Mail*Link(r) SMTP FWD>RE>Conductor widths
Ray and Doug --
I hope the following information will be of help to you for you "conductor
width" concerns. Though I'm not a NEC compliance legal-eagle, the following
is my understanding and knowledge of the NEC and other related requirements.
The three keys to current carrying capacity in conductors is voltage drop,
surge (transient) currents, and thermal rise.
The US's National Fire Protection Association's "National Electric Code",
NFPA-70, is primarily focused on industrial and premises wiring and the scope
of applicability is for the distribution of electrical power into you facility
or home, and essentially terminates (regulatory wise) at the wall outlet
(receptacle) or at some equipment, such as a directly wired motor, controller
or heater. Within the NEC, some specialized equipment is included. In recent
years, some communications "wired" services that are "electrical like" have
been added to the NEC.
In the main, the NEC is a common-sense based set of requirements to promote
electrical safety, for facilities, wiring and people. Other regulatory
groups, national standards organizations, insurance underwriters, and industry
has extended the basic concepts of the NEC into other products, such as
electrical power extention cords, multiple outlet boxes, electrical/electronic
equipment. This extension of NEC-like requirements by others has been done
because it makes-common-sense to continue NEC-like requirements where the
requirements are similar in nature, for example through out the "primary"
electrical power wiring of electrical/electronic equipment.
The conductor sizes as mentioned by Doug does present a problem for printed
board designs, and in general the NEC does not apply.
IMO (in my opinion) what you need to do is the following:
The NEC's conductor sizes are based on voltage drop and thermal rise.
The voltage drops are based on the total loop wiring in the parlence of the
NEC "the ungrounded" (commonly called the hot or line) and "grounded" (the
neutral)" conductors. In general, one of the requirements is the voltage drop
across the load shall not be less than ~97% of the supply voltage. You need
to do a simlar thing for printed boards.
Current carrying capacity - the conductor needs to be sized to carry the
necessary current and meet the voltage drop requirements. In addition the
conductor needs to be "sized" to carry the "worst case anticipate surge
current" without blowing out. In particular the grounding (that's the green,
green/ yellow, or bare safety grounding) conductor. Comment -- in the NEC,
the smallest wire they allow is a #16 awg wire. Determining the worst case
"fault" current can be a problem. You must determine (or a darn good
estimate) the worst case transient voltage and the impedance of you electrical
power distribution system -- not simple task to do. What you generally do is
to figure a possible surge current 10-100X the normal current carrying
capacity of the conductors, and you use a "fusing current" calculation like
was posted by Bill Gains on technet on 4/8/96. Within the NEC, conductors are
"sized" acording to the thermal characteristics of the insulation and ambient.
Wires/cabling are de-rated according to the proximity of other sources of
heat (including wires). These application requirements are different than
printed boards.
You do not want to equate cross-sectional equivalence between round wire
conductors and rectangular (printed board) conductors except for equivalent
series resistance for voltage drop determinations. The "surface" area of the
conductors and it's associated thermal coupling to heat transfer mechanisms
are different. In the case of round wires, there is generally an electrical
and thermal insulative material around the wire, this increases accumulation
of heat in the wire. Also, a cylindrical body is a very compact shape, and
the analysis of heat transfer is a text book example in most text books. If
the wire (and it's insulation) are in contact with a planar surface, the heat
transfer is significantly different and the wire will run significantly hotter
without the "free air circulation".
Printed boards are different - we have relatively large surface areas in
direct contact with an electrical insulator that has relatively lousy thermal
conductivity (better than nothing). So there is a limited ability to spread
heat laterally across the surface of the base material. More importantly
though, with multilayer printed boards we can transfer heat through the
dielectrical to an adjecent layer of conductive patterns that will function as
heat spreaders. In addition, you have two conditions, the "long-term"
average/constant power thermal considerations and the "short-term" transient
conditions, which for brevity I'll conveniently avoid addressing. All-in-all
you can have a lot of fun (;-) doing the thermal analysis and modeling.
Current carrying capacity of conductors for printed boards. Several decades
ago, (to my rememberence) the US's NBS (National Bureau of Standards)
conducted some of the first documents current carrying capacity and thermal
rise above ambient tests for UL. The results of this testing and subsequent
testing has resulted in the current-carrying capacity -vs- thermal rise above
ambient in the two main design standards, the old MIL-STD-275, and IPC's
D-275, Table 3-4. The original NBS/UL studies used a test procedure and test
speciment much like IPC-TM-650, Method 2.5.4.1. Memory seems to recall though
that the first tests were performed with the test conductors horizontally, but
the test printed board was "hung" vertically. Then in the second set of
testing (and I believe all subsequent testing to date), the test printed board
was suspended horizontally with the test conductor located on the lower side.
The reason for the change was that with vertical mounting of the printed board
there was preferential convective cooling of the test specimen that was not a
worst-case like condition of an in-use horizontally mounted printed board.
Care must be exercised in looking at the test data and results because most
have used 0.8 mm [0.03 inch] and others have used 1.5 mm [0.06 inch] thick
base materials. CAUTION must also be used when you use the tables because
they do not include any polymeric coatings over the conductive patterns (such
as solder resist).
IMO, the key to understanding the IPC-D-275, Table 3-4 current-carrying
capacity -vs- thermal rise, like most of the other tables, is to realize the
table is the result of "avg" test data, and that many of the "tables" are a
redraw of a redraw of a redraw. I've collected about a dozen variations of a
Table 3-4 like table, they're all similar, but each illustrator has taken some
liberty by "cleaning up the drawing" in the redraw. Aside from this, at home
on my own is I've collected all of the raw test data that I've been able to
find from both published articles and personal correspondence, I've also
back-digitize all of the tables I've got, then all of this has been entered
into a spread sheet data base, now I'm in the process of overlaying the heat
transfer modeling for conductive, convective and radiative heat transfer
functions and develop a best reasonable set of coefficients and factors for a
best curve fit. I'm also attempting to include the effects of solder resist.
The goal is to have something based on physics and correlation to test data,
and not redrawing the drawings for publishing clarity. And yes, I've been
derelict in my duties to comment on some IPC documents, I've observed most
tables contain a unique physical capability --- (;-) (;-) (the smilies are to
indicate the following is presented in jest, though serious, and not to tweek
too many of you off at me) both Table 3-4a and 4c have a unique electrical
capability. Look at the origin and you will note that with one exception you
can have current flow of up to 250 mA with a conductor of 0 (zero) cross
sectional area. Now that's one heck of a current source -- I wish I had one
that worked that way. (;-) (;-)
In theory, the "power density per unit area and thermal rise above ambient"
should be a constant. Also in theory, if you double the current and quadruple
the cross sectional area (the ol' electrical power I squared R power law) the
power density and thermal rise should be a constant. However it's not, and
IMO is because the additonal heat and area changes the thermal (heat transfer)
characteristics (coefficients). At work, we have found it is a reasonable
approximation for you to extend the slope of the tables thermal rise above
ambient. We've done this for surface conductors limited to 20 degrees C above
ambient and currents up to 50 Adc, though you need to verify it for your
application.
Ray and Doug, I hope this is enough of a start to help you out.
Ralph Hersey
Lawrence Livermore National Laboratory
email: [log in to unmask]
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Date: 4/11/96 1:00 AM
From: [log in to unmask]
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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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.....
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