Jim, 

UL is based on approx. 0.6 volts. 

I've seen others based on approx 0.25 volts. 

How much is too much?  

Well, for UL-1950 approval, follow their Table J.1 (page 187) in the 3rd edition. 

Otherwise, I believe there will be a trade-off among several factors: 

  1. What will be imposed upon the design, 

  2. Wet/dry environment and temperature, 

  3. Cost of material to comply, 

  4. The level of electrical current being run through the junction, 

etc, etc, ...

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Doug McKean
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The comments and opinions stated herein are mine alone,
and do not reflect those of my employer.
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