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urjaengrs (Electrical)
14 Mar 08 2:12
I would like to know regarding standard cable size. Who defines it, i mean which standar has defined this. Is somebody is aware why the standard concluded for a particular size. For exampl, why cable size is of 70 sq.mm. only and not 75 sq.mm.

In advanse thanks..........
ScottyUK (Electrical)
14 Mar 08 3:51
The series is loosely a geometric series with a factor of 1.5, with the numbers somewhat rounded off to convenient values. 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630...

The values follow the series less closely as sizes get larger because copper cost at those sizes becomes significant and the jumps in ampacity become large between sizes if the series is followed.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

Marmite (Electrical)
14 Mar 08 4:45
Ampacity Scotty?
Have you crossed the pond to the large island in the West?!
Marmite
ScottyUK (Electrical)
14 Mar 08 10:03
Just trying to use words that are familiar to the masses, because those of us who speak English are in a minority!

My spell checker is not impressed by 'ampacity'.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

itsmoked (Electrical)
14 Mar 08 15:17
opacity?  None of mine are either.  It is such a useful word too.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

rbulsara (Electrical)
14 Mar 08 16:26
This might be one of the cases where "standard" follows the commona practices that has evolved overtime due to various factors. The most new comers sort of follow these "standards".

I as far as I know, there are no standards for conductor sizes for manufactureres. No one prevents you from making a odd size conductor, except you may not find many buyers! If you are a user you can specify any size condutor you want, if it is a odd size, you may not find it in the market!

Many cable sizes also have to do with standard ratings of overcurrent protections. Then there is the market forces of supply and demand. Making sizes that can not be sold is not profitable.
alehman (Electrical)
15 Mar 08 0:00
If you think the mm sizes don't make sense, try figuring out the system we Americans use.

We use American wire gauge for sizes #40 through #0000 (4/0), but only even AWG numbers are available in sizes 4 through 40. 4/0 through 4 are available in odd and even sizes?? And we're not quite sure where these sizes came from anyway.

As if that weren't confusing enough, if we want wire bigger then #4/0, we identify it in terms of cross-sectional area instead of AWG.  Cross-sectional area seems OK, but to keep it confusing to the uninitiated we decided to measure it in the units of circular mils (being the area of a circle of diameter one mil (0.001 inch). ponder

Then we can talk about aerial conductor sizes if you want. We name those for birds and they come a strange variety of circular mil and AWG sizes.

So i don't want to hear any complaining about 70 vs. 75 sq. mm. wink
rbulsara (Electrical)
15 Mar 08 10:27
alehman now will be subjected to disciplinaty actions fro giving away american secrets to stay ahead of the game, by keeping others confused!!

Now that the beans are spilled, I have heard that those AWG wires sizes came into play because those were the sizes of steel wires that could be "drawn" (as in manufacturing). They used the similar dies or whatever machines to "draw" copper wires. ( I have very little idea as to how the wires are made).

As for the name of the birds, I believe the uninitiated world has accepted some animal names too like squirrle, zebra, moose, panther etc. for ACSR conductors, probably originated by ANSI. I am sure many animals and birds may have grief for being left out!!
rovineye (Electrical)
15 Mar 08 12:55
I could have a cable manufactured in between AWG 10 and 12 and call it AWG 11. But my standard (IEEE Std 45) wouldn't give me expected ampacit... errr... current carrying capacity, weight, jacket thickness etc. And as pointed out by rbulsara, I couldn't accurately or economically match them up with protective devices, lugs and entry tubes etc.
7anoter4 (Electrical)
15 Mar 08 13:52
HI urjaengrs
There is an IEC [International Electrical Commission] standard no.228 /228A:
Conductors of cables, wire and flexible cords for power installation
Or DIN VDE [German standard] NO. 0295 .
The standard states the cross sectional area, diameter for solid and stranded wire and the electrical resistance [for 20 degree C] for copper and aluminium wire [from 0.5 to 1000 sqr.mm]
I don't know how these cross sectional area are chosen, but I think the ScottyUK explanation is very close.
Best Regards
alehman (Electrical)
15 Mar 08 14:41
Oops. blush

I actually must confess that I have a spool of #13 wire for connecting motors in electric powered model airplanes. It's very distinctly labeled "13 AWG" on the insulation. I don't remember where I got it. It has very fine strands.
electpower (Electrical)
19 Mar 08 16:15
How could one calculate the cross section area of the cable by knowing the current?
rbulsara (Electrical)
19 Mar 08 17:51
If you look up explanation of AWG and its history in wikeipedia, it sheds some light. If you look up the formulae people have come up with there, it becomes very evident that those were not the 'standard' set by any one, but people tried to fit the equations to what had become accepted sizes of wires.

Also try googleing "AWG wire size history" etc.,
rcwilson (Electrical)
19 Mar 08 20:41
A utility in the North Eastern part of the US required all metering to be connected with #9 awg wire with a very specific color code.  Each voltage wire, current wire, kyz contact outputs, and other functions all had different colors (black red blue white pink purple orange black/red + others).  No wiring diagrams were needed because the meter men all knew where each color wire got connected.

 Ever try to buy 50 feet of 12 conductor #9 awg, special strand, special color wire?  Of course, the only source was the utility.

Standard wire sizes are standard not because they are the best design but because they are readily available.
alehman (Electrical)
19 Mar 08 21:33
electpower - the electrical codes specify the size of wire required based on the maximum current. Is that your question?


electpower (Electrical)
20 Mar 08 4:20
alehman- My question is "How to calculate the size(cross section) of the cable, Knowing the maximum current and the voltage level ?"
rbulsara (Electrical)
20 Mar 08 9:44
electpower:

You cannot! It is otherway around. The ampacity tables of a given type of condutors are  published by the manufacturers and many are incorporated in Codes. These ampacity are arrived at by testing  (measuring the temprature rise, etc.) and the following factors affect them:

1. Type of material and size of the conductor.
2. Type of insulation, for maximum temperature
3. Ambient temperature.
4. Type of installation: in air, conduit, underground,cable tray, concrete encased or direct burial, etc.
5. deratig factors based on no. of condutors in a raceway.
conduit
6. Type of load served, higher harmonic content can cause more heating and required furhter derating.
7. Sizing of condcutor also needs to be checked against thermal withstand during a fault current, especially for medium and higher voltage feeders.

I may have missed some factors but you get the idea. In the end you have to meet both the Code and manufacurer's recommendations and last but not the least, need to use engineering judgement.
alehman (Electrical)
20 Mar 08 21:39
I agree with rbulsara. There is no simple formula for such calculations.

One possible idea - there is a commercial software package called Ampcalc that is used for calculating underground duct bank conductor capacities. It takes into account many of the factors mentioned by rbulsara. I don't know its capabilities in detail. I suppose it might be able to give you conductor temperature based on load for underground installation. It might be worth a look to see if it will meet your needs.
rbulsara (Electrical)
21 Mar 08 20:16
electpower:

We can only read what you write and interprete on its face value. If  you really meant how to size a cable based on amps and voltage and not necessarily look for a formula, all you need to do is refer to a Code such as NEC (or applicable codes depending upon the location of the application) and it has almost everything you need, unless you are talking about utility distribution and transmission systems.

1x1x1x (Electrical)
22 Mar 08 15:34
The geometric series factor for IEC cables are calculated as: (5)sqrt(10) = 1.585 where the numbers  rounded off
1x1x1x (Electrical)
22 Mar 08 15:44
Additionally the factor a (7)sqrt(10) = 1.39 is also used e.g. 25mm^2*1.39 = 35 mm^2 ;  50mm^2*1.39 =70mm^2; etc.

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