Harry,
A related subject to the ampacity (safe current-carrying capacity) of
printed circuit board traces is the ampacity of wires. I've been
studying both topics seriously since June 1999. I will be including the
results of these studies in the book I am writing for Kluwer (Robust
Electronic Design Reference, to be published late this year) in:
* Appendix F: Important Properties of Wires and Cables.
* Appendix G: Important Properties of PCB Traces, Flat Cables, and
Busbars.
I've come up with a simple way to estimate the ampacity of wires with
about +/-34% accuracy (+/-2 standard deviations, should cover 95% of
real data). I would like some SI-LIST'ers and EMC-PSTC'ers to try my
method, see how it compares to data they have seen/gathered/used, and
give me feedback. From various sources I found over 1300 datapoints of
wire ampacity versus:
* Type of conductor (Kcond, from Table F-6).
* Environment (Kenv, from Table F-7).
* Number of wires in bundle/cable (N).
* Wire cross-sectional area (CSA, in mm^2 = 10^-6 m^2).
* Wire temperature (Twire, in degrees C).
AND
* Ambient temperature (Tamb, in degrees C) (or temperature rise, in
degrees C).
For American Wire Gage (AWG),
solid wire diameter = 0.0082515 * 0.89053^AWG meters
CSA = (pi/4) * diameter^2
I had to use two equations, because plotting and regression analyses of
the data showed a sharp bend at about 0.5-0.7 mm^2 CSA. For large wires
changing the CSA has a strong effect on the wire resistance, and thus
the heat produced in the wire by current (P = I^2 * R), but a relatively
small effect on the surface area of the wire. For small wires changing
the CSA seems to have a much greater relative effect on surface area,
and thus the wire's ability to conduct/convect/radiate heat away. I
decided to make the break-point between the equations at 0.5 mm^2,
because this is the smallest size wire permitted for linecords. So the
first equation covers linecords and other power wiring, while the second
equation basically covers signal wiring.
Here is my proposed method for estimating the ampacity of wires.
For CSA >= 0.5 mm^2 (20AWG and larger)
I = Kcond * Kenv * N^-0.2 * CSA^0.642 * sqrt(Twire - Tamb) amperes
For CSA <= 0.5 mm^2 (21 AWG and smaller)
I = Kcond * Kenv * N^-0.2 * 1.122 * CSA^0.808 * sqrt(Twire - Tamb)
amperes
Table F-6: Ampacity Factor for Base Conductor Material
* Copper has Kcond = 1.00.
* Aluminum has Kcond = 0.80.
* Nickel has Kcond = 0.56.
Table F-7: Ampacity Factor for Environment
* Free air has Kenv = 2.60.
* Cables have Kenv = 2.24.
* Equipment has Kenv = 2.06.
* Wire in conduit has Kenv = 1.77.
Many standards specify the wire size to be used for a given current in a
specified environment. So Appendix F also (already) includes 23 tables
summarizing these requirements from:
* IEC 950.
* SAE AS50881.
* MIL-W-5088.
* HS-1738.
* NFPA 79.
* UL 486.
* UL 817.
* CAN/CSA C22.2 No.21-95.
* IEC 60799.
* National Electrical Code (NFPA 70).
* Canadian Electrical Code.
My list of references for Appendix F is 5 pages long, so I won't include
it with this post. But I can post it later if there is interest.
So, if you are working on something that requires you to choose a wire
size/temperature rating based on the current through the wire, how about
giving my proposed method a try? Please see how it compares to your
present method(s), and let me know the results.
I would like to specifically acknowledge Doug Brooks' article
"Temperature Rise in PCB Traces" ( http://www.ultracad.com/pcbtemp.pdf
) as the inspiration for the method I used to come up with these
equations.
Thanks!
John Barnes KS4GL, PE, NCE, ESDC Eng., SM IEEE
dBi Corporation
http://www.dbicorporation.com/
(859)253-1178 phone
(859)252-6128 fax
[email protected]
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