Terry,
I can certainly appreciate that and we have considered that as a
possibility. Under analysis, the higher current rated solid wire has
MORE surface area than the IPC recommended trace width!
For instance,
Say, I have 10 amps on a 2 oz. trace and I can handle no more than a
30 degree rise. IPC crosses over to (as best my eyesight can handle)
a cross-sectional area of about 175 square mils. Dropping down to
trace width, that correlates to about 70 mils. Assume the trace width
is much much more radiative than the trace sides. The total radiative
area is 70 mils.
10 amps for NEC correlates to 16 gauge solid wire. 16 gauge solid
wire has a nominal diameter of 50 mils. The available surface area
for heat radiation is 2*pi*(50/2) = 157 mils!!!
Now, under analysis, the IPC recommends 175 square mils to handle 10
amps whereas the NEC is recommending a cross-sectional area of 2027
square mils to handle 10 amps!!! These numbers differ by a factor of
11. Basic electricity tells me that two conductors that differ in
cross-sectional area by a factor of 11 also differ in current carrying
capability by a factor of 11.
This does not make me feel good. What am I missing here?
Doug McKean
ADC Video Systems
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______________________________ Reply Separator _________________________________
Subject: Re: Conductor Widths
Author: [log in to unmask] at internet-mail
Date: 4/11/96 8:57 AM
Doug has queried the difference in recommended cross-section between the NEC
specs. for solid wire and IPC specs for PCB tracks for the same current rating.
I would suggest that since temperature rise is the limiting factor, the
difference may be explained by the different surface area between a wire and
a track. The larger surface area of the track will give a lower thermal
resistance to ambient, and thus allow a larger current flow for the same
permitted temperature rise (viz. same Amps and X-section area).
Terry Davey
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___________________________________________________________________________
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Is your board being directly connected to AC power distribution of a
building? If so, you'll have to make sure that it conforms to the NEC
(National Electrical Code). This is where I have a problem with IPC
specification of trace construction.
The NEC rates 14 gauge wire for 20 amps and 10 gauge wire for 35
amps. Since this is for solid wire, 14 gauge wire has a
cross-sectional area of 3227 square mils, 10 gauge wire has 8156
square mils. If your 2oz. copper is 2.8 mils thick, then your
corresponding widths are 1.2 inches and 2.9 inches!!!
Cross-sectional areas that are specified by the IPC as opposed to the
above procedure can lead to a difference in areas of up to of 5 TIMES.
Someone have an answer for this?
Doug McKean
ADC Video Systems
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______________________________ Reply Separator _________________________________
Subject: Conductor widths
Author: [log in to unmask] at internet-mail
Date: 4/10/96 9:54 AM
Help,
We are laying out a power distribution board. We have current
requirement of 35 amps and 15 amps. Our design is using 2oz copper,
2 layers, and 20 degrees temperature rise.
I am not sure of the conductor width required for both 35 & 15 amps.
Can anyone help answer this question?
Any help would be appreciated....
Ray.....
--
_/ _/ _/ _/ _/_/_/
_/ _/ _/_/ _/ _/ _/ Hughes Network Systems
_/ _/ _/ _/ _/ _/ 11717 Exploration Lane
_/_/_/_/ _/ _/ _/ _/_/_/ Germantown, Md. 202876
_/ _/ _/ _/ _/ _/
_/ _/ _/ _/_/ _/ _/
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Ray McKinnon Email: [log in to unmask]
ph (301) 212-7958 fx (301) 212-2099
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Terry Davey, Reliability Engineer,
European Development Engineering (108),
Motorola Ltd, GSM Products Division, Tel: +44 1793 545390
16 Euro Way, (switchboard: 541541)
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England, SN5 8YQ. E-mail: [log in to unmask]
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