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 properties

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 Crosslinked Polyethylene R (rubber – replaced by:) Thermoset  ?? Why is H & W identified in THWN2 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) (7045
 Temperature Limitations
 AWG
 18 to 4/0 (4ought)
 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)
 60°C
 ≥ 100 amps
 75°C
 110.14(C)(1)(B)(1)
 Larger than 1 AWG
 Must be sized to table 310.15(B)(16)
 75°C
 ≤ 100 amps
 110.14(C) (7045

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)
 class 1 remote controls

Overcurrent
 Article 240
 Transformers secondary circuit can use the primary circuit for overcurrent 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 ontheline side of terminals
 110.9
 Must have short circuit protection
 110.10 note
 Causes by
 Overload
 short circuit
 ground fault

Overcurrent ≤ 800 amps
 240.4(B)
 Next higher rating permitted providing:
 240.6(A)
 not supplying more than 1 receptacle cordandplug
 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
 ambient temperature correction
 not over 800 amps

Overcurrent≥ 800 amps
 240.4(C)
 must have a rating not less than the overcurrent device

Overcurrent Small Conductors
 240.4(D)
 Exceptions
 240.4(E)
 240.4(G)
 Must not exceed:

Wire Size, AWG Amperage 18Cu 7 16Cu 10 14Cu 15 12Al 15 12Cu 20 10Al 25 10Cu 30

 Exception 240.4(G)
 Must comply with table 240.4(G)
 Motors, Article 430
 Overcurrent protection, Article 430
 A/C equipment, Article 440
 Overcurrent 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/currentcarrying 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)
 No more than 3 hot/ungrounded/currentcarrying conductors
 **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 currentcarrying 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 currentcarrying conductors installed in any crosssectional 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)
 Conductor Ampacity Adjustment after table 310.15(B)(16)

Wet locations
 300.5(B)
 300.9
 310.10(C)
 300.6(D)

Rooftops
 310.15(B)(3)(6), pg 70148
 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 XHHW2 not subject to this rule

Cable Types AC and MC
 Special amperage, adjustment rules do not apply
 No outer jacket
 No more than 3 currentcarrying 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 currentcarrying conductor according to 310.15(B)(5)
 Equipment conductors
 Never considered currentcarrying 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 CurrentCarrying Determination

Circuit Type Neutral Carrying Current Notes 2wire Yes 310.15()()() 3wire, 1 Ø, 120/240V, Wyeconnect No 310.15(B)(5)(a) 4wire, 3 Ø, 120/208V or 277/480V, Wyeconnect No 310.15(B)(5)(a) 3wirefrom4wire, 3 Ø, Wyeconnect Yes 310.15(B)(5)(b) 4wire, 3 Ø, 120/208V or 277/480V, Wyeconnect
(if more than 50% of neutral is nonlinear load)
Yes 310.15(B)(5)(c)  NonLinear load is when the current wave form does not follow the voltage wave form
 i.e.: Triplen Harmonic Currents
 Unbalanced 3wire Wye Secondary Neutral Current Formula
 (I_{Neutral} = √(I_{Line1}^{2} + I_{Line2}^{2}) – (I_{Line1} * I_{Line2})

 Neutral conductor


Miscellaneous Overcurrent Protection and Conductor Sizing

Branch Circuits
 Overcurrent protection
 210.20(a)
 Must have a rating of not less than 125% of continuous loads, plus 100% of noncontinuous loads
 Conductor sizing
 Must have an ampacity of not less than the maximum load served
 Must be the larger of these 2 options:
 conductors must be sized with a rating of not less than 125% of continuous loads, plus 100% of noncontinuous loads, based on terminals
 Temperature rating ampacity of table 310.15(B)(16)
 Minimum size allowable amperage not less than maximum load to be served
 Overcurrent protection

Appliance OverCurrent 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)pg7095)
 Rated ≤ 13.30 amps

Water Heater Branch Circuit OverCurrent 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
 Overcurrent protection device
 sized to 422.11(E)(3)
 Electric Space Heating Branch Circuit
 424.3(B)
 considered a continuous load
 Branch circuit and overcurrent protection
 210.19(A)(1)
 210.20(A)
 Must be sized not smaller that 125% of the total load (heater and motor)
 REVIEW MHPG 269QUESTIONSFIG. 6643 STEPS


Feeders

Feeder OverCurrent Protection
 215.3
 Must have a rating of not less than 125% of continuous loads, plus 100% of noncontinuous 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 noncontinuous 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 noncontinuous loads based on the 90°C column of table 310.15(B)(16)
 110.14(C)(2), pg 7045
 ADD PHONE PICTURE
 or
 215.2(A)(1)(B)
 determined after ampacity adjustment & correction & not less than the load to be served
 215.2(A)(1)(A) (has exceptions)

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
 overcurrent protection to 240.4(B)
 Rated ≥ 800 amps
 overcurrent 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)
 125% of continuous loads for circuits ≥ 100 amps
 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
 overcurrent protection to 240.4(B)
 Rated ≥ 800 amps
 overcurrent 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)
 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
 Exception 3
 Neutral conductors must have an ampacity of not less than 100% of continuous and noncontinuous loads
 Feeder Overcurrent Protection
 215.3
 Device sizing requirements for continuous and noncontinuous loads
 215.3
 Feeder Taps
 Tap is a conductor (not a service conductor) that has overcurrent protection rated higher than normally allowed in 240.2
 Overcurrent protection must be placed where branch circuits or feeder conductors receive power
 Except as permitted by (a)to(h)
 10′ feeder tap conductors
 No overcurrent protection needed provided:
 Ampacity is greater than the:
 Calculated load according to article 220
 rating of overcurrent protection device equipment and termination containing overcurrent 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 overcurrent protection device protecting the feeder
 Ampacity is greater than the:
 No overcurrent protection needed provided:
 NOTE: 408.36 = overcurrent protection for panelboards
 25′ feeder tap conductors
 No overcurrent protection needed provided:
 Ampacity is not less than 1/3rd the rating of the overcurrent protection device protecting the feeder
 terminate in an overcurrent protection device rated not more than the tap conductor ampacity from 310.5, table 310.15(B)(16)
 No overcurrent protection needed provided:
 Feeder Tap Conductors with unlimited length outside
 No overcurrent protection needed for unlimited length provided:
 protected from physical damage within a raceway
 Terminates at a single overcurrent protection device limiting load to the ampacity of the outside feeder tap conductors
 overcurrent protection device for outside feeder tap conductors is part of building feeder disconnect
 Disconnecting means is readily accessible
 No overcurrent protection needed for unlimited length provided:
 Sized to 100% of continuous load according to 90C column on table 310.15(B)(16), 110.14(C)(2)
 75°C


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 currentcarrying 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 noncontinuous loads
 Noncontinuous load is not added when not mentioned
 NEC tables are based on 86°F/30°C