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.

• The second chart says 1.4A for "chassis wiring," which is about consistent with the 1.5A rating from the L-Com "tip." What kind of currents will these cables be seeing (not values, but DC/AC, etc)? – Shamtam Jun 9 '12 at 2:36
• Both specs are probably correct. One is for individual wire, another is for wire in flat cable. If in doubdt, imagine the SCSI carrying ~70A in total. .. Then recall how real 70A wire looks. – user924 Jun 9 '12 at 3:12
• en.wikipedia.org/wiki/American_wire_gauge – kenny Jun 9 '12 at 13:24
• @Kenny, I'm not sure how that helps...if you read the table, they don't list anything for 28 AWG "ampacity", nor does it list fusing current. – ajs410 Jun 9 '12 at 15:56
• One pair will be good enough you don't need to use more than one pair. – user45583 Jun 14 '14 at 6:05

1 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.

• The length of the wire has a big impact on how much current it can carry. I get the impression this will not be a SCSI interface. – Gil Feb 8 at 21:01
• True, voltage drop due to the resistance of the wire is a factor for low-voltage applications. A 5-foot length of 28 AWG twisted pair (wire length of 10 feet) results in a voltage drop of 0.7 V at 1 A current. – davidmneedham Mar 1 at 14:32