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] -------------------------------------- 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 [log in to unmask] ______________________________ 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..... --