The "Skin Effect" largely explains why ampacity does not increase in a linear
fashion with increasing wire size. (http://en.wikipedia.org/wiki/Skin_effect)

Dick
Solarwind Electric



--- You wrote:
Taking Jarmo's example a step further, if you unroll the 3 wires so that 
they are flat, place those 3 end to end and
then unroll the larger 975 KCMIL wire and lay it next to those 3 placed 
end to end,
the 3 smaller circumferences will be "square root of 3 times" longer 
or,  about 1.732 times longer than
the larger 975 KCMIL wire.  That's 1.732 times more breathing (cooling) 
area.
Was that about the amount of difference you saw in the NEC table, Larry 
?  1.7 times or so ?

boB


On 12/2/2014 4:45 PM, boB@midnitesolar.com wrote:
>
> VERY nice example, Jarmo !
>
> Hey, one day, probably after we are all gone, maybe everybody will be 
> using high temperatue
> superconductors and we won't have to worry about wire size any more.
>
> If they can make them work well with AC....
>
> boB
>
> On 12/2/2014 4:26 PM, Jarmo.Venalainen@schneider-electric.com wrote:
>> Hi:
>>
>> Look at it this way,
>>
>> The three conductors in parallel can handle more current because they 
>> have more surface area with which to dissipate heat.  Or to put it 
>> another way, the heat from the middle of the thick 975 kCM cant get 
>> out and so the wire stays hotter.
>>
>> See picture,
>>
>>
>>
>> JARMO
>>
________________________________________________________________________________
_____
>> *
>> Jarmo Venalainen*  | *Schneider Electric **  |  Solar Business*  | 
>> *CANADA*  | *Training & Development Specialist - Senior* *
>> Phone:* +604-422-2528  | *Fax:* +604-420-1591  | *Mobile:* 
>> +604-505-0291 *
>> Email:* _jarmo.venalainen@schneider-electric.com_ 
>> <mailto:jarmo.venalainen@schneider-electric.com> | 
>> *Site:*_www.SEsolar.com_ <http://www.sesolar.com/> | *Address:* 3700 
>> Gilmore Way, Burnaby, BC V5G4M1
>> _
>> _<http://%20www.sesolar.com/>
>> *** Please consider the environment before printing this e-mail
>>
>>
>>
>> From: 	Larry <larry@starlightsolar.com>
>> To: 	RE-wrenches <re-wrenches@lists.re-wrenches.org>,
>> Date: 	12/02/2014 03:30 PM
>> Subject: 	Re: [RE-wrenches] Parallel Wire combining
>> Sent by: 	"RE-wrenches" <re-wrenches-bounces@lists.re-wrenches.org>
>>
>>
>> ------------------------------------------------------------------------
>>
>>
>>
>> Hey boB,
>>
>> This is the reference table I was using. They show 325 AWG and 325kCM 
>> refer to the same diameter cable but I should have used kcm for clarity.
>>
>> If all insulation/temp rating is the same we are back to my original 
>> question. Anyone else able to explain this? To be safe I am leaning 
>> toward using the combined CM number to size for current rather than 3 
>> times the ampacity as that just makes no sense to me.
>>
>>
>> On 12/2/14 3:54 PM, _boB@midnitesolar.com_ 
>> <mailto:boB@midnitesolar.com>wrote:
>>
>> The "area" of the conductor will be 3 times and you would think that 
>> the NEC Ampacity would also be 3 times
>> that of one conductor.   But one LARGE conductor with the same area 
>> might not be as high as you think
>> because of insulation.  I would think that ampacity of 3 cables in 
>> parallel would be 3 times.  But probably
>> not when you take a single cable of the 3X area out of the NEC 
>> table.  (I haven't looked at this to verify)
>>
>> Also,  Larry,  325 AWG 750 AWG (gauge) wire is a bit too small for 
>> this, don't you think ?
>>
>> I know...  You mean circular mils...
>> boB
>>
>>
>>
--- end of quote ---

