under floor wiring

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metaldome88

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The company I work for manufacturers pre-fab steel shelters for various utilities and recently we had a request I've never dealt with before.....two 240vac 1 phase, AC panels, on exact opposite walls (400amp MB in each), with one feeding the other through feedthru lugs. The customer doesn't want to use the cable tray to get across the room, but rather go through the floor. It's a steel floor, roughly 10" deep (C-channel) and insulated with 1/4" steel plates on top for the finished floor. Originally, we planned on setting PVC in an open channel, before installing the floor,90 degree elbows coming out of the floor below each panel. I say originally because it was going to be 225amps. Now with the 400amp requirement, we're looking at parallel runs of 4/0 and a larger conduit. My question is, can (6) 4/0 wires be bundled up and set to lay loose in the channel in the floor, with either a PVC pipe or wireway coming down from the panels to the 1/4" steel floor for entry into them? (I'd put a wireway adapter or a coupling on the underside of the deck plate to protect the wires as they came up through the floor, we're just trying to avoid an elbow since a 3" pipe would have a rather large radius coming up from the floor if we ran continuous pipe). Sorry for the novel, those still awake, I'd appreciate any suggestions! Thanks! :confused:
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Re: under floor wiring

I don't think that the floor in the original post would fit the definition of a Cellular Metal Floor Raceway as described in article 374. And even if it did 374.4 limits the size of a conductor installed within it to #1/0 unless you receive special permission.

You could install 2-1 1/2" EMT conduits each with one set of 3-#3/0 Cu conductors for a 400 amp parallel feeder.
 
jwelectric

jwelectric

Senior Member
Location
North Carolina
Re: under floor wiring

Are these things required to have a third party listing and label?

Are they being tested by a testing lab?

Are you asking us to help design a death trap?

Am I asking the wrong questions?

Are we going to get paid for this design?
:)
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: under floor wiring

I would suggest a couple of things. First off, this type of thing seems to me to not be covered by the NEC, being as it is a prefabricated assembly.

If it is OK by UL, than it is OK.

I believe if you look closely, UL does not prohibit you from using unlisted structural parts as wireways inside such an assembly, as long as the wiring is protected from damage.

If you are not building a UL listed assembly, than I do not know what to tell you.
 
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metaldome88

Member
Re: under floor wiring

You could install 2-1 1/2" EMT conduits each with one set of 3-#3/0 Cu conductors for a 400 amp parallel feeder.

That may be the best solution, but according to my calcs, at 40% pipe fill, even 1 1/2" would be too small for (3) 3/0 with a ground in each conduit....am I missing something? Also, in the original post I didn't mention the length of the run across the floor would be roughly 15'. With using EMT, in concealed areas, I'm a little concerned about set screw connectors (in case they aren't installed tight or the screw loses the metal to metal contact over time), is this warranted or is EMT with set screw connectors perfectly fine generally, when installed properly? Thanks for the replies so far.
 
infinity

infinity

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Location
New Jersey
Occupation
Journeyman Electrician
Re: under floor wiring

Your original post mentioned 6 conductors total so I split it into 2 conduits with 3 in each. EMT when installed correctly does not require a EGC. If you choose to use one than you would probably need 2". Two sets of 1 1/2' EMT with 3-#3/0 THHN cu conductors would be code compliant without an additional grounding conductor.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: under floor wiring

Frank you think like I do. :)

There is a problem with parallel MC cables though.

The EGC in each MC would be sized for a 200 amp OCPD.

250.122(F) requires the EGC in each paralleled set to be sized for the 400 amp OCPD.

There is a way around this if the feeder is protected by a GFP device.
 
B

benaround

Senior Member
Location
Arizona
Re: under floor wiring

Bob,

Good point,In that case FMC with properly sized EGC (#3) would work!

LFMC would also do a good job.

frank
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: under floor wiring

Yeah for what sounds like fairly short runs either FMC or LFMC would be a good product to use. :cool:
 
ed downey

ed downey

Senior Member
Location
Missouri
Re: under floor wiring

I might be missing something but two options that I come up with are:
1.) install 2" emt in a straight run and at each panel install a skirt on the bottom of the panel (Basically a wireway that extends to below the floor). Then terminate your conduits directly into the skirt and you do not need a 90.
2.) install Alcan Stabiloy MC Cable they usually stock a cable that has a ground sized for parallel feeders. Check with your local ALCAN rep.
-Ed
 
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