As a hobby project I'm building a power line monitor to detect when a load is switched on and turns on some additional loads. I have 120 V 220 V running through my board and was curious what the standard board spacing is for high voltages.

I'm sure there's some specs by UL or other agencies for this, but I'm cheap.


Correction, the voltage I'm working with is 220 V, powers a cabinet saw. In any case, is there a general formula, possibly even for higher voltages (e.g. flyback for fluorescent lights)

  • \$\begingroup\$ I don't know a rule off the top of my head, if you decide to look at actual regulations then IEC 60950 is what your after but i don't think you can get that for free. \$\endgroup\$ – Mark Nov 24 '10 at 2:26
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    \$\begingroup\$ It pisses me off that IEC, ISO and the like want money for their standards documentation. They already get membership fees, don't they? And if I had the rights to some standard, I would love to see it spread as much as possible. A job for wikileaks, maybe? :-) \$\endgroup\$ – stevenvh Nov 28 '10 at 8:08
  • \$\begingroup\$ @steven - You can register for free with the JEDEC standards body; their motto is "Leading developer of standards for the solid-state industry, all being available online, free of charge." Not applicable here, but useful in many other cases like memory and layout design! \$\endgroup\$ – Kevin Vermeer Feb 1 '11 at 6:49

Minimum 2.5mm for standard insulation (across mains), 5mm for reinforced ("double") insulation (mains to low-voltage)

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    \$\begingroup\$ Allowances for board creepage distances will make this larger, too. \$\endgroup\$ – Connor Wolf Nov 24 '10 at 1:02
  • \$\begingroup\$ Wait, Is this for insulation thickness? Cause it's insane for adjacent traces on a PCB (even with soldermask). \$\endgroup\$ – Connor Wolf Nov 24 '10 at 9:34
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    \$\begingroup\$ Sounds about right for adjacent tracks and pads. Not insane, required to prevent flashover. Dirt and moisture levels affect the distance required. If you don't use these sort of creepage distance, you are creating a safety hazard. In Europe, that comes under the Low Voltage Directive and associated standards; equivalent in US is UL. \$\endgroup\$ – Martin Nov 29 '10 at 13:22
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    \$\begingroup\$ IIRC IEC needs 5.5mm for double insulation (may even be 6mm). An DIL optocoupler with standard pads doesn't comply. \$\endgroup\$ – stevenvh Jul 16 '11 at 7:02

Consider looking at footprint of smallest 400-600V capacitor (which is minimum DC for 220V AC, with peak-to-peak 310V, so overrate is extra 33..100%). If its pins are 15..20 mm apart (I have not seen less than this), then with traces it makes about 11..17 mm. gap.


Various standards I have read seem to indicate 1.5-2mm for mains-mains, and 6-8mm for mains-low voltage.

(Note I mean mains = "low voltage", low voltage = "safety extra low voltage in standards terminology)

  • \$\begingroup\$ References could make this a much better answer. \$\endgroup\$ – akaltar Mar 13 '16 at 14:55

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