[RE-wrenches] Parallel Wire combining

[Daniel Young]

Three things:
1. The skin effect would only apply in and AC situation, correct?. So we
need to keep AC/DC separated when talking in detail about voltage drop for
example, which we already know is true since the NEC gives different
resistance #'s for DC vs AC in NEC tables 8 and 9. (but the band should stay
together IMHO)

2. Doesn't the stranding of the conductor essentially minimize the skin
effect? A 250kcmill wire is actually 37 #12solid wires, and a 1000kcmill is
actually 61 #8solid wires.

3. Temperature is still the driver for the NEC and the surface area really
drives heat dissipation capacity/temperature. This is how I've always
understood it to be at least.

With Regards,

Daniel Young, 
NABCEP Certified PV Installation ProfessionalTM: Cert #031508-90
NABCEP Certified Solar Heating InstallerTM: Cert #SH031409-13
 
-----Original Message-----
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Richard L Ratico
Sent: Wednesday, December 03, 2014 6:47 AM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Parallel Wire combining

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 at 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 at 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 at schneider-electric.com_
>> <mailto:jarmo.venalainen at 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 at starlightsolar.com>
>> To: 	RE-wrenches <re-wrenches at 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 at 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 at midnitesolar.com_
>> <mailto:boB at 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 ---
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