Re: calculating max current through wire?
- From: Terry Given <my_name@xxxxxxxx>
- Date: Wed, 27 Jul 2005 13:46:22 +1200
Paul E. Schoen wrote:
I have made a crude Excel spreadsheet that calculates current carrying capacity for wires as well as bus bar. For wire or bus bar in free air, the current carrying capacity is closely related to watts per square unit of surface area, so smaller wires can carry more current per cross sectional area than larger wires. For multiple conductors in close proximity (such as in a wound coil or multi-conductor cable), the capacity is more closely related to the cross sectional area, or watts per unit of volume. The temperature rating of the insulation also comes into play, so you need to determine the ambient temperature and the temperature rise due to power, factoring in the temperature coefficient of copper, and also the rate at which heat will be conducted or radiated from the wire. I made my wire chart based on the NEC ratings of 30 A for #10, 20 A for #12, and 15 A for #14, and found a constant 0.0037 A/sq mil of cross section area and 0.240 W/sq in of surface area. The surface area calculations correlate fairly closely with the published ratings for bus bar, so I am comfortable with those ratings for single larger wires.
Using this chart (which I just updated with approximate data for smaller wire sizes), the #28 AWG wire should be able to handle 1.3 amps, and the #30 AWG should handle 0.9 A. Based on cross sectional area, these ratings would be 0.5 and 0.3 A. I normally use this chart for much higher currents and very large wires and bus bars. My work with circuit breaker test sets involves continuous currents up to 6000 amperes and short pulse currents up to 100,000 amperes. At those levels, there are other considerations such as skin effect and both mutual and self inductance, as well as mechanical constraints to prevent conductors from jumping around due to magnetic effects.
The Excel spreadsheet is on my website at http://www.smart.net/~pstech/WireSize.xls. Feel free to make any improvements or suggestions. Good luck.
[snip]
Hi Paul,
thats a damn good way of working it out. My only comment would relate to the thermal characteristics of the wire insulation, which is probably not constant over varying wire sizes. Radox, for example, uses a X-linked polymer with a nice low Rtheta, allowing even higher current densities (and it wont melt when you run the soldering iron across it, nor does it outgas Chlorine when hot).
Conversely it is highly unlikely that ribbon cable insulation is optimised for heat transfer (the insulation can be surprisingly thick), so I'd expect it to go horribly wrong with small wire sizes. But it certainly gives a reasonable upper limit, which can easily be followed up with suck-it-and-see experimentation.
Cheers Terry .
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