Don't know if you already have your answer (I'm in "digest" mode, once a day) and I can't tell you how that chart was derived, but here are a few clues that might help you out: 1) the voltage breakdown you mention is THROUGH the dielectric, in the Z-axis, in other words. This is far different than the value across the surface of the board, trace edge to edge. 2) IPC is a guideline for predicting what is needed for the operation of the board, and the board will work using those values. UL is a safety requirement, and will have MUCH different values. For example, your coffeepot might draw 6amps, and you can calculate a value of copper that will operate your coffeepot, and it WILL work. The building SAFETY code, however, requires a wire gauge that is much much much bigger. Don't confuse a minimum design guideline with a customer's safety requirement. good luck, Jack ----- Message from - Bogert <[log in to unmask]> on Wed, 2 Feb 2005 18:55:57 -0500 ----- Subject: Technical basis for Table 6-1 of IPC-2221 February 2, 2005 Table 6-1 defines minimum electrical spacing requirements between traces on a PWB. What is the technical basis for the Table? Is there a formula associated with the values in the table? For example, if I have a PWB operating at 30 volts DC, the minimum spacing required between internal traces is 0.05 mm. What if a PWB operates at 30 VDC but only has 0.04 mm spacing, is there a technical concern? Based on what formula or technical consideration? The minimum spacing in the Table are much less than the electric strength requirements for FR4. For example, my understanding is that FR4 is about 1250 volts/mil. Therefore, at 1 mil of spacing the PWB should be able to withstand 1250 volts between conductors. The values in the table are also different than those included in UL 840. Why? Any help you can offer on this point would be appreciated. --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------