what is the resistance per metre value for 25mm meter tails? I have 2 rolls of old colour meter tails and would like to work out how many meteres there are on each roll without physically unrolling and measuring with a tape! A simple end to end measurement with a ohm meter would be much quicker.
many thanks in advance
Too low to measure with reasonable accuracy with a standard instrument,
A special low ohms meter will be needed, presumably with a Kelvin connection.
Alternatively pass a significant current through the cable and measure the voltage drop.
Agree, at quite a bit less than 1 milliohm per metre per tail, unless it is a very long roll you will struggle to get an accurate answer with simple 2 probe metering as contact resistances can easily be tens of milliohms and then dominate the answer giving an error of tens of metres.
However, an ammeter, a volt meter and a suitable load, perhaps like the workshop kettle, may be a good way to demonstrate the effect.
Generally for any odd cross-section, to estimate resistance, copper can be taken as 16 to 19 milliohms per metre length per mm cross section. That is to say take 16 milliohms, multiply by the there and back distance (assuming that is a pair of wires being measured with a short at the far end) and divide by the cross section in mm2.,
The 16 milliohm figure is almost always too low, and assumes cold conductors of very pure copper, and is good for worst case PSSC estimates The 19m ohm is more likely to be correct for cables in use. If you have it, maker's data is best.
Mike
Yes, less than 1 mΩ per metre, so you might be able to get within 5 m.
If you can get a reasonable estimate of the weight of the drum, you might be better off weighing it.
Mass per unit length is available in the appropriate datasheet.
One or more of your wholesalers may have a dedicated test meter, that is made for the job. Ask nicely and one of the wholesalers may let you take their meter out to check the rolls of cable, whilst your van is parked outside their trade counter.
I have a Megger TDR meter that I bought through EBay for a specific fault finding job on a farm, where there was an underground fault on a buried SWA, probably a fault in a buried connection, but then another electrician sold the farmer a couple of hundred meters of SWA in a single length cheap having used it for a temporary supply at an event on a show ground, so the cable was replaced and I have never used the meter in earnest.
I tried using the Megger TDR to measure the lengths of cables on drums, but it’s not completely accurate because the cable is tightly wound on the drum, rather than being stretched out in a long length.
From Table I1 of the OSG, or the equivalent table in GN3, or BS 60228, the DC resistance at 20 ºC is 0.727 mΩ/m.
A simple end to end measurement with a ohm meter would be much quicker.
Quicker, yes, but not at all anywhere near accurate enough for even a basic idea, unless either you have well over 100 m on the reel, or unless the instrument used is a 4-wire micro-ohm meter ( 'kelvin' type, sometimes called a 'ductor tester' by those who use such instruments to check high current terminations at, or within, switchboards and transformers). The reason is that the contact resistance of a 2-wire instrument can be in excess of 10 mΩ (equivalent to around 14 m of 25 mm2 conductor at 20 ºC).
With the resolution and accuracy of a standard continuity tester used for electrical installations, you might need to add on another "uncertainty" of perhaps another 14 m or even more.
As an aside, this is the reason why tests for "bonding" in GN 3, and tests for most appliance product standards, and PAT testing, have a "pass" limit of 0.1 Ω (100 mΩ) in cases where the expected reading is well below this value - it's the resolution and accuracy of the test instrument.
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