PRINTED CIRCUIT BOARD (PCB) SPACING / CLEARANCE vs. VOLTAGE

IPC-2221A REQUIREMENTS

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IPC-2221 is generally accepted in electronic industry as a generic PCB design standard. However, in my view, the IPC-2221 stepwise limits for distances between the PC traces are baseless: the spacing vs. voltage curve should be linear. Of course, it is not the only standard that defines the electrical clearance. For power conversion circuits IPC-9592 provides the following linear circuit board spacing recommendations: SPACING (mm) = 0.6 + Vpeak x .005. In general, a linear relationship makes more sense. However, again, in my view, IPC-9592 requirements are also too conservative and sometimes are not even practically doable. Note that all IPC standards are voluntarily rather than mandatory. Conversely, for the products covered by safety standards the creepage and clearance requirements of a respective UL/IEC standard are mandatory. For example, for most ITE applications you need to use UL 60950-1 Tables 2K to 2N.

One would think that UL norms should be most stringent than IPC. However, it is interesting to note that when it comes to functional insulation, UL requirements are not necessarily more stringent than IPC. See our guide to PCB creepage vs. voltage for a comparative analysis of different IPC and UL requirements.
Note that if your ITE is manufactured or sold in China, their standard GB 4943.1-2011 assumes your unit must be suitable for use at altitudes up to 5000 m. This will require clearance limit 1.48 times of IEC/UL 60950-1 unless your device marked as suitable for use only up to 2000 m. Also note that the discussed requirements here are related only to breakdown safety. From the standpoint of PC trace temperature rise you may want to significantly increase all the distances (see this for more info and a calculator).

Vpk, V	mm	inches
15	0.1	0.0039
30	0.1	0.0039
50	0.6	0.024
100	0.6	0.024
150	0.6	0.024
170	1.25	0.0492
250	1.25	0.0492
300	1.25	0.0492
500	2.5	0.0984
1000	5	0.197
2000	10	0.394
3000	15	0.591
4000	20	0.790
5000	25	0.984

The chart to the left is adapted from IPC 2221A. It lists recommended minimum spacing between external uncoated conductors as a function of working voltage level for intended use at altitude below 3050 meter. The official IPC table 6-1 gives the spacings only up to 500V and provides formulas for calculating the spacings above 500V. For your convenience, I've added calculated distances for higher voltages based on these formulas. All my calculated numbers of course are rounded.

Different applications of may require different distances. Internal layers as well as external tracks with conformal coating need lower spacings. Converesely, operation with elevations >3050 meter requires greater separation between the conductors.

Note: This guide is provided for informational purposes only and does not constitude a professional or a legal advice- see the Disclaimer linked at the bottom of this page. Consult with respective standards for final design decisions.

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