Commercial Electrical 100%

2023 NEC Chapter 9 and Article 314

Lesson 9: Raceways, Cables, and Boxes

Chapter 9 Tables 1, 4 and 5, NEC 314.16, 314.28, 334.10, 358.30 · 14 practice questions

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Reading

Once the conductors are sized, they have to fit in something. Conduit and tubing have a fill limit, cables have support rules and building-type limits, and every box has a volume that has to hold the wires, devices and clamps inside it. This lesson works through conduit fill from Chapter 9, the raceway and cable rules for EMT, RMC, PVC, MC and NM, and box fill and pull box size from Article 314 of the 2023 NEC. Local adoption governs which edition is enforced (Lesson 1).

Two fill checks: conduit fill in EMT and box fill in a device box Panel A adds three 1/0 THHN conductors at 0.1855 square inches each and one 6 AWG THHN at 0.0507 for a total of 0.6072 square inches. The 40 percent limit of 1-1/4 inch EMT is 0.598, so the bar is over it and fails. The limit for 1-1/2 inch EMT is 0.814, so it passes. Panel B adds four 12 AWG conductors at 2.25 cubic inches for 9.00, one equipment grounding allowance of 2.25 and a device allowance of 4.50, for 15.75 cubic inches. A 3 by 2 by 2-3/4 inch box holds 14.0 and fails, and a 3 by 2 by 3-1/2 inch box holds 18.0 and passes. Two fill checks (Chapter 9 Tables 1, 4, 5 and NEC 314.16) A. Conduit fill: three 1/0 THHN and one 6 AWG THHN ground in EMT, 40 percent limit (sq in.) 1/0 THHN 0.1855 1/0 THHN 0.1855 1/0 THHN 0.1855 Total 0.6072 6 THHN ground 0.0507 1-1/4 in. EMT: 0.598, bar is over it, fails 1-1/2 in. EMT: 0.814, passes B. Box fill: 12 AWG feed-through receptacle with ground, in a device box (cu in.) 4 conductors 9.00 EGC 2.25 Device 4.50 Total 15.75 cu in. 3 x 2 x 2-3/4 box: 14.0, too small 3 x 2 x 3-1/2 box: 18.0, passes Conduit: add the Table 5 areas and compare with the Table 4 area at 40 percent. Box: add the Table 314.16(B)(1) allowances.
In both checks the load must not exceed the limit. Three 1/0 conductors and a 6 AWG ground just miss the 1-1/4 in. EMT, and one receptacle with four 12 AWG conductors and a ground does not fit a 14.0 cu in. box.

Conduit fill

Chapter 9, Table 1 limits the total cross-section of the conductors to 53 percent of the conduit for one conductor, 31 percent for two, and 40 percent for more than two. Equipment grounding and bonding conductors count at their real size (Note 3). A multiconductor cable counts as a single conductor for fill (Note 9). A nipple 24 in. or shorter between boxes or cabinets, not counting connectors, may be filled to 60 percent, and the adjustment factors do not apply to it (Note 4). EMT, RMC and PVC all point back to this table for the number of conductors (358.22 for EMT).

The areas come from two tables. Table 4 gives the conduit's total area and its 40 percent area, and Table 5 gives the area of each insulated conductor. Rows for EMT and THHN, THWN and THWN-2 are shown, in square inches.

EMT trade sizeTotal area40 percent
1/20.3040.122
3/40.5330.213
10.8640.346
1-1/41.4960.598
1-1/22.0360.814
23.3561.342
THHN sizeArea (sq in.)THHN sizeArea (sq in.)
14 AWG0.00974 AWG0.0824
12 AWG0.01331/00.1855
10 AWG0.02114/00.3237
8 AWG0.03666 AWG0.0507

Heavier insulation takes more room. The area of 1/0 THW is 0.2223 against 0.1855 for THHN, so always check the insulation type. For same-size conductors, divide the 40 percent area by one conductor's area. A decimal of 0.8 or more is rounded up, and anything below is dropped (Note 7).

Worked example: a feeder in EMT

A 3-phase feeder uses three 1/0 THHN conductors and a 6 AWG THHN ground in EMT.

  1. Phases: 3 x 0.1855 = 0.5565 sq in.
  2. Ground counts too: 0.5565 + 0.0507 = 0.6072 sq in.
  3. Compare with the 40 percent column: 1-1/4 EMT is 0.598, and 0.6072 is more than that. It fails, though only by 0.0092.
  4. 1-1/2 EMT is 0.814, and 0.6072 is under it. Use 1-1/2 in. EMT.

Same-size check: 3/4 in. EMT with 12 AWG THHN holds 0.213 / 0.0133 = 16.02, which is 16 conductors. For 4 AWG in 1-1/2 in. EMT, 0.814 / 0.0824 = 9.88, and since the decimal is 0.8 or more, 10 conductors are allowed.

EMT, RMC and PVC

EMT is not smaller than trade size 1/2 (3/8 only for motor leads), and never threaded (358.20, 358.28). It can go in concrete, in earth with fittings identified for direct burial, and in dry, damp and wet locations (358.10(A)). It may not hold luminaires or other equipment except conduit bodies (358.12). Bends between pull points total no more than 360 degrees (358.24(B)), which is four 90 degree bends. EMT is fastened within 3 ft of each box or termination and at intervals of 10 ft or less (358.30(A)). RMC follows the same 3 ft and 10 ft pattern (344.30).

PVC conduit expands and contracts, so it is supported closer, by trade size: 3 ft for 1/2 to 1, 5 ft for 1-1/4 to 2, 6 ft for 2-1/2 to 3, 7 ft for 3-1/2 to 5, and 8 ft for 6 (Table 352.30(B)), plus a fastening within 3 ft of each box. Where the expected length change is 1/4 in. or more between fixed points, expansion fittings are required (352.44).

MC and NM cable

MC cable is secured at intervals of 6 ft or less, and within 12 in. of every box for cables of four or fewer conductors of 10 AWG or smaller (330.30). NM and NMC are supported at intervals of 4-1/2 ft or less and within 12 in. of each box (334.30), and their ampacity may not exceed the 60 degree C column even though the conductors are 90 degrees C, although the 90 degree C rating can be used for adjustment and correction (334.80).

The bigger question in commercial work is where NM may go. It is allowed in one- and two-family dwellings, in multifamily dwellings permitted to be Types III, IV or V construction, and in other structures permitted to be Types III, IV or V, where it is concealed behind a thermal barrier with at least a 15-minute finish rating (334.10). Outside those lists it is allowed only in Type I and Type II construction, and only where installed within raceways permitted there (334.10(5)), so open NM cable in a Type I or Type II office is out. It is also not allowed exposed above a dropped or suspended ceiling other than in dwellings, in commercial garages with hazardous locations, in theaters, in storage battery rooms, in hoistways, or embedded in concrete (334.12(A)). A building's construction type comes from the building code, so this lesson connects to the construction-type lessons in Commercial Building.

Box volume and box fill

A box must have at least the volume of everything inside it (314.16). Boxes not marked with a volume use Table 314.16(A). The rows most used are in cubic inches.

Box (inches)Volume
3 x 2 x 2-3/4 device14.0
3 x 2 x 3-1/2 device18.0
4 square x 1-1/221.0
4 square x 2-1/830.3
4-11/16 square x 2-1/842.0

Fill is figured by adding the parts in 314.16(B), using the Table 314.16(B)(1) allowance for the size involved: 14 AWG is 2.00 cu in., 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, and 6 AWG 5.00.

  • Each conductor that starts outside and ends or is spliced in the box counts once, and one that passes through counts once. A conductor that never leaves the box, such as a pigtail, does not count (314.16(B)(1)).
  • One or more internal cable clamps together count as one allowance for the largest conductor (314.16(B)(2)).
  • Each strap or yoke with devices counts double, from the largest conductor connected to it (314.16(B)(4)).
  • Up to four equipment grounding conductors count as one allowance, and each additional one adds one quarter of an allowance (314.16(B)(5)).
  • Small fittings such as locknuts and bushings are not counted.

Worked example: box fill

A feed-through receptacle sits in a 3 x 2 x 3-1/2 in. device box (18.0 cu in.) with 12 AWG THHN in EMT. Four conductors are counted (hot in, hot out, neutral in, neutral out) at 2.25 each, which is 9.00. The grounds count as one allowance, 2.25. The receptacle yoke counts double, 4.50. The total is 15.75 cu in., which fits. A 3 x 2 x 2-3/4 in. box holds only 14.0 and does not.

Second example: a 4 in. square box, 2-1/8 in. deep (30.3 cu in.), holds eight 12 AWG conductors (18.00), one internal clamp (2.25), three grounds (2.25) and one switch yoke (4.50). The total is 27.00 cu in., which fits with 3.3 cu in. to spare. The same box only 1-1/2 in. deep holds 21.0 and would fail.

Pull boxes and conduit bodies

Where conductors of 4 AWG or larger are involved, a pull or junction box is sized by the raceways (314.28(A)). In a straight pull the box is at least 8 times the trade size of the largest raceway (314.28(A)(1)), so a 3 in. conduit needs 24 in. In an angle or U pull, or with a splice, the distance from each raceway entry to the opposite wall is 6 times the largest raceway in the row, plus the sum of the other entries in the same row on the same wall (314.28(A)(2)). With a 3 in. and a 2 in. raceway entering one wall, that is 6 x 3 + 2 = 20 in. Conduit bodies with 6 AWG or smaller conductors must be at least twice the area of the largest conduit that attaches, and only volume-marked bodies can hold splices, taps or devices (314.16(C)).

Key numbers
  • Conduit fill: 53 percent for 1 conductor, 31 for 2, 40 for more than 2 (Chapter 9 Table 1); nipples of 24 in. or less: 60 percent
  • EMT 40 percent areas: 3/4 = 0.213, 1 = 0.346, 1-1/4 = 0.598, 1-1/2 = 0.814 sq in.
  • EMT: bends total 360 degrees or less between pull points; fasten within 3 ft of boxes and every 10 ft
  • PVC supports: 3 ft for 1/2 to 1 in., 5 ft for 1-1/4 to 2 in.; expansion fitting at 1/4 in. of movement
  • NM: every 4-1/2 ft and within 12 in. of boxes; as open cable only in Types III, IV, V (raceway only in I and II); 60 degree C ampacity limit
  • MC: every 6 ft; within 12 in. of boxes for four or fewer conductors of 10 AWG or smaller
  • Box allowance per conductor: 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 cu in.
  • Straight pull: 8 times the largest trade size; angle pull: 6 times plus the other entries in the row

In the field

A GCHS rehab converts a church annex into a community office. The plans show a wall of receptacles on 12 AWG in EMT. The inspector counts a few boxes: the shallow 3 x 2 x 2-3/4 in. boxes often do not have room for a feed-through receptacle with a ground, and a 4 in. square box with clamps and two devices is often full. At the panel, look at the conduit fill of the feeder ground. It is easy to forget when the ground is 6 AWG or larger. And ask the building construction type before any NM cable is accepted.

Practice questions

1. A Group B office feeder uses three 1/0 THHN copper conductors and a 6 AWG THHN copper ground in one run of EMT. Using the 40 percent column of Chapter 9 Table 4 and the THHN areas of Table 5, what is the smallest EMT trade size?

  1. 1-1/4 in., because the total of 0.6072 sq in. rounds down to the 0.598 limit
  2. 1 in., because only the three phase conductors are counted
  3. 1-1/2 in., because the total of 0.6072 sq in. is under 0.814 sq in.
  4. 2 in., because grounding conductors require the next larger size

2. Two 4/0 THHN copper conductors, with no separate ground, run in one EMT to a Group B pump. Using the two-conductor limit of 31 percent, what is the smallest trade size, given 1-1/2 in. EMT at 31 percent is 0.631 sq in. and 2 in. at 31 percent is 1.040?

  1. 2 in., because two 4/0 THHN total 0.6474 sq in., which is over 0.631
  2. 1-1/2 in., because the 40 percent area of 0.814 applies to two conductors
  3. 1-1/4 in., because the two conductors take only 0.3237 sq in. together
  4. 2-1/2 in., because two conductors always need one size more than three

3. A 20 in. EMT nipple between two panelboards in a Group B electrical room holds four 3/0 THHN conductors. The 40 percent area of 1-1/2 in. EMT is 0.814 sq in. and the 60 percent area is 1.221. THHN 3/0 is 0.2679 sq in. Is this fill allowed?

  1. No, because four conductors total 1.0716, which is over the 40 percent limit
  2. No, because nipples may never hold more than three conductors
  3. Yes, but only if the nipple is shorter than 12 in. between the enclosures
  4. Yes, since a nipple of 24 in. or less may be filled to 60 percent

4. An installer plans a run of EMT between two junction boxes in a Group B building that has five 90 degree bends and no pull point between them. What does 358.24(B) say?

  1. It is allowed, because the limit is 450 degrees for EMT
  2. Not allowed, since bends between pull points total 360 degrees at most
  3. It is allowed, because only bends over 90 degrees are counted
  4. It is allowed, if the conduit is one trade size larger than needed

5. A 30 ft horizontal run of EMT joins two boxes in a Group B warehouse. What is the maximum distance from a box to the nearest support under 358.30(A)?

  1. 3 ft
  2. 1 ft
  3. 5 ft
  4. 10 ft

6. A 2 in. trade size PVC conduit run is supported on the exterior wall of a Group B community building. What is the maximum spacing between supports under Table 352.30(B)?

  1. 3 ft
  2. 6 ft
  3. 10 ft
  4. 5 ft

7. A straight run of PVC conduit is exposed along the sunny south wall of a Group S-1 warehouse and is expected to change length by 1/2 in. between fixed boxes over the year. What does 352.44 require?

  1. No fitting, because PVC does not expand enough outdoors to matter for wiring
  2. An expansion fitting, because the length change is 1/4 in. or more
  3. An expansion fitting only if the change is over 2 in.
  4. A rigid metal conduit sleeve instead of an expansion fitting

8. A new two-story Group B office building of Type II-B (noncombustible) construction is being wired. Can Type NM cable, run in walls and above ceilings and not in a raceway, be used for its branch circuits under 334.10?

  1. Yes, if the cable is concealed in walls behind gypsum board
  2. Yes, if the building is under 3 stories and has a sprinkler system
  3. No, since open NM is allowed only where Type III, IV or V is permitted
  4. Yes, if the circuits are 20 A or less and use 12 AWG copper

9. In a Group B office building of Type V-A construction, an electrician runs Type NM cable exposed above a suspended acoustical tile ceiling. What does 334.12(A) say?

  1. Not permitted, since NM may not be exposed in a suspended ceiling cavity outside dwellings
  2. It is permitted, because Type V construction allows NM everywhere, including exposed in ceiling cavities
  3. It is permitted, if the cable is supported every 4-1/2 ft and within 12 in. of boxes
  4. It is permitted, if the cable is 14 AWG or smaller and runs under 25 ft

10. Type NM cable runs along the side of joists in a Group R-2 apartment building of Type V construction. What are the maximum spacing of supports and the distance to the first support at a box under 334.30?

  1. 6 ft between supports and within 12 in. of each box
  2. 4-1/2 ft between supports and within 36 in. of each box
  3. 4-1/2 ft between supports and within 12 in. of each box
  4. 3 ft between supports and within 6 in. of each box

11. MC cable with three 12 AWG conductors and a bonding conductor is run above a ceiling in a Group B office. Under 330.30(B), where must it be secured?

  1. Within 3 ft of every box, and at intervals of 10 ft or less
  2. Within 12 in. of every box, and at intervals of 6 ft or less
  3. Within 12 in. of every box, and at intervals of 4-1/2 ft or less
  4. Only at the boxes, because it is armored cable

12. A Group B junction box is a 4 in. square box, 1-1/2 in. deep (21.0 cu in.). It holds ten 12 AWG THHN conductors that end in the box, one 12 AWG ground and no devices or clamps. What is the fill, and does it pass?

  1. 22.5 cu in., so it passes in the 21.0 box after rounding down
  2. 24.75 cu in., so it passes because ground wires are not counted
  3. 20.25 cu in., so it passes with 0.75 cu in. to spare in the 21.0 box
  4. 24.75 cu in., so it fails; a 2-1/8 in. deep box (30.3) is needed

13. A feed-through duplex receptacle on 12 AWG THHN in EMT is planned for a 3 x 2 x 2-3/4 in. device box (14.0 cu in.) in a Group B office. What does the fill calculation show?

  1. 11.25 cu in., so the box is large enough for the four conductors and device
  2. 13.5 cu in., so the box is large enough because the device counts once
  3. 15.75 cu in., too small; a 3 x 2 x 3-1/2 in. box (18.0) is needed
  4. 15.75 cu in., but the box passes because grounds are exempt

14. A Group B pull box receives 4/0 copper conductors in a 3 in. conduit in one wall and leaves through the opposite wall in a straight line. What is the minimum length of the box under 314.28(A)(1)?

  1. 18 in., which is 6 times the trade size of the largest raceway
  2. 24 in., which is 8 times the trade size of the largest raceway
  3. 12 in., which is 4 times the trade size of the largest raceway
  4. 30 in., which is 10 times the trade size of the largest raceway

Glossary

Conduit fill
The share of a conduit's cross-section taken up by conductors; 53, 31 or 40 percent depending on how many conductors (Chapter 9 Table 1).
Nipple
A short piece of conduit, 24 in. or less, joining two enclosures; it may be filled to 60 percent.
Trade size
The standard name of a conduit size, such as 1-1/2, used for identification and not an actual dimension.
Pull point
A box or conduit body where conductors can be pulled; bends in a run are limited to 360 degrees between pull points.
Type NM
Nonmetallic-sheathed cable, outside dwellings it is allowed as open cable only in buildings of Type III, IV or V construction.
Type MC
Metal-clad cable with an interlocked armor, secured every 6 ft and near each box.
Box fill
The total volume allowance for conductors, clamps, devices and grounds in a box, compared with the box's volume.
Straight pull
A pull where the conductors enter and leave a box on opposite walls; the box must be 8 times the largest trade size long.
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