NM Cable or conduit for feeders on 80 unit apartment complex?

[storeytime]

I'm a commercial contractor. I've never done an apartment complex but am thinking about bidding on one that caters to senior citizens. Branch wiring is specced for NM cable. Each unit has a 150A panel, fed from mechanical rooms via the hall way plenum throughout the complex. Would you use cable for
the feeders or would it be more cost efficient to use conduit and THHN? I'm thinking I won't even be close if I use conduit, even PVC.

 

[JDBrown]

Do they even make NM cable that big? I'm no Contractor, so I don't really spend time at Electrical supply houses, but I've never heard of NM larger than #2/3 (someone please correct me if I'm wrong).

You have to use the 60 deg. column of Table 310.16 to determine the ampacity of NM cable (see NEC 334.80), so you're going to need a minimum of #2/0 CU or #3/0 AL for 150A feeders, even if you take advantage of 240.3(B). We don't do a lot of residential, but when we do we use EMT with THHN wiring for feeders that size. Then again, part of the reason we don't do much residential is because we're a little more expensive than most Engineers in the area. Perhaps someone else can suggest a cheaper but still acceptable alternative.

 

[Dennis Alwon]

I would use SER Aluminum for the feeders as conduit may be very costly.

 

[Dennis Alwon]

Check the plans as they may spec copper

 

[infinity]

Can SE cable be installed above a hung ceiling?

 

[Dennis Alwon]

Can SE cable be installed above a hung ceiling?

I didn't see anything about a hung ceiling? I see he mentioned a plenum in the hall area but that would need to be avoided.

As far as I can tell SE cable can be installed above a dropped ceiling

 

[GoldDigger]

I didn't see anything about a hung ceiling? I see he mentioned a plenum in the hall area but that would need to be avoided.

It sounds like the design specifically calls for use of the hallway overhead space.
If it really is a plenum, the options will be very limited. If the space is actually an environmental air-handling space, the options are slightly better. If it is just a chase and has nothing to do with air handling, then it should be OK.
I did not see specifically that the space is over a suspended ceiling. It could be a hard ceiling with access hatches.
If the wiring goes over individual tenant spaces instead of over common area like the hallway, there may be issues regarding putting the feeder for one unit across the private space of another. Including local amendments beyond NEC.

 

[storeytime]

The design calls for the feeders to run in the space above the hallway suspended ceiling and each individual feeder turns into the apartment ceiling that it
feeds. Apartments are sheetrock ceilings. So, the main part of the feeder runs is above suspended ceiling, finishing above tenant sheetrock ceiling.

I will check and see if they spec copper, but the specs are very small. I didn't notice anything other than #12 minimum wire and that branch circuits could be NM cable. I'll go back and review them to see if I missed something.

 

[GoldDigger]

The design calls for the feeders to run in the space above the hallway suspended ceiling and each individual feeder turns into the apartment ceiling that it
feeds. Apartments are sheetrock ceilings. So, the main part of the feeder runs is above suspended ceiling, finishing above tenant sheetrock ceiling.

I will check and see if they spec copper, but the specs are very small. I didn't notice anything other than #12 minimum wire and that branch circuits could be NM cable. I'll go back and review them to see if I missed something.

If you are lucky, there will be no air ducts (either supply or return) that open into the space above the suspended ceiling. If there are no air ducts to the hall or they all come from a duct directly to a grate in the ceiling, then it is not a plenum or an air handling space. Your only limitation will be what wiring methods you can use for feeders above an ordinary suspended ceiling.

 

[storeytime]

If you are lucky, there will be no air ducts (either supply or return) that open into the space above the suspended ceiling. If there are no air ducts to the hall or they all come from a duct directly to a grate in the ceiling, then it is not a plenum or an air handling space. Your only limitation will be what wiring methods you can use for feeders above an ordinary suspended ceiling.

There is a duct in every hallway feeding the hall registers. I don't think there are any that open into the space above the suspended ceiling.

I noticed in the project manual it said, "All branch circuit wiring to be NM cable.". In the electrical notes it said minimum #12 THWN in conduit. The THWN makes it sound like to me that this is just some standard note in the drawing template that won't apply here. I would prefer that they require conduit on the feeders because that would put me on more of and even footing with the romex people that will be bidding the job. I don't even do houses. The only thing residential we ever do is remodeling. If it weren't in my back yard I wouldn't even consider bidding it.

 

[storeytime]

I just answered my own question. A note on the riser drawing specs (3)# 1/0 and (1) # 6 in 2" conduits for the panel feeders. Hooray!
Thanks for the quick and helpful answers.

 

[GoldDigger]

I don't think there are any that open into the space above the suspended ceiling.

I noticed in the project manual it said, "All branch circuit wiring to be NM cable.". In the electrical notes it said minimum #12 THWN in conduit.

Don't forget that from the strict NEC definition, if there are OCPDs in each space the long runs to those panels are feeders, not branches. So they may be expecting you to use conduit for the feeders, or they may just be giving you the option and telling you what you can and can't do. It is hard to infer too much from boilerplate notes, but they are still part of the contract.

 

[mstrlucky74]

Never really seen to many drawings/ specs where they tell you where/routing the feeders MUST be run?..exet a riser showing the closets/shafts. Your job seems very particular. Sometimes there are conduit routing drawing but rarely.

 

[Strathead]

I didn't see anything about a hung ceiling? I see he mentioned a plenum in the hall area but that would need to be avoided.

As far as I can tell SE cable can be installed above a dropped ceiling

You are right Dennis, it can be. I have done an Assisted Living Facility exactly this way. SE on a strut rack above ceiling.

 

[Strathead]

I just answered my own question. A note on the riser drawing specs (3)# 1/0 and (1) # 6 in 2" conduits for the panel feeders. Hooray!
Thanks for the quick and helpful answers.

I would offer a Value Engineered option for the SE cable,or a least a "deduct for cost savings per our design $xxx​)

 

[infinity]

IMO a possible alternate to conduit and wire would be MC cable. Running multiple long runs of SE cable can be an installation nightmare when it comes time to support the cable.

 

[cdslotz]

Don't forget that from the strict NEC definition, if there are OCPDs in each space the long runs to those panels are feeders, not branches. So they may be expecting you to use conduit for the feeders, or they may just be giving you the option and telling you what you can and can't do. It is hard to infer too much from boilerplate notes, but they are still part of the contract.

There still needs to have OCPD's at the source in the mech room or exterior, wherever it's located.


I would do what strat says. Bid it as specified and drawn in your base bid (which is what you are required to do).
Then provide alternate deducts for using SE and anything else you can think of.
Good luck competing with resi contractors who specialize in multi family. They use sub crews that flat-rate rough-in and trim out. And also skirt taxes and workman's comp by doing so.

 

[Fulthrotl]

I just answered my own question. A note on the riser drawing specs (3)# 1/0 and (1) # 6 in 2" conduits for the panel feeders. Hooray!
Thanks for the quick and helpful answers.

fwiw, my little iphone app says you could put three 2/0 aluminum, and a ground in a 1 1/2" emt, and that is good to go....
fill would be 31.9% ampacity is 150 amps.

2/0 ser aluminum with a #1 ground is about $2.90 a foot now....... if thats an option....

as long as you hypressed on lugs, you shouldn't have any issues. and a significant cost savings.

 

[storeytime]

After meeting with the GC, they want SER cable.

When you say "hypressed on lugs", are you talking about the lugs you crimp on the aluminum wire and they have posts that go under the lugs on the gear?

 

[curt swartz]

as long as you hypressed on lugs, you shouldn't have any issues. and a significant cost savings.

Why would you even consider using compresion lugs for a residential application? Standard residential loadcenters and meterstacks don't have compression options. You would have to use commercial panelboards and multi-meter switchgear. The cost increase for this equiptment would be more that just pulling copper feeders.

As long as the standard mechanical lugs are torqued properly they should perform equaly as well with copper or aluminum conductors.