By Mike Holt for EC&M Magazine

Here's the follow up to yesterday's newsletter. This includes all of the answers to the questions sent, so you can see how you did.
Note: These questions are based on the 2014 NEC^®. Any underlined text indicates a change to the Code rule for the 2014 NEC^®.

Q1. What are the NEC requirements for sizing raceways?

A1. Raceways must be large enough to permit the installation and removal of conductors without damaging the conductor’s insulation [300.17]

Example 1: When all conductors in a raceway are the same size and of the same insulation type, the number of conductors permitted can be determined by Annex C.

Question: How many 12 THHN conductors can be installed in trade size ¾ electrical metallic tubing?

(a) 12         (b) 13        c) 14        (d) 16

Answer: (d) 16 conductors [Annex C, Table C1]

Example 2: When different size conductors are installed in a raceway, conductor fill is limited to the percentages in Table 1 of Chapter 9.

Author's Comment: The above percentages are based on conditions where the length of the conductor and number of raceway bends are within reasonable limits [Chapter 9, Table 1, Note 1].

Step 1:     When sizing a raceway, first determine the total area of conductors (Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors).

Step 2:     Select the raceway from Chapter 9, Table 4, in accordance with the percent fill listed in Chapter 9, Table 1.

Question: What trade size Schedule 40 PVC conduit is required for the following conductors?

3—500 THHN

1—250 THHN

1—3 THHN

(a) 2      (b) 3         (c) 4         (d) 6

Answer: (b) 3

Step 1: Determine the total area of conductors [Chapter 9, Table 5]:

500 THHN                   0.7073 x 3 =  2.1219 in.²

250 THHN                   0.3970 x 1 =  0.3970 in.²

3 THHN                      0.0973 x 1 =  + 0.0973 in.²

Total Area =                                            2.6162 in.²

Step 2: Select the raceway at 40 percent fill [Chapter 9, Table 4]:

Trade size 3 Schedule 40 PVC = 2.907 sq in. of conductor fill at 40%.

Q2. What are the NEC requirements for sizing boxes for conductor fill?

A2. Boxes containing 6 AWG and smaller conductors must be sized in an approved manner to provide free space for all conductors, devices, and fittings. In no case can the volume of the box, as calculated in 314.16(A), be less than the volume requirement as calculated in 314.16(B) [314.16].

Conduit bodies must be sized in accordance with 314.16(C).

Author’s Comment:

The requirements for sizing boxes and conduit bodies containing conductors 4 AWG and larger are contained in 314.28. The requirements for sizing handhole enclosures are contained in 314.30(A).

(A) Box Volume Calculations. The volume of a box includes the total volume of its assembled parts, including plaster rings, extension rings, and domed covers that are either marked with their volume in cubic inches (cu in.), or are made from boxes listed in Table 314.16(A).

(B) Box Fill Calculations. The calculated conductor volume determined by 314.16(B)(1) through (5) and Table 314.16(B) are added together to determine the total volume of the conductors, devices, and fittings. Raceway and cable fittings, including locknuts and bushings, aren’t counted for box fill calculations.

(1) Conductor Volume. Each unbroken conductor that runs through a box, and each conductor that terminates in a box, is counted as a single conductor volume in accordance with Table 314.16(B).

Each loop or coil of unbroken conductor having a length of at least twice the minimum length required for free conductors in 300.14 must be counted as two conductor volumes. Conductors that originate and terminate within the box, such as pigtails, aren’t counted at all.

Author’s Comment:

    According to 300.14, at least 6 in. of free conductor, measured from the point in the box where the conductors enter the enclosure, must be left at each outlet, junction, and switch point for splices or terminations of luminaires or devices.

Ex: Equipment grounding conductors, and up to four 16 AWG and smaller fixture wires, can be omitted from box fill calculations if they enter the box from a domed luminaire or similar canopy, such as a ceiling paddle fan canopy.

(2) Cable Clamp Volume. One or more internal cable clamps count as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box. Cable connectors that have their clamping mechanism outside of the box aren’t counted.

(3) Support Fitting Volume. Each luminaire stud or luminaire hickey counts as a single conductor volume in accordance with Table 314.16(B), based on the largest conductor that enters the box.

Author’s Comment:

    Luminaire stems don’t need to be counted as a conductor volume.

(4) Device Yoke Volume. Each single-gang device yoke (regardless of the ampere rating of the device) counts as two conductor volumes, based on the largest conductor that terminates on the device in accordance with Table 314.16(B).

Each multigang-device yoke counts as two conductor volumes for each gang, based on the largest conductor that terminates on the device in accordance with Table 314.16(B).

(5) Equipment Grounding Conductor Volume. All equipment grounding conductors in a box count as a single conductor volume in accordance with Table 314.16(B), based on the largest equipment grounding conductor that enters the box. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)], count as a single conductor volume in accordance with Table 314.16(B).

Author’s Comment:

    Conductor insulation isn’t a factor that’s considered when determining box volume calculations.

Question: How many 14 AWG conductors can be pulled through a 4 in. square x 2½ in. deep box with a plaster ring with a marking of 3.60 cu in.? The box contains two receptacles, five 12 AWG conductors, and two 12 AWG equipment grounding conductors.

(a) 3          (b) 5         (c) 7         (d) 9

Answer: (b) 5

Step 1: Determine the volume of the box assembly [314.16(A)]:

Box 30.30 cu in. + 3.60 cu in. plaster ring = 33.90 cu in.

A 4 x 4 x 21⁄8 in. box has an interior volume is 30.30 cu in., as listed in Table 314.16(A).

Author's Comment: The gross volume of a box base on Table 314.16(A) dimension (4 in. x 4 in. x 21⁄8 in. = 34 cu in.) doesn't take into account the actual volume of the material used to make the box. This why the interior volumes on this table are smaller than the overall volumes.

Step 2: Determine the volume of the devices and conductors in the box:

Two—receptacles               4—12 AWG

Five—12 AWG                   5—12 AWG

Two—12 AWG Grounds       1—12 AWG

Total 10—12 AWG x 2.25 cu in. = 22.50 cu in.

Step 3: Determine the remaining volume permitted for the 14 AWG conductors:

33.90 cu in. – 22.50 cu in. = 11.40 cu in.

Step 4: Determine the number of 14 AWG conductors permitted in the remaining volume:

14 AWG = 2.00 cu in. each [Table 314.16(B)]

11.40 cu in./2.00 cu in. = 5 conductors

Q3. What are the Code’s installation requirements for raceway seals in Class I locations?

A3. Raceway and cable seals must comply with 501.15.

501.15 Note 1: Raceway and cable seals must be installed to:

•   Minimize the passage of gases and vapors from one portion of electrical equipment to another through the raceway or cable.

•   Minimize the passage of flames from one portion of electrical equipment to another through the raceway or cable.

•   Limit internal explosions to within the explosionproof enclosure.

(A) Raceway Seal—Class I, Division 1. In Class I, Division 1 locations, raceway seals must be located as follows:

(1) Entering Enclosures. A raceway seal is required in each raceway that enters an explosionproof enclosure if either (1) or (2) apply:

(1) A raceway seal fitting must be installed in each raceway that enters an explosionproof enclosure that contains make-and-break contacts. A seal must also be provided where the enclosure contains equipment that operates at over 80 percent of the autoignition temperature of the gas or vapor in the area.

Ex: A raceway seal isn’t required if the make-and-break contacts are:

a. Within a hermetically sealed chamber

b. Immersed in oil in accordance with 501.115(B)(1)(2)

c.  Enclosed within a factory-sealed explosionproof chamber

(2) A raceway seal fitting isn’t required for trade size ½, ¾, 1, 1¼, or 1½ raceways that enter an explosionproof enclosure that doesn’t contain any make-and-break contacts (junction and splice boxes). An example is an enclosure that only contains terminals, splices, or taps. However, a trade size 2 or larger raceway that enters any explosionproof enclosure must have a raceway seal fitting installed within 18 in. of the explosionproof enclosure.

Author’s Comment:

    When a raceway enters an enclosure that doesn’t contain make-and-break contacts, the raceways only require a seal if they’re trade size 2 or larger.

The raceway seal fitting must be installed within 18 in. of the explosionproof enclosure.

Only explosionproof unions, couplings, reducers, elbows, capped elbows, and conduit bodies are permitted between the raceway seal and the explosionproof enclosure.

(2) Pressurized Enclosures. A raceway seal fitting must be installed in each raceway that isn’t pressurized where the raceway enters a pressurized enclosure. The raceway seal fitting must be installed within 18 in. of each raceway that terminates to the pressurized enclosure.

(3) Between Explosionproof Enclosures. A single raceway seal is permitted between two explosionproof enclosures containing make-and-break contacts if the raceway seal fitting is located not more than 18 in. from either explosionproof enclosure.

(4) Class I, Division 1, Boundary Seal. A raceway seal fitting must be installed in each raceway that leaves a Class I, Division 1 location within 10 ft of the Class I, Division 1 location on either side of the boundary.

There must be no fitting, except for a listed explosionproof reducer installed at the raceway seal fitting, between the raceway seal fitting and the point at which the raceway leaves the Class I, Division 1 location.

Ex 1: A raceway boundary seal fitting isn’t required for a raceway that passes completely through the Class I, Division 1 area unbroken with no fittings installed within 1 ft of either side of the boundary.

Ex 2: If the raceway boundary is below grade, the raceway seal can be located above grade, after the raceway emerges from below grade.

(B) Raceway Seal—Class I, Division 2. In Class I, Division 2 locations, raceway seals must be located as follows:

(1) Enclosures with Make-and-Break Contacts. A raceway seal fitting must be installed in each raceway that enters an enclosure that must be explosionproof and that contains make-and-break contacts. The seal fitting must be installed within 18 in. of the explosionproof enclosure.

(2) Boundary Seal at Unclassified Location. A raceway seal fitting must be installed in each raceway leaving a Class I, Division 2 location. It can be installed on either side of the boundary within 10 ft of the Class I, Division 2 area.

Except for listed explosionproof reducers installed at the raceway seal fitting, there must be no union, coupling, box, or fitting between the raceway seal fitting and the point at which the raceway leaves the Division 2 location.

Raceway boundary seals aren’t required to be explosionproof, but must be identified for the purpose of minimizing the passage of gases permitted under normal operating conditions, and they must be accessible.

Author’s Comment:

    See the definition of “Accessible” as it relates to wiring methods in Article 100.

    The raceway boundary seal at unclassified locations is used to minimize the passage of gases or vapors, not to contain explosions in the raceway system.

Ex 1: A raceway boundary seal fitting isn’t required for a raceway that passes completely through the Class I, Division 2 area unbroken with no fittings installed within 1 ft of either side of the boundary.

Ex 2: A raceway boundary seal fitting isn’t required for raceways that terminate in an unclassified location where the metal conduit transitions to cable trays, cablebus, ventilated busways, MI cable, or open wiring if:

(1) The unclassified location is located outdoors or the unclassified location is indoors and the conduit system is entirely in one room.

(2) The raceways must not terminate at an enclosure containing an ignition source in normal operation.

Ex 3: A boundary seal fitting isn’t required for a raceway that passes from an enclosure or a room that’s unclassified, as a result of pressurization, into a Class I, Division 2 location.

(C) Raceway Seals—Installation Requirements. If explosionproof sealing fittings are required in Class I, Division 1 and 2 locations, they must comply with the following:

Ex: Boundary seals for Class 1 Division 2 locations that aren’t required to be explosionproof [501.15(B)(2)] aren’t required to comply with 501.15(C).

(1) Fittings. Raceway seal fittings must be listed for the specific sealing compounds and Class I location, and must be accessible.

(2) Compound. The raceway seal compound must be mixed and installed in accordance with manufacturer’s instructions so it minimizes the passage of gases and/or vapors through the sealing fitting.

Author’s Comment:

    The sealing compound must be from the same manufacturer as the raceway seal.

(3) Thickness of Compounds. Except for listed cable sealing fittings, the thickness of the raceway seal compound installed in completed seals, other than listed cable sealing fittings, must not be less than the trade size of the seal fitting, but in no case less than 5⁄8 in.

(4) Splices and Taps. Splices and taps must not be made within a raceway seal fitting.

(6) Number of Conductors or Optical Fiber Cables. The conductor or optical fiber tube cross-sectional area must not exceed 25 percent of the cross-sectional area of rigid metal conduit (RMC) of the same trade size, unless the seal is specifically identified for a higher percentage fill.

Author’s Comment:

    The cross-sectional area of intermediate metal conduit (IMC) is approximately 7 percent greater than that of RMC because the wall thickness of IMC is less than RMC. If the cross-sectional area of IMC is used for conductor fill calculations, the 25 percent of “RMC conductor fill” can be exceeded.

(D) Cable Seal—Class I, Division 1. In Class I, Division 1 locations, cable seals must be located as follows:

(1) Terminations. Type MC-HL cable is inherently gas/vaportight by the construction of the cable, but the termination fittings must permit the sealing compound to surround each individual insulated conductor to minimize the passage of gases or vapors.

Ex: The removal of shielding material or the separation of the twisted pairs isn’t required within the raceway seal fitting.

(2) Cables Capable of Transmitting Gases or Vapors in a Raceway. Raceways containing cables must be sealed after removing the jacket and any other coverings so that the sealing compound surrounds each individual insulated conductor or optical fiber tube in a manner so as to minimize the passage of gases and vapors.

Ex: The removal of shielding material or the separation of the twisted pairs isn’t required within the raceway seal fitting.

(3) Cables Not Capable of Transmitting Gases or Vapors in a Raceway. Each multiconductor cable installed in a raceway is considered to be a single conductor if the cable is incapable of transmitting gases or vapors through the cable core.

(E) Cable Seal—Class I, Division 2. In Class I, Division 2 locations, cable seals must be located as follows:

Ex: Seals aren’t required in a Class 1 Division 2 location if the cable passes through the location and has an unbroken gas or vaportight sheath.

(1) Multiconductor Cable. Multiconductor or optical fiber cables that enter an explosionproof enclosure must be sealed after removing the jacket and any other coverings so that the sealing compound will surround each individual insulated conductor or optical fiber tube in a manner so as to minimize the passage of gases and vapors.

Multiconductor cables or optical fiber cables installed in a raceway must be sealed in accordance with 501.15(D)(2) or (3).

Ex 2: The removal of shielding material or the separation of the twisted pairs isn’t required within the cable seal fitting.

(4) Cable Seal—Boundary. Cables without a gas/vaportight continuous sheath must be sealed at the boundary of the Class I, Division 2 location in a manner so as to minimize the passage of gases or vapors into an unclassified location.