| electrician@electrician2.com 2006-05-14, 1:21 am |
| 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=B0C (140=B0F)
conductor temperature rating. The 90=B0C (194=B0F) rating
shall be permitted to be used for ampacity derating purposes,
provided the final derated ampacity does not exceed
that for a 60=B0C (140=B0F) 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.
|