Wire ratings question
Submitted: Wednesday, Aug 17, 2005 at 16:33
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baza
Hi, just wondering about the ratings given to current carrying wire. Say it is rated as 50Amps at 50Volts, does this mean its rating at 12V would be a higher amperage? Is it related to the power it can carry, i.e 50A at 50V = 2500w, so 12V * 208A = 2500w? I guess this is not correct as 208 amps sounds like a lot. Thanks.
Reply By: G-wizz - Wednesday, Aug 17, 2005 at 18:01
Wednesday, Aug 17, 2005 at 18:01
Basically you are correct. 50 amps is also a lot of current at 50 volts which is why 200 amps at 12v looks high.
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Follow Up By: Member - Jeff M (WA) - Wednesday, Aug 17, 2005 at 18:15
Wednesday, Aug 17, 2005 at 18:15
G-wizz, I was always under the impression the the max amperage was just that, and had no link to the the voltage. I mean 200amps is 200amps, weather it's at 50v or 240v isn't it?? I mean obviously the the total amount of power will be different but that's why we use 240v @ 10amps, because otherwise we'd need MASSIVE wire to cope with the high amperage if our houses ran of 12v?? That's why my inverter has HUGE wire on the 12v side and only normaly 240 cable is fine for the other.
I may very
well be wrong here, but that's what I thought anyway.
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Follow Up By: baza - Wednesday, Aug 17, 2005 at 18:34
Wednesday, Aug 17, 2005 at 18:34
That's sort of what is confusing me. I always thought of current as the amount of 'electric stuff', electrons I guess, that come through the wire, and voltage as the pressure it comes through as. Maybe this is not correct, but I am not sure what effect the pressure (Voltage) has on the wire at different currents.
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Follow Up By: G-wizz - Wednesday, Aug 17, 2005 at 20:09
Wednesday, Aug 17, 2005 at 20:09
Yes 200 amps is 200 amps but the 'power' is a product of current & voltage and the heating effect is a direct result of the power. Think of lightbulbs as an example, they are rated in watts and the greater the wattage the hotter the lamp gets (as a generalisation).
The size of the cable is actually measured in cross sectional area, automotive cables have more strands than domestic house cables which makes them more flexible and less prone to breakage due to vibration. This also means that identical rating cables can have different physical sizes depending on their designated use.
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Reply By: Max - Sydney - Wednesday, Aug 17, 2005 at 18:49
Wednesday, Aug 17, 2005 at 18:49
Bazza
The ratings are a bit more complicated than just ohm's law because v (total voltage drop) = I (current) * R.
First - if the cable is rated 50 volts - that means the maximum voltage the insulation will take.
Then, the maximum amps will be determined by the maximum acceptable voltage drop. For example, maybe they are saying you get 1 volt drop over a certain length (not specified) at 50 amps. 1 volt is 2% of 50 volts, but is 8% of 12 volts, and could be more of a problem.
I think most of the ratings on automotive cable are pretty dodgy, and they use different systems for things like cable size than for 240 volt power. There are endless discussions on the subject on the Caravanners'
Forum (
http://groups.msn.com/Caravanersforum/ ), but 6 mm squared cable seems to be about the biggest you can easily use in a car, and that seems to give fairly low volts drops for fridges etc at the backl of a fourby.
That confuse you more?
Max
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Follow Up By: Max - Sydney - Wednesday, Aug 17, 2005 at 21:26
Wednesday, Aug 17, 2005 at 21:26
Woops - you don't have to go away from this site for a good article on cable sizes - look at the article on
Solar Power.
And I should have said 6 mm dia, not 6 mm squared.
Max
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