Electronic – Current carrying capacity of 28 AWG wire


This SCSI cable is composed of 28 AWG twisted pairs. According to an L-Com "tip", such a twisted pair should be able to carry at least \$3 \times 0.8=2.4\text{ A}\$ per conductor. However, this other web site which references the Handbook of Electronic Tables and Formulas suggests that you can carry only 226 mA per conductor. This is an order of magnitude difference! Can anyone explain which one is correct?

The application requires the conductors to carry 1 A at 5V DC. So the question really comes down to whether I need one pair or more than one pair.

Only one pair will be carrying this current, and 1 A is the most current I expect that pair to carry. All other pairs will carry at most 50 mA worst case per wire, typically much less than that, and all the other pairs combined are current limited to 150 mA.

Best Answer

I am assuming you are going to use only one twisted pair (out of the 34 available) for 1 A. In that case, you should be fine for your application using 28 AWG copper assuming that your cable is surrounded by open air at 30 °C or cooler. The first link's recommendation is close to the (United States) National Electric Code formula. Note that the fine print for the second link says that the power transmission values are "very very conservative."

If more than one twisted pair is carrying a significant amount of current or the cable will not allow heat dissipation, use multiple pairs to carry your current.

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