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Illustrated Changes to the NEC, 2005 Edition - Part 1

Article 310

Conductors for General Wiring

310.15 Conductor Ampacity

Text added to clarify how each current-carrying conductor of a parallel conductor set is to be treated when applying the ampacity adjustment factors of Table 310.15(B)(2)(a). Additional text clarifies that the reduced size feeder conductors permitted in Table 310.15(B)(6) only applies for individual dwelling units of one-family, two-family and multifamily dwellings.

(B) Ampacity Table. The allowable conductor ampacities listed in Table 310.16 are based on conditions where the ambient temperature isn’t over 86°F and no more than three current-carrying conductors are bundled together. Figure 310-3

Author’s Comment: When conductors are installed in an ambient temperature other than 78–86°F, ampacities listed in Table 310.16 must be corrected in accordance with the multipliers listed in Table 310.16.

(2) Ampacity Adjustment
(a) Conductor Bundle.
Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled in lengths exceeding 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a).

Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.
Figure 310-4

Table 310.15(B)(2)(a) Conductor Adjustment Factors for Conductors in a Raceway or Cable
Current–Carrying Ampacity Multiplier
1–3 Conductors     1.00
4–6 Conductors     0.80
7–9 Conductors     0.70, Figure 310-5
10–20 Conductors 0.50

(6) Dwelling Unit Feeder/Service Conductors. For individual dwelling units of one-family, two-family and multifamily dwellings, Table 310.15(B)(6) can be used to size 3-wire, single-phase, 120/240V service or feeder conductors that serve as the main power feeder to a dwelling unit. Figure 310-6


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