MikeHolt_Section03_Raceway, Conductors, and Box sizing


Unit 05: Raceway, Conductors, and Box Calculations

NEC Chaper 9, Annex C, Article 300, 314, 376

NEC Code Book

  • Annex C
    • raceway interior cross-sectional area maximum fill-limit percentages by conductors of the same size
  • NEC Ch.09, Table 1
    • raceway interior cross-sectional area maximum fill-limit percentages by conductors
    • raceways 2′ or less can be filled to 60%
    • When raceway ID is between 2.8 to 3.2 x the conductor diameter, jamming can occur
  • NEC Ch.09, Table 4
    • Raceways cross-sectional area and fill percent area
    • Various raceway types with columns representing allowable percentage fills
    • Use caution when selection raceway
      • divided into numerous tables for each raceway type
      • must choose the correct table section for the raceway type performing calculations on
  • NEC Ch.09, Table 5
    • Conductor cross-sectional area, round, concentric-lay-stranded, giving worst-case scenario
      • Note: concentric is multiple circles sharing the same center
  • NEC Ch.09, Table 5A
    • Conductor cross-sectional area, round, compact-stranded, giving worst-case scenario
  • NEC Ch.09, Table 8
    • Conductor cross-sectional area, round, concentric-lay-stranded, uninsulated, giving worst-case scenario
    • Circular mils table
    • number of strands per conductor
      • 4th column – ‘Stranding’, ‘Quantity’
      • ‘1’ denotes solid conductor
    • DC resistance @ 75°
  • NEC Ch.09, Table Notes
    • Note (1) Use Annex C with same size conductors
    • Note (2) Table 1 doesn’t apply to raceways used for conductor protection
    • Note (3) When calculating raceway fill, use equipment grounding and bonding conductors area too
    • Note (4) Raceways 2′ or less can be filled to 60%
    • Note (5) With Chapter 9 not-included conductors (multi-conductor & optical fiber), use actual dimensions
    • Note (6) Tables 4, 5, 5A summaries
    • Note (7) When using Annex C, calculating # of same-size conductors to fill conduit/tubing, round up to whole # when decimal  ≥ 0.8
    • Note (8) Table 8, bare conductors
    • Note (9) With elliptical cables (multi-conductor & optical fiber), use larger cross-section
      • If only cable diameter is given, determine the conductor Sq in. area using this forumla
        • Conductor Area = Area = 3.14 x (Dia. x ½)2
    • Note (10) Tables 5, 5A summaries
  • Table 310.104(A)
    • Conductor Applications and Insulation
    • differences in conductor insulation thickness
  • Table 314.16(A)
    • outlet boxes conductor limits

Conductors

  • Conductor Area
    • Area = 3.14 x (Dia. x ½)2
  • Raceway fill Limitation Ratio
    • Ratio = Raceway Diameter / Conductor Diameter

Raceways

  • Distance between raceways enclosing the same conductor must not be closer than 6 times the trade size of the largest raceway measured from nearest edges
  • Conductors 8 AWG & larger must be stranded when installed in a raceway
    • 310.106(C)
  • Raceway Trade Size

    • use Ch.9-Table 4
    • Metal Wireways

      • Article 376
      • Limits conductor cross-sectional area , number of conductors, wire-bend radius,
  • Raceway Sizing

    • 300.17
    • Raceway Sizing StepsNEC-2017 ReferenceNotesShorthand
      Determine each conductors cross-sectional areaCh9-Table 5Insulated conductorsconductor x-sec area
      Ch9-Table 8Bare conductorsconductor total x-sec area
      Determine total cross-sectional area for all conductors
      Size raceway according to percent fillCh9-Table 1raceway % fill
      Ch9-Table 4see notes above
  • Raceways with multiple Rows

    1. Calculate each row individually as shown above
    2. Use row with largest distance
  • Raceway fill Limitation Ratio

    • Ratio = Raceway Diameter / Conductor Diameter
    • Ratio between 2.8 to 3.2 can cause jamming

Electrical Outlet Box Fill

  • 314.16
  • Calculations:
  1. Determine conductor number & sizes within the box
  2. Determine conductor volume equivalents from Table 314.16(B)
  3. Size box using Table 314.16(A)
    • Conductors same-size = add together & size box using Table 314.16(A)-AWG size columns
    • Conductors different sizes = Finds area of each conductor using Table 314.16(B), add them up, size box using Table 314.16(A)-cu” column
    • Draw out picture to problem solve
    • Volume of box calculated by 314.16(A) can never be less than the volume required by calculations based on 314.16(B)

Go over pages 209-214, this is complicated

  • Insulation type is not a sizing factor with Box Fill calculations
  • Table 314.16(A)
    • Determine outlet box size required for given number of same-sized conductors and number of conductors permitted within
    • Table does not reflect need for allowance for switches, respectacles, etc.
  • Box Volume Barriers
    • if unmarked:
      • ½ in3 if metallic
      • 1 inif non-metallic
  • 314.16(B)(1) through (5)
    • determines total volume of conductors. devices, & fittings
      1. Conductor Volume
        • Count as a single conductor every conductor running through a box and each conductor terminating in a box (314.16(B)
        • Count loops 12″ or longer as 2 conductors (300.14)
        • Conductors looping through box 12″ or less are not counted
      2. Cable Clamp Volume
        • based on largest entering conductor, all internal cable clamps count as 1 conductor
        • Clamping mechanisms outside the box count for 0
      3. Support Fitting Volume
        • based on largest entering conductor, each luminaire stud or hickey is counted as 1 single conductor volume
      4. Device Yoke Volume
        • based on largest entering conductor
          • each single-gang device yoke is counted as 2 conductor volumes
          • each multigang device yoke is counted as 2 conductor volumes
      5. Equipment Grounding Conductor Volume
        • Count all entering as 1 using size from largest entering box
        • Use all attached to receptacles as 1
  • 314.28
    • Conductors 4 AWG and larger
  • Box Sizing Formulas
    • Remaining Space = Total space less the area required for existing conductors
    • Conductors added = Remaining space / Added Conductors Volume

Junction Boxes, Pull Boxes, and Conduit Bodies

  • Junction box & pull box calculations only calculated when conductor sizes are 4 AWG & larger
  • 314.28
    • Conductors 4 AWG and larger requiring insulation
    • Minimum sizes
      • Straight Pulls
        • When  a conductor enters and than exits on the opposite wall
        • Minimum box dimension must not be less than 8 times the trade size of the largest raceway
        • Straight Pull calculation required when conductor enters one side and exits on the opposite wall
      • Angle Pulls
        • When  a conductor enters and than exits 90° from the entrance wall
        • Minimum box dimension must not be less than 6 times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall
      • U Pulls
        • When  a conductor enters and exits on the same wall
        • Minimum box dimension must not be less than 6 times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall
      • Splices
        • When  a conductor is spliced
        • Distance from where the raceways enter to the opposite wall must not be less than 6 times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall
    • To size boxes, calculate:
      1. Minimum box size, and
      2. Distance between raceways, and
      3. possibly multiple row calculations
        • MH-pg 215-216
    • Box sizing tips
      • Draw out problem
      • Calculate horizontal
        • Left-to-right, straight
        • Right-to-left, straight
        • Left-to-right angle &/or U-pull
        • Right-to-left angle &/or U-pull
      • Calculate Vertical
        • Top-to-Bottom, straight
        • Bottom-to-Top, straight
        • Top-to-Bottom angle &/or U-pull
        • Bottom-to-Top angle &/or U-pull
      • Calculate raceway distances enclosing same conductor
        • Problem examples in MH-pg 217