[RE-wrenches] Cable Sizing - revisited, Ambient Temp

[John Wadley]

Mr. Brooks,
     You replied to Mr. Parrish back in 2009 with this example (below) on properly applying all the deratings to ampacity for wire sizing.  I have a bit of confusion and a question about the definition of "ambient temperature".  You define it below as the ASHREA 2% high temp.  My NEC 2008 (310.15 (2) Except No. 5 (3) (b) FPN) mentions it being an "average ASHREA" number.  The only definition for "ambient temp" in NEC I could find was 310.10 FPN (1) which says it "varies along the length of the conductor by time and place".  In the Photovoltaic Systems by Dunlop, p. 288, he cites a sizing example without saying where ambient comes from but uses 61C-70C (142F-158F) derate factor (0.58) for a sunlit roof top conduit.  He does not say how he arrives at that tempature range, but I suspect he started with the 90F rating of the USE-2 conductor in the example and added a Table 310.15 (B) (2) (c) adder of 33C.  Other articles I've read talk about conditions like an unventilated attic or a sunlit jbox on a roof where ambient temps could reach 150F.  I can also think of a situation where on a flat roof with a surrounding parapet wall, the sunlight shining into a corner would act like a solar oven on any conduit running close to the corner.  So, given all these definitions and possible exceptions to the definition of "ambient temperature", does your original definition (ASHREA 2% high temp) still stand as standard practice for most conditions and are there situations where one should use something other than that defined value?  If one is unsure of an exceptional situation, would it make sense to use an IR thermometer to measure free air temp on a sunny, calm day and then the air temp exactly where conduit might run and use the temp delta as an adder (like Table 310.15 (B) 2 (c)) to the ASHREA 2% high temp to arrive at a new, situational ambient temp before applying the other factors cited?
 
Thanks in advance,
John Wadley, PE
Wadley Engineering
NABCEP Certified Solar PV Installer (TM)
Dallas, TX
Peter,

We cannot use load diversity to increase the number of conductors in a PV
conduit since there generally is little diversity among the conductors,
particularly on large arrays.

The more traditional conduit adjustment table to use is Table
310.15(B)(2)(a). The value from this table is multiplied by the temperature
adjustment factor in Table 310.16. The key is what to use as the ambient
temperature in Table 310.16. We also have the third adjustment of Table
310.15(B)(2)(c) in the 2008 NEC for conduit close to rooftops. Even if you
are excused from using the 2008 NEC by a jurisdiction, the 2005 NEC has
310.10 FPN2 that generally recommends a 17C adder on ambient temperature.

The NEC has not had any explanation as to what ambient temperature to use
until the 2008 NEC in the FPN to Table 310.15(B)(2)(c) when it referenced
ASHRAE data in an incorrect way. To be consistent with the Copper
Development Industry, we have put a proposal into the 2011 NEC to use the
ASHRAE 2% design temperatures. These values can be downloaded at
www.copper.org.

Summarizing in an example:

Assume that 8 current carrying conductors, with and Imax of 10 amps [as
defined by 690.8(a)], are in a conduit in direct sun 4" off the roof deck in
Palm Springs, California. What must be the 30C ampacity of the conductor to
meet the requirement?

Answer:

I(30C) = 10A/(conduit fill adjustment)/(Temp adjustment--direct sunlit
conduit)

Conduit fill adjustment factor = 0.7 (70%)

Direct sunlit conduit temperature = +17C above ambient
2% Design Temp for Palm Springs = 44.1C (ASHRAE 2005 Fundamentals)
Design temp = 44.1 + 17 = 61.1C --corresponds to a 0.58 factor for 90C
conductors

I(30C) = 10A/0.7/0.58 = 24.63 amps -- minimum conductor size is 14 AWG
(barely)

Most inspectors will quickly cite the fact that the ampacity cannot be
greater than the 75C column, so we check to make sure (nearly always is just
fine). The 75C column says that 14 AWG wire can handle 20 amps (Imax is
10amps) at 30C but the asterisk limits our overcurrent protection to 15 amps
(since the module has a 15 amp max fuse rating, we are already using the
required 15 amp device).

The upshot is that even a 14 AWG 90C conductor works in almost the hottest
climate in the U.S. as long as only 8 conductors or less in conduit, conduit
is at least 4" above roof, and no more than 10 amps flowing through it. Most
contractors will use 10AWG for small systems and occasionally 12AWG. 10AWG
makes it simple since it meets all wiring options in today's smaller
systems. 12AWG works in many cases, and, as our example shows, even 14 AWG
works in some circumstances (we're talking ampacity, not voltage
drop--that's a different issue). In large systems, generally we specify the
minimum wire since it adds up after a few miles of conductor.

Now wasn't that fun--I can't believe anyone could be put to sleep by that
(maybe want to commit suicide, but no sleeping here). The short answer is
that it is complicate and not well organized in the code because the
majority of wiring systems are indoor. PV and HVAC systems are the two most
common outdoor wiring systems requiring these calculations. Most electrical
engineers doing HVAC wiring are just now learning this stuff.

Most inspectors will quickly cite the fact that the ampacity cannot be
greater than the 75C column, so we check to make sure (nearly always is just
fine).

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