as well as from the face of the busplug?

Some of you may tend to disagree with me, but my stance, which is conservative, is that we need to maintain the clearances listed in table 110.26(A)(1). In our scenario we would fall under condition #2 as I would consider the ceiling supports grounded metallic objects. This clearance would apply to both the busduct face as well as the actual bus plug face.

Reasoning: Usually there is a reason why a structural support is in a certain location. Can you imagine if we didn’t follow the clearances of the actual bus and then added a bus plug down the road? Now what. Just move the ceiling support…? Sounds expensive…lets be proactive!!

Many times, the busduct has already claimed the real estate and in comes a new ceiling below it. Maintenance and operation will be more of a concern when a ceiling is added below, so many owners are opting to just relocate the busduct to somewhere nearby where “better accessibility” can be established. During design it does need to be brought to the owner’s attention if existing and considered during the design if everything is a new installation.

Hopefully this article finds you well and can be used as a reference for your project needs.

If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director

res - technical corner JANUARY 2021 The ROCHESTER ENGINEER |