how to calculate csa (cross-sectional areas)

Postby mark smith » Fri Mar 26, 2010 12:25 pm

a 50metre drum of 'singles'cables has a resistance of 0.15ohms.if the resistivity of the metal is 18 nono ohmmetres,calculate the csa(cross sectional area)
mark smith
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Postby kbrownie » Mon Mar 29, 2010 6:54 am

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Postby sparx » Mon Mar 29, 2010 7:44 pm

just checked it's not 1st APRIL.....

What the hell is a nono ohm?

who would use such a measurement?

I must be getting old, 50+ years of electrics/electronics never heard of such a numpty name, it is a joke yes?

Any of the wiring guides publish lists of CSA-resistance tables.
From memory they quote per kilometre, so since there are 20 lots of 50m per kM
20 X 0.15=3.0 ohms, 6mm2 = 3.08 ohms @ 20Deg C it would seem without resorting to wierd numbers 6mm2 would fit the bill?
BTW it is imperative to state temp. so this may account for slight difference.
would love to see other responses, come on Ericmark! hi hi!
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Postby ericmark » Tue Mar 30, 2010 7:18 pm

I do remember reading the resistance of copper and because it is in a big lump it is a really low figure in nano ohms range (where nano = 10 to -9). However we use wires and the regulations as Sparx has said gives us handy volt drop figures. As I am sure you realise volts / Amps gives ohms so mV/A/m could be considered as mΩ/m. So 150/50 gives us 3mV/A/m but this is return value so really 6mV/A/m looking in tables 6mm² is 7.3 and 10mm² is 4.4. This is where the problem arises, once you start to look at it you realise how accurate the measurements would need to be, to be able to with any certainty calculate it’s cross sectional area. If we consider 0.01 ohm out would change reading enough to drop into the next group plus of course could be imperial. So we tend to keep a few lugs in our box. If yellow will not fit then over 6mm² if tight then is 6mm² if slack and will not fit blue 4mm² etc.
OK to be accurate one would measure the cores with micrometer but we normally assume that it will be metric and so go and no go is near enough.
Normally most meters are calibrated at 20ºC and all cable measurements are also at 20ºC so you would need a room at 20ºC to do all measurements in. You would need a low ohm ohmmeter able to measure so low these normally use at least 200ma and even so it is so easy to have a bad connection and of course will been to be zeroed to account for lead resistance.
Wheatstone bridge would really be order of the day rather a complicated method to measure cable size.
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Postby bartkusal » Wed May 05, 2010 12:18 pm

mark smith wrote:a 50metre drum of 'singles'cables has a resistance of 0.15ohms.if the resistivity of the metal is 18 nono ohmmetres,calculate the csa(cross sectional area)

Ok the simple one: csa=(resistivity of the condactor x length) / resistance

so: (18to the power of -9 x 50) / 0.15= 6 to the power of -6 or if you like 0.000006 m squared or as earlier mentioned 6 mm sqared. hope it helps
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