Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Can anyone explain to me why there are 2 maximum voltages/amperage for both AC and DC like below ?

10A 250VAC 10A 125VAC

10A 30VDC 10A 28VDC

I was thinking that the one on the left is for normally open and the one on the right for normally closed ?

enter image description here

share|improve this question
Can you upload a picture?. One will be european and one US/UL I guess. – Spoon Jul 6 '14 at 19:05
Check edit pls. – drinu16 Jul 6 '14 at 19:07
i never heard of european / US DC voltages ?! – drinu16 Jul 6 '14 at 19:09
My guess is that these are voltages that are common in the application of these relays, so the ratings are specified at those points. DC ratings are much lower because AC voltage crosses zero and extinguishes arcing, where DC arcs at similar voltages can persist causing much more rapid wear-out of the contacts. – John D Jul 6 '14 at 19:14
Yes that I know, but why different DC voltages ? – drinu16 Jul 6 '14 at 19:16
up vote 2 down vote accepted

NC vs NO is is usually a different current at the same voltage (if the ratings are, in fact different).

More likely the differences you cite would refer to compliance with different safety agencies.

For example, this one:

enter image description here

Should not be used at more than 125VAC if you need compliance with Canadian and US safety requirements-- the cURus component registration. The other rating refers to other safety agencies.

share|improve this answer
Lol I picked the same one by chance. – Spehro Pefhany Jul 6 '14 at 19:12
Yeh :), the DC is my main concern. 30v and 28v. what difference would it make as long as your not using your country AC – drinu16 Jul 6 '14 at 19:14
Use the 28V rating in the US/Canada because that is where it was tested (to cURus standards) to not cause a fire. It may not make 100% sense, but this is CYA sometimes rather than EE. – Spehro Pefhany Jul 6 '14 at 19:16
Ahh, so CQC saftey limits are 250vAC or 30vDC and cURus safety limits are 125VAC or 28VDC ? – drinu16 Jul 6 '14 at 19:17
So in other words the max limit for DC is 30V, but just to be safer the max limit is 28V ? – drinu16 Jul 6 '14 at 19:19

Don't know about the compliance thing mentioned here however mechanical relays have different AC and DC ratings - this is standard! Now why... When the relay opens there will be an arc between the contacts.

If you are running AC (say 50Hz) the current goes down to 0A A 100 times per second, after this happened once the arc is gone. When you run DC there is no interruption to the current flow so the arc eats away on the contacts for longer. This is why the DC rating is always much lower.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.