MC cable or conduit in existing plaster wall?

[zemingduan]

Hi, guys. I'm an electrical designer, not an electrician, and not familiar with the wiring method. The architect is asking these two questions below for a delicate historical building. They want to know what they’re dealing with.

"We plan to cut a channel in the existing plaster/brick walls, install the wire to some new light fixtures, then patch the channel with plaster to hide the wire. Is MC cable used, or conduit per NEC? We want to know what we’re dealing with and the dimension of the wires "

What is the required/usual dimension of the channel (depth and width) for one 20AMP lighting branch circuit?


"A final question has to do with the junction boxes at our surface mounted lights that are channeled into the plaster. This applies primarily to exit signs and emergency lights. Do these need to be full depth boxes, or can they be shallow boxes? What size will these boxes be?"

Can anyone give me some help? Appreciate it!

 

[ritelec]

Just a thought..... maybe you can hire a local electrician to help you with wiring techniques and box sizes. Just a thought.

 

[zemingduan]

Just a thought..... maybe you can hire a local electrician to help you with wiring techniques and box sizes. Just a thought.

Yes. Sure. They will definitely hire an electrician. We are still in the design phase right now. And the architect wants to know this information ahead of time. So that they can do some delicate, fancy design, arrange the locations, consider the feasibility because of the extremely dense walls, etc... It's gonna be a fancy/delicate historical ballroom. I think that is the reason they ask now.

 

[infinity]

I would run MC cable. Just make the chase deep enough for the cable to be covered with the wall finish.

 

[ritelec]

I would run MC cable. Just make the chase deep enough for the cable to be covered with the wall finish.

then be extra careful installing pictures or moldings

Yes. Sure. They will definitely hire an electrician. We are still in the design phase right now. And the architect wants to know this information ahead of time. So that they can do some delicate, fancy design, arrange the locations, consider the feasibility because of the extremely dense walls, etc... It's gonna be a fancy/delicate historical ballroom. I think that is the reason they ask now.

Yes you will hire an electrician for the install, maybe hire one to advice you of what you would need to do to achieve your delicate design.

 

[winnie]

I agree with ritelec, hire an electrician at this stage to consult, look at what you are actually dealing with, and provide info about how to practically and most efficiently do the job.

However MC cable can be embedded in plaster in dry locations; for 12-2 MC you are looking at about 1/2" in diameter. Conduit will be larger; nominal 1/2" conduit has an OD of about 0.7" and if there are any joints the connector will have an even larger diameter.

Shallow 'pan' boxes can be used for supplying a _single_ lighting fixture. These are only 1/2" deep and 4" in diameter, and by normal standards don't have the cubic inch capacity for the wires...but I believe there is still an exception that permits their use. You would need to find a version that has built in MC cable clamps to be practical. If you are running a single circuit from box to box then you need to do box fill calculations to determine the minimum size box to use.

-Jon

 

[synchro]

Is the plaster on top of wooden lath or is it directly on top of the brick? If it's on lath then that will give you some extra depth for cable or conduit.

 

[Chamuit]

I agree with everyone above. Definitely get an EC or electrician to advise you on the front end for planning and routing.