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Everywhere I dig around the web (in forums etc.) I see people not recommending more than 12v to go on an automotive relay.

Let's look at this one for example:

enter image description here

I understand they have been designed for 12v systems but the vast majority, if not all, have rated amps on them.

AFAIK conductor cross section requirements have no relation to voltage so if it can carry 40A at 12v it should be able to carry 40A at 50v or any arbitrary, reasonably high, voltage. That is until we get to the point where the voltage gets so high that arcs could jump through gaps between the inside components of the relay but that would be way out of the league of anyone like me.

I'm suspecting the posts I found which discourage going above 12v are regarding the energizing voltage, and not the carried voltage.

Is my guess reasonable and would it be possible to use higher voltage on the NO/NC paths of relays such as the one shown above?

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  • \$\begingroup\$ Most likely. Check the datasheet. \$\endgroup\$ – winny Jul 12 at 20:00
  • \$\begingroup\$ a relay does not carry voltage when closed, it carries current ... the voltage rating is how much voltage it is able to hold back without arcing when the contacts are open \$\endgroup\$ – jsotola Jul 12 at 20:19
  • \$\begingroup\$ @jsotola the required voltage to jump 1mm of air is about 3000V or that's not what you mean? \$\endgroup\$ – php_nub_qq Jul 12 at 20:21
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    \$\begingroup\$ when the contacts open, higher voltage differential could produce a sustained arc \$\endgroup\$ – jsotola Jul 12 at 20:23
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Yes, more than the rated contact rating of 14V, but not with 40A.

If you read the datasheet for that relay, maximum voltage it can switch is 75V. However, it can't be used to switch 40A at that voltage, there is a 630W limit for the load.

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  • \$\begingroup\$ But why is that? The NC path could be looked at as a regular conductor. A conductor designed to carry 40A can carry 40A at almost any voltage? \$\endgroup\$ – php_nub_qq Jul 12 at 20:15
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    \$\begingroup\$ It's about contact arcing as you mentioned. There's no zero crossing with DC to extinguish any arc, so the higher the voltage the longer the arc will occur as the contacts open or bounce on closing. So the relay's lifetime design is specified for the given conditions, and violating them could cause drastically shortened lifetime or contact welding. \$\endgroup\$ – John D Jul 12 at 20:20
  • \$\begingroup\$ Yes the problem is not the current when it is flowing, it's the breaking of it. It is far more difficult to break 630W load than a 1W load. Some relays can pass much more current than break, and this is indicated in the datasheet. \$\endgroup\$ – Justme Jul 12 at 20:27
  • \$\begingroup\$ Thank you, I think I see a clearer picture now. I'm thinking of building a poor man's H bridge with two DPDT relays to reverse a 1kW motor (~30A @ 36VDC) and I might just get away with using those if I only do the switching when the motor is stopped, which would by the way be the normal use case. \$\endgroup\$ – php_nub_qq Jul 12 at 20:31

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