This article was posted 02/23/2007 and is most likely outdated.

Article 310 Conductors for General Wiring
 

 
Subject - Article 310 Conductors for General Wiring

February 23, 2007  

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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


Figure 310-3
(Click on image to enlarge)


Figure 310-4
(Click on image to enlarge)


Figure 310-5
(Click on image to enlarge)

 
 

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Comments
  • I am confused about Figure 310-6. It depicts what I consider the grounded neutral current carrying conductor, yet references 250.66, which talks about the grounding electrode conductor. Am I missing something here? Thanks, Sam

    Sam Williams
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  • Can Table 310.15(B)used for a sub-panel in a single family dwelling?

    Romeo Ethier
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  • I really wish the NEC would assume 40C/104F or even 50C/122F as their starting point for all this rather than 30C/86F.

    How hot is your attic in the summer? How hot is it in Phoenix? I worked in a factory in MT in the summer and it was well over 100F inside. When the sun beats down, it beats down, even in Alaska.

    I sometimes think I'm the only person who temperature de-rates conductors.

    Most instruments and machines are stamped for 40-50C max operating temperature and you know the inside will be hotter still.

    Yet when you get these little handy wheel chart shortcut toys and such, they have no de-rating.

    I had a friend who was chronically late, so we'd just tell him the (fill in blank) started 45 minutes earlier than it really did, and it worked. This is the same deal.

    Matt
    Reply to this comment

  • Just woundered if cables built with heat carring materials installed were ever concidered for general use?

    Ralph Beaster
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  • THIS MAKE'S THING'S SO MUCH CLEARER THANK,S

    DAVID KLEIN
    Reply to this comment

  • On Figure 310-4, aren't the paralleled conductors all supposed to be in the same conduit run? Is the bottom figure correct? I thought I saw this somewhere but can't remember or find it.

    Matt
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  • From the way I see 310.15(B)(6), the reduced feeder size does not apply in a multifamily situation where the unit feeder is derived from a 120/208 volt, 3-phase service whether the unit feeder is single phase or three phase since the voltage is not 120/240. Is this true?

    John McGovern
    Reply to this comment

  • In a dwelling with romex run through bored holes , does the neutral of a 2 wire 120v circuit count as a current carring conductor or does it fall under 310.15(B)(4)(a).

    Ray
    Reply to this comment

  • This is insane this is never enforced anywhere that I know of it was important about 15 yrs ago when the new electronic ballasts were throwing off sick harmonics and proportional heat but nowadays the ees have gotten better with power supply and ballasts to eliminate harmonics. The old cables used to cook under full load but those ballasts are long gone

    sherman
    Reply to this comment

  • As I understand it, the neutral of a 3ph-4W circuit is considered conductor #4 and requires that an 80% multiplier be used for just one 4W circuit (plus gnd)? True or false?

    Jeff
    Reply to this comment

  • Hi Mike. I have a question,concerned to the word -PHASE.

    I understan the word how is applicable inside de motors,maybe is more easy cacht the idea in my brain,but I can't to diggestive in Service Box,when the cables enter the box, for example three cable enter in the box , each one is called Phase: PHASE A PHASE B PHASE C

    Please,explain to me,because I have a little confuse about this, I will appreciate.

    Alejandro Somarriba
    Reply to this comment


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