MikeHolt_Section02_Conductor Sizing and Protection


Unit 06: Conductor Sizing and Protection Calculations

NEC Article 240, 310, 400, 402

  • Conductor heat is directly proportional to wire resistance and the square of current flow
  • Conductors 8 AWG & larger must be stranded when installed in a raceway
    • 310.106(C)
  • Conductor sizing determining factors:

    • current flow & resulting heat (ampacity adjustment)
    • number of conductors in same raceway (bundle amp adjustment)
    • ambient temperature
    • terminal temperature ratings
    • continuous load factor
    • overcurrent protection
    • conductor insulation temperature rating
  • Table 310.104(A)

    • Conductor insulation properties
      • letter type
      • operational temperature
      • application
      • insulation
      • outer cover properties
    • In general, only conductors in this table can be used unless permitted elsewhere in the NEC
  • Conductor Insulation Letter Ratings

    • Letter Designation Description
      No H 60°
      H 75°
      HH 90°, Dry
      -2 90°, Wet
      N Nylon Outer Cover
      T Thermoplastic
      U Underground
      W Wet or Damp
      X Cross-linked Polyethylene
      R (rubber – replaced by:) Thermoset
    • ?? Why is H & W identified in THWN-2 cable?  Isn’t this redundant??
  • Article 400

    • Tables 400.4 / 400.5(A) / 400.5(B)
    • Flexible cords and cables
  • Article 402
    • Tables 402.3 / 402.5
    • Fixture wires
  • Conductor sizes

    • 110.14(C) (70-45
      • Temperature Limitations
    •  AWG
      • 18 to 4/0 (4-ought)
      • larger than 4/0 is expressed in kcmil
        • 250 kcmil & up
    • kcmil
      • kcmil = 1000’s (K) of (C)ircular (M)ils
      • Mil means 1/1000 inch
      • Circular Mil means the area of a circle 1/1000 inch in diameter
      • 20 AWG = 1 kcmil
    • Table 310.15(B)(16)(A & B)
      • **Conductor Insulation Temperature Ratings
      • Equipment terminations
      • Conductor sizing
    • Equipment Ratings
      • ≤ 100 amps
        • 60°C
          • 110.14(C)(1)(A)(1)
          • Must be sized to table 310.15(B)(16)
        • 75°C
          • 110.14(C)(1)(A)(3)
          • Must be sized to table 310.15(B)(16)
          • Motor terminals
            • must be design letters B, C, D (on nameplate)
            • 110.14(C)(1)(A)(4)
            • Can be used within 75° column of table 310.15(B)(16)
      • ≥ 100 amps
        • 75°C
          • 110.14(C)(1)(B)(1)
          • Larger than 1 AWG
            • Must be sized to table 310.15(B)(16)
  • Branch Circuits Conductor sizing

    • Minimum 14 AWG
    • Smaller than 14 AWG permitted for:
      • class 1 remote controls
        • 725.43
      • Fixture wire
        • 402.6
      • Motor Control Circuits
        • Table 430.72(B)
  • Over-current

    • Article 240
    • Transformers secondary circuit can use the primary circuit for over-current protection
    • Conductors must be protected at the point receiving the supply
      • 310.15(B)(16)
      • Except for 240.4(A) to (G)
      • 310.15
        • Conductor amperage correction and adjustments
    • Must be on-the-line side of terminals
      • 110.9
    • Must have short circuit protection
      • 110.10 note
    • Causes by
      • Overload
      • short circuit
      • ground fault
    • Over-current ≤ 800 amps

      • 240.4(B)
      • Next higher rating permitted providing:
      • 240.6(A)
        • not supplying more than 1 receptacle cord-and-plug
        • conductor sizing requires
          • ambient temperature correction
            • 310.15(B)(2)(A)
          • conductor bundling adjustment
            • 310.15(B)(3)(A)
          • last two points don’t correspond with fuse nor circuit breaker ratings
        • not over 800 amps
    • Over-current≥ 800 amps

      • 240.4(C)
      • must have a rating not less than the over-current device
    • Over-current Small Conductors

      • 240.4(D)
      • Exceptions
        • 240.4(E)
        • 240.4(G)
      • Must not exceed:
        • Wire Size, AWG Amperage
          18-Cu 7
          16-Cu 10
          14-Cu 15
          12-Al 15
          12-Cu 20
          10-Al 25
          10-Cu 30
      • Exception 240.4(G)
        • Must comply with table 240.4(G)
        • Motors, Article 430
          • Over-current protection, Article 430
        • A/C equipment, Article 440
          • Over-current protection, Article 440.22
  • Conductor Ampacity

    • Conductor Ampacity Calculation

      • Table 310.15(B)(16)
      • Table 310.15(B)(2)(a)_ambient temp above 86°F/30°C
      • Table 310.15(B)(3)(a)_4+ hot conductors
    • Table 310.15(B)(16)

      • allowable ampacity of insulated conductors
      • Provided:
        • No more than 3 hot/ungrounded/current-carrying conductors
          • if 4 or more, correct using Table 310.15(B)(3)(a)
        • ambient temperature of 86°F/30°C
          • if not correct using Table 310.15(B)(2)(a)
      • **When using this table, if the ambient temperature where installed is outside 78°-86°C, the ampacity must be adjusted using table 310.15(B)(2)(a)
      • **When using this table, if there are more than 3 bundled current-carrying conductors, the ampacity must be adjusted using table 310.15(B)(3)(a)
    • Table 310.15(B)(2)(a)

      • Ampacity Correction
      • Ambient temperature correction formula
        • Corrected Amps = Table 310.15(B)(16) amps * Ambient Temp. Correction Factor table 310.15(B)(2)(a)
    • Table 310.15(B)(3)(a)

      • Conductor Ampacity Adjustment after table 310.15(B)(16)
        • Used when more than 3 bundled current-carrying conductors installed in any cross-sectional wireway
      • Conductor Bundling Ampacity adjustment formula
        • Adjusted Amps = Table 310.15(B)(16) amps * Bundled amp Adjustment Factor table 310.15(B)(3)(a)
      • Reduce conductor ampacity  when:
        • 4 or more hot conductors
        • raceway or cable is longer than 24″
        • adjust according to table 310.15(B)(3)(a)
    • Wet locations

      • 300.5(B)
      • 300.9
      • 310.10(C)
      • 300.6(D)
    • Rooftops

      • 310.15(B)(3)(6), pg 70-148
      • When exposed to sunlight and less than 7/8″ above a roof, add 60°F/33°C to table 310.15(B)(2)(a)
        • Exception: Cable XHHW-2 not subject to this rule
    • Cable Types AC and MC

      • Special amperage, adjustment rules do not apply
      • No outer jacket
      • No more than 3 current-carrying conductors
      • Conductors are 12 AWG copper
      • No more than 20 conductors installed without spacing, with length ≥ 24″
    • Conductor Ampacity Notes:

      • Neutral conductor
        • Only considered a current-carrying conductor according to 310.15(B)(5)
      • Equipment conductors
        • Never considered current-carrying conductors according to 310.15(B)(6)
      • Corrections based on conductors insulation temperature rating
        • determined by Table 310.15(B)(16)
        • not by terminal rating, 110.14(C)
      • Correct for multiple conditions when:

        • more than 3 hot conductors, and
        • ambient temperature is not between 78°-86°F
        • Combine adjustments of ambient temperature and conductor buldling
        • Adjusted & Corrected Ampacity = Table 310.15(B)(16) amps * Temperature factor * Bundled adjustment factor
      • Lower Ampacity Rule

        • When more then 1 ampacity applies, use the lower amperage rating
      • Higher ampacity Exception

        • 310.15(A)(2)(Ex)
        • If different amperage’s apply to different areas of one circuit, the higher amperage rating can be used if the lower amperage cable area :
          • length is not over 10′ or
          • ≥ 10% of total circuit length
      • Neutral, Bonding, and Grounding Conductors

        • refer to 310.15(B)(5) and (6)
        • Does not apply to conductors in cable trays
        • refer to 392.80
      • Neutral Conductor Current-Carrying Determination

        • Circuit Type Neutral Carrying Current Notes
          2-wire Yes 310.15()()()
          3-wire, 1 Ø, 120/240V, Wye-connect No 310.15(B)(5)(a)
          4-wire, 3 Ø, 120/208V or 277/480V, Wye-connect No 310.15(B)(5)(a)
          3-wire-from-4-wire, 3 Ø, Wye-connect Yes 310.15(B)(5)(b)

          4-wire, 3 Ø, 120/208V or 277/480V, Wye-connect

          (if more than 50% of neutral is non-linear load)

          Yes 310.15(B)(5)(c)
        • Non-Linear load is when the current wave form does not follow the voltage wave form
        • Unbalanced 3-wire Wye Secondary Neutral Current Formula
          • (INeutral = √(ILine12 + ILine22) – (ILine1 * ILine2)
  • Miscellaneous Over-current Protection and Conductor Sizing

    • Branch Circuits

      • Over-current protection
        • 210.20(a)
        • Must have a rating of not less than 125% of continuous loads, plus 100% of non-continuous loads
      • Conductor sizing
        • Must have an ampacity of not less than the maximum load served
        • Must be the larger of these 2 options:
          1. conductors must be sized with a rating of not less than 125% of continuous loads, plus 100% of non-continuous loads, based on terminals
          2. Temperature rating ampacity of table 310.15(B)(16)
            • Minimum size allowable amperage not less than maximum load to be served
    • Appliance Over-Current Protection

      • 422.11(E)
      • Not to exceed marked rating
      • If rating is unmarked
        • Rated ≤ 13.30 amps
          • not to exceed 20 amps
        • Rated ≥ 13.30 amps
          • 422.11(E)(3)
          • Not to exceed 150% of rated current
            • Note: Next higher rating permitted when amperage falls inbetween (2406(A)-pg70-95)
    • Water Heater Branch Circuit Over-Current Protection

      • 422.13
      • ≤ 120 gallon is a continuous load
        • not less than 125% continuous loads, refer to 210.19(A)(1)
      • Branch circuit conductor
        • 422.10
        • 125% of amperage of continuous load
      • Over-current protection device
        • sized to 422.11(E)(3)
    • Electric Space Heating Branch Circuit
      • 424.3(B)
      • considered a continuous load
      • Branch circuit and over-current protection
        • 210.19(A)(1)
        • 210.20(A)
        • Must be sized not smaller that 125% of the total load (heater and motor)
    • REVIEW MH-PG 269-QUESTIONS-FIG. 6-64-3 STEPS
  • Feeders

    • Feeder Over-Current Protection

      • 215.3
      • Must have a rating of not less than 125% of continuous loads, plus 100% of non-continuous loads
    • Feeder Conductor Sizing

      • 215.2(A)(1)
      • Must be sized to larger of:
        • 215.2(A)(1)(A) (has exceptions)
          • minimum feeder conductor ampacity is not less than 125% of continuous loads, plus 100% of non-continuous loads
          • 215.2(A)(1)(A)(Ex2)
            • Feeder conductors with a junction box at both ends into 90°C terminals can have an ampacity of not less than 100% of continuous loads and 100% of non-continuous loads based on the 90°C column of table 310.15(B)(16)
            • 110.14(C)(2), pg 70-45
            • ADD PHONE PICTURE
        • or
        •  215.2(A)(1)(B)
          • determined after ampacity adjustment & correction & not less than the load to be served
    • Feeder Circuits

      • 75°C
        • 125% of continuous loads for circuits ≥ 100 amps
          • 215.2(A)(1)(A)
        • Conductors sized to 75°C on table 315(B)(16) / 110.14(C)(1)(b)
        • Rated ≤ 800 amps
          • over-current protection to 240.4(B)
        • Rated ≥ 800 amps
          • over-current protection to 240.4(C)
        • Continue from pg 12
        • ADDED FROM PHONE
        • Neutral sized to 100% of continuous load to 75°C column of table 310.15(B)(16), 110.14(C)(1)(B), based on maximum unbalanced load according to 220.61 and not smaller then required for equipment grounding conductors by 220.122
        • 215.2(A)(1)(Ex3)
      • 90°C
        • Sized to 100% of continuous load according to 90C column on table 310.15(B)(16), 110.14(C)(2)
          • 215.2(A)(1)(a)(Ex1)
        • Rated ≤ 800 amps
          • over-current protection to 240.4(B)
        • Rated ≥ 800 amps
          • over-current protection to 240.4(C)
        • Neutral conductor
          • Sized to 100% of continuous load to 75C column on table 310.15(B)(16) based on maximum unbalanced load according to 220.61 and not smaller than 250.122 for equipment grounding conductors
            • 215.2(A)(1)(Ex3)
            • 110.14(C)(1)(b)
        • Exception 3
          • Neutral conductors must have an ampacity of not less than 100% of continuous and non-continuous loads
        • Feeder Over-current Protection
          • 215.3
            • Device sizing requirements for continuous and non-continuous loads
        • Feeder Taps
          • Tap is a conductor (not a service conductor) that has over-current protection rated higher than normally allowed in 240.2
          • Over-current protection must be placed where branch circuits or feeder conductors receive power
            • Except as permitted by (a)-to-(h)
          • 10′ feeder tap conductors
            • No over-current protection needed provided:
              • Ampacity is greater than the:
                • Calculated load according to article 220
                • rating of over-current protection device equipment and termination containing over-current protection supplied by tap conductors
              • must not extend beyond the equipment they supply
              • are installed within a raceway leaving an enclosure
              • must have an ampacity of not less than a 10% rating of the over-current protection device protecting the feeder
          • NOTE:  408.36 = over-current protection for panelboards
          • 25′ feeder tap conductors
            • No over-current protection needed provided:
              • Ampacity is not less than 1/3rd the rating of the over-current protection device protecting the feeder
              • terminate in an over-current protection device rated not more than the tap conductor ampacity from 310.5, table 310.15(B)(16)
          • Feeder Tap Conductors with unlimited length outside
            • No over-current protection needed for unlimited length provided:
              • protected from physical damage within a raceway
              • Terminates at a single over-current protection device limiting load to the ampacity of the outside feeder tap conductors
              • over-current protection device for outside feeder tap conductors is part of building feeder disconnect
              • Disconnecting means is readily accessible
  • Unit 6 Conclusion

    • NEC tables are based on 86°F/30°C
      • Conductor amperages must be corrected for other temperatures
    • NEC requires ampacity adjustments when bundling more than 3 current-carrying conductors
    • Section 110.14(C) is a confusing part of code
      • Do not size conductors to a higher rating then the terminals
    • With requirement of a rating of not less that 125% of continuous loads, plus 100% of non-continuous loads
      • Non-continuous load is not added when not mentioned