[RE-wrenches] Cable Sizing

Thu Jan 8 17:11:22 PST 2009

Hi Bill,

Who puts this into a spread sheet or computer program?

thanks,

jay

peltz power
On Jan 8, 2009, at 11:56 AM, Bill Brooks wrote:

> 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).
>
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark  
> Frye
> Sent: Thursday, January 08, 2009 11:08 AM
> To: 'RE-wrenches'
> Subject: Re: [RE-wrenches] Cable Sizing
>
> In any case,I believe "load diversity" refers to the duty cycle of the
> various circuits with constant load levels, as opposed to the instant
> current flowing in the conductors to any given load.
>
>
> Mark Frye
> Berkeley Solar Electric Systems
> 303 Redbud Way
> Nevada City,  CA 95959
> (530) 401-8024
> www.berkeleysolar.com
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
> Parrish
> Sent: Thursday, January 08, 2009 11:01 AM
> To: 'RE-wrenches'
> Subject: Re: [RE-wrenches] Cable Sizing
>
> NEC 2005 Handbook, Annex B, page 1204, Table B.310.11
>
> I see now that 100% diversity applies to 4-6 and 7-9 ccc's but 50%  
> above
> that. Sorry.
>
> - Peter
>
> Peter T. Parrish, President
> California Solar Engineering, Inc.
> 820 Cynthia Ave., Los Angeles, CA 90065
> Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779,
> NABCEP Cert. 031806-26 peter.parrish at calsolareng.com
>
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill  
> Brooks
> Sent: Thursday, January 08, 2009 10:19 AM
> To: 'RE-wrenches'; exeltech at yahoo.com
> Cc: 'Will Logan'
> Subject: Re: [RE-wrenches] Cable Sizing
>
> Peter,
>
> Where do you read the 50% diversity? My 2005 NEC handbook clearly  
> says that
> the raceway adjustment table is based on NO diversity--all  
> conductors loaded
> at maximum.
>
> Bill.
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
> Parrish
> Sent: Thursday, January 08, 2009 9:47 AM
> To: exeltech at yahoo.com; 'RE-wrenches'
> Cc: 'Will Logan'
> Subject: [RE-wrenches] Cable Sizing
>
> Here is an issue that has been bothering me for some time and I  
> finally was
> asked to explain it to a junior member of our Company and had to admit
> ignorance...
>
> I am referring to the issue of derating the ampacity of wire in a  
> raceway or
> conduit with other current carrying wires. I refer to Table B. 
> 310.11, Annex
> B of the 2005 NEC Handbook, but the same information occurs  
> elsewhere in the
> HB as Table 310.15(B)(2)(a) and the CB.
>
> Table B.310.11 requires a 20% derating for four to six current  
> carrying
> conductors. There is a note that this derating includes the effects  
> of a
> "load diversity" of 50%. Am I to assume that 50% load diversity  
> means that
> each current carrying wire is operating at 50% of its temperature- 
> corrected
> ampacity (or less)?
>
> Does this table apply equally well to DC and AC circuits? What about
> 3-phase/3-conductor wiring?
>
> In specific: if we have two DC PV circuits (4 wires: 2 positive, 2  
> negative)
> in EMT and we have taken into account the "PV 125% circuit current
> calculation", "cable sizing of 125% for proper operation of  
> overcurrent
> devices", and cable derating for temperature -- do we now apply the  
> derating
> from B.310.11 for 4-6 wires?
>
> This is what we have been doing, and I'm looking for confirmation.
>
> - Peter
>
> Peter T. Parrish, President
> California Solar Engineering, Inc.
> 820 Cynthia Ave., Los Angeles, CA 90065
> Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779,
> NABCEP Cert. 031806-26 peter.parrish at calsolareng.com
>
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