Hi - I'm looking at this page: - and am trying to figure out the thinnest guage wire I can use. Is there a good rule of thumb for this? I was thinking about using some 28 AWG wire (as I would like to use some .05" pitch ribbon cable). According to that page it's 232ohm/km. I need it to go about 15cm, so (232/1000)*.15 = .0348ohms. I'd like it to be able to handle 1.5A max current (though I'd be incredibly surprised if current spiked above 1A, and normally it should be under 200ma. So, .0348 * 1.5 = 0.0522V drop at peak current. Supply voltage is 6V, so .0522/6 = 0.87% of power (.0783W) dropped over the wire. Is this OK? Or is this pushing things? Am I reading the table correctly and are my calculations correct?Thanks!-MJ Noone

Ben Bradley wrote innews: Well - ideally I'd like to use some .05" ribbon cable for this - and all the .05" ribbon cable I've seen is 28 or 30 AWG. My choice of .05" ribbon cable is due to me wanting as fine as possible of a ribbon cable to connect a couple boards. I'm thinking that what I could probabaly do, if necessary, is use multiple wires in the ribbon cable for this power line . Everything else except for the ground will be very low current - it's just this one wire and ground that will be carrying substantial amounts of current, so this makes more sense to me than using thicker ribbon cable.It will be stretched out in open air.-Michael

calculating max current through wire?

Jul 24, 2005 21 Replies

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The ))) type numbers were added long after the original system was developed. How often do you wind a transformer with ))) wire, anyway?

Circular mils is very simple, if you think about it. You select the rating you want: E.G.: 750 circular mils/amp * the current required and take a look at the chart to select the wire, or a small range that will work. Its worked for generations, whether you like it or not.

Use whatever you can wrap your mind around.

I guess you would like to round pi off to 3.000? Personally, I just write small programs to plug the numbers in and let it spit out the answer. Do it once, and you're done. AWG wire is what is available to me in the US. No matter how you feel about your metric wire gauges, its irrelevant if I can't pick up a spool without ordering it from another county.

if you deal with large currents, all the time :)

It forces you to use a cookbook/ROT approach, starting with a current density in circular mils per amp. What current density to use? That depends entirely on the thermal environment of the wire. Odds on such a design can be improved. Personally, I'd just do it right the first time.

very true.

whats rounded off in E = wNBA? NOTHING.

whats rounded off in E = 4.44fNBA? (note the hidden RMS)

4.44 4.44288293817..... = pi*sqrt(2)

Personally, I do almost all of my maths by hand, with a pen and paper. Admittedly I use an HP calculator for evil trig etc. I also use mathcad and matlab (I keep trying to wean myself off mathcad, its slow and buggy, but its so darned convenient),.

Now thats a great reason to use AWG wire sizes. The abundance of US-made gear means I have to be able to get it too (the lack of stuff made in US nowadays means this problem will disappear in the next decade or so).

IMO Its still not a good reason to use circular mils, let alone in conjunction with Rule-Of-Thumb current density figures. And yes, I understand how to use circular mils. Just like I understand how to use BTUs (Bloody Terrible Units), slugs etc. but of course never do.

Cheers Terry

calculating max current through wire?

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