Cable Tray Under Raised Floors, Outdoor Space Wiring and More

If you have a National Electrical Code (NEC)-related problem, are experiencing difficulty in understanding an NEC requirement, or are wondering why or if such a requirement exists, ask Charlie and he will let the Code decide. Send your questions to

Underfloor cable management

I’m wondering if a cable tray (under 600V nominal) installed below a nonplenum raised floor is considered exposed and accessible per 392.6(H)? There will also be heating, ventilating and air conditioning (HVAC) supply ducts below the raised floor, so are there special clearance requirements between the ducts and our cable tray?

Underfloor cable management systems are available and permitted by 392.3. See Exhibit 392.1 in the NEC Handbook where cable tray is used in an accessible underfloor application. Cable tray systems cannot be used in plenum or other air-handling spaces except to support wiring methods recognized for use in such spaces (392.4). Cable tray can be used in installations of more than 600 volts (V) in accordance with 392.6(F).

The NEC does not have any specific requirements relating to clearance requirements between HVAC supply ducts and cable tray installed below raised floors other than 392.6(I) where sufficient space must be provided and sustained about cable trays to permit adequate access for installing and maintaining the cables.

MC cable and nonmetallic boxes

If MC cable is run to and then connected to the nonmetallic outlet box of a central vacuum system for the 120V power source, I would have a Code violation based on 314.3. There is no need for a ground for the VOH connection, only a hot and neutral. The box is listed, and a ground is not used. Why is a nonmetallic-type connection required?

NEC 314.3 does not permit the use of metal-armored cables with nonmetallic boxes unless a means is provided for internal bonding of all entries to the box. The concern here is for continuity of the ground-fault path, which apparently is not a concern in your installation. Check with the authority having jurisdiction for special permission by 90.4 to waive specific requirements.

Multiwired branch circuits

I’m pulling 10-3 MC cable from my panel to a junction box. From there, I’m pulling 12-2 MC cable for each of my two receptacle branch circuits. If I do it this way, is it still considered a multiwired branch circuit, and do I still have to pigtail my neutrals at the receptacles? Please explain why or why not.

You left me to assume too many parts in your question. To start, why did you use 10 AWG conductors? What size overcurrent device are you using to protect these conductors? If you are using 30-ampere (A) protection as you are permitted for the 10 AWG conductors, then the 12 AWG conductors are not being properly protected. They are required by 240.4(D) to be protected by 20A overcurrent protective devices. However, each of the 2-wire branch circuits has its own neutral and is not a multiwire branch circuit, and the requirement in 300.13(B) for device removal and the continuity of a grounded (neutral) conductor is not necessary.

Transformer requirements

We have a building with 208/120V, three-phase wye system. The HVAC contractor purchased a 25-ton, 480V, three-phase unit for the building. Will the Code permit us to use a 480V primary 208/120V secondary 75-kilovolt-ampere, dry-type transformer and back feed the transformer to connect the unit? We have enough incoming service for the load of the HVAC unit.

I suppose the simple answer would be to use NEC 110.3(B) and refer you to the manufacturers’ instructions in the listing and labeling. Transformer designers compensate the windings ratio to account for voltage drop, and this will work against the user when a transformer is back-fed. Problems with high inrush current and increased potential fault current may occur. The best solution is to use a transformer designed for the application.

Water pipe as grounding electrode

I have a question with a grounding electrode in a house. There is an old water pipe system that used to serve this lake community when the houses were only summer houses. The houses all have their own wells now. The system is empty and has not been in use for at least 20 years. I did a service upgrade, 200A, and drove two ground rods and bonded the cold water pipe in the house. The well is piped in plastic pipe to the storage tank and then changes to copper inside the house. Do I need to bond the old empty water pipe, and/or should that be my grounding--electrode conductor?

The old metal underground water piping system is no longer considered as a metal underground water piping system and cannot be used as a grounding electrode. If any part of it is located where it is accessible to people and where it is likely to become energized by a circuit in that vicinity, it should be bonded to the equipment-grounding conductor in accordance with 250.104(B).
The two driven ground rods are sufficient for your grounding-electrode system, if there are no other qualified electrodes present at the building.

Is Scotch tape a mechanical barrier?

Provided that there is enough capacity, can an AWG 18 multiconductor cable, (UL Listed “Communications Cable,” No. NJ 307848) occupy the same J-box containing a 208V or 120V (AWG 12 THWN) circuit without a mechanical separation? Is Scotch tape considered a mechanical barrier? The AWG multiconductor cable has a CMP/CL3P/FPLP rated jacket but no voltage rating marking anywhere. It is a 24V control circuit for PAM relays (also contained in the same J-box as mentioned above) used to control HVAC equipment. I would say the higher voltage circuits are for the associated equipment, although they pass through the J-box continuing onto other units on the same circuit. Please help settle some confusion about Class 1, 2 and 3 wiring.

NEC 800.133(1)(c) tells us that communications conductors shall not be placed in any raceway or outlet box with conductors of electric power or light unless separated by a permanent barrier or listed divider. I don’t know of any Scotch tape listed for that purpose.

Outdoor use of NM cable

Can NM cable be used in an outdoor location? My situation is an outdoor pavilion. Everything is piped up in PVC and terminated into a WP box and is ground-fault circuit interrupter-protected, but I want to install a light under the roof. All four sides of the pavilion are open and exposed to the elements. The roof is an open truss construction, and the lighting fixture will be located 8 feet above the floor, which is concrete. I’m thinking that the trusses need to be covered with plywood to be considered a dry location.

NEC 334.12(B)(4) does not permit Type NM cable to be used in wet or damp locations. Check out the definition of damp location in Article 100. Even if the trusses were covered with plywood, it wouldn’t work.

Bonding in a wood frame building

In a wood frame building using metal studs or steel beams, is bonding or grounding them required?

There are no requirements in the NEC to bond or connect metal studs or remote steel materials to the grounding system. NEC 250.4(A)(4) requires electrically conductive materials that are likely to become energized to be connected together and to the supply source in a manner that establishes an effective ground-fault current path. Unless there is some substantiation that these metal materials are likely to become energized, it is not necessary to connect them to establish an effective ground-fault path.

Limiting conduit runs

Is there an NEC requirement limiting the length of a run of conduit without a junction box or pull box being installed?

There are no NEC requirements limiting the length of a run of conduit without a junction box. Good design would be to establish pull points to facilitate the installation of conductors in a raceway. The NEC is not intended as a design specification and leaves this up to the designer or the well-trained journeyman electrician. Requirements, such as 300.18, prevent installing conductors before the run is complete and subsections .24 and .26 in the raceway articles restrict the bend radius and the number of bends between pull points.

TROUT answers the Code Question of the Day on the NECA NEIS Web site. He can be reached at

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

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