| **THE-RFI-EMI-GUY** 2006-05-14, 2:21 am |
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electrician@electrician2.com wrote:
>TAble 310.16 of the National Electrical Code and applicable notes and
>sections is where you find the ampacity of NM building cable. Although
>NM is rated 90 degrees C. you are required to use NM at no greater than
>the 60 degree C ampacity given in Table 310.16 (per section 334.80
>copied below.) However the 90 degree C. ampacity given in Table 310.16
>is allowed to be used for derating purposes.
>Table 310.16 has derating factors for high ambient temperatures at the
>bottom of the table. For example a No. 12 NM copper has a 90 degree C.
>ampacity of 30 amperes. However it cannot be used at greater than the
>60 degree C. ampacity rating of 25 amperes. But there is also one more
>requirement given in 240.4(D) (also quoted below) that No. 12 be
>protected at no greater than 20 amperes. So what does all this mean.
>In short a No. 12 in an ambient of 132 degrees F to 140 degrees F. has
>a derating factor of .71. Since .71 x 30 is greater than 20 amperes
>this No.. 12 can be protected by a 20 ampere circuit breaker. However
>for a continnous loads that lasts for 3 hours or more branch-circuit
>conductors shall have an ampacity not less than the maximum load to be
>served. Where a branch circuit supplies continuous loads or any
>combination of continuous and noncontinuous loads, the minimum
>branch-circuit conductor size, before the application of any adjustment
>or correction factors, shall have an allowable ampacity not less than
>the noncontinuous load plus 125 percent of the continuous load..
>
>If you don't understand all this then hire a competent electrician ,
>master electrician, or electrical engineer.
>
>References:
>334.80 Ampacity. The ampacity of Types NM, NMC, and
>NMS cable shall be determined in accordance with 310.15.
>The ampacity shall be in accordance with the 60°C (140°F)
>conductor temperature rating. The 90°C (194°F) rating
>shall be permitted to be used for ampacity derating purposes,
>provided the final derated ampacity does not exceed
>that for a 60°C (140°F) rated conductor. The ampacity of
>Types NM, NMC, and NMS cable installed in cable tray
>shall be determined in accordance with 392.11.
>Where more than two NM cables containing two or
>more current-carrying conductors are bundled together and
>pass through wood framing that is to be fire- or draftstopped
>using thermal insulation or sealing foam, the allowable
>ampacity of each conductor shall be adjusted in
>accordance with Table 310.15(B)(2)(a).
>
>240.4 (D) Small Conductors. Unless specifically permitted in
>240.4(E) or 240.4(G), the overcurrent protection shall not
>exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG,
>and 30 amperes for 10 AWG copper; or 15 amperes for
>12 AWG and 25 amperes for 10 AWG aluminum and
>copper-clad aluminum after any correction factors for
>ambient temperature and number of conductors have
>been applied.
>
>210.19 Conductors - Minimum Ampacity and Size.
>(A) Branch Circuits Not More Than 600 Volts.
>(1) General. Branch-circuit conductors shall have an ampacity
>not less than the maximum load to be served. Where
>a branch circuit supplies continuous loads or any combination
>of continuous and noncontinuous loads, the minimum
>branch-circuit conductor size, before the application of any
>adjustment or correction factors, shall have an allowable
>ampacity not less than the noncontinuous load plus
>125 percent of the continuous load.
>
>
>
Thanks, it looks like I am safe, my #6 CU rated 75A (at 90C), derated x
..71 (at 140F/60C) is good for 53A (closely agreeing with 60C rating of
55A). In case my 22A total Jacuzzi and misc loads all run continuous (>3
hours, mult x 1.25) I will need at least 30A rated breaker, however I
will need to use a 40A for the disconnect breaker as that is minimum for
fitting the #6 CU conductor. The branches are further protected 15A each
at the subpanel.
--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."
"Follow The Money" ;-P
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