Amp rating question

Hi, a screw terminal block rated at 15 Amp 300 volts just like this Molex pn: WM5766-ND should allow 15amp x 300volts = 4500 W per bus and should allow 4500W / 12 V = 375 Amp per bus @ 12v. Is this a correct assumption?

thank's

No. Do not exceed either the ampere or the voltage rating.

Absolutely not. The maximum current is the maximum regardless of the voltage -- 15A at 300V, 12V or 1V. The current carrying capacity doesn't increase with the voltage and you would only manage 12V x 15A = 180W.

Also check carefully on the relay rating to see if the 15A is for AC. There may be another, lower, maximum current capacity for DC.

This is what I suspected and why I asked.
Thank you

As mentioned above the voltage and current ratings are not related.

The voltage rating is that voltage that will not conduct from one section to the next. Note it will not flashover at 310 volts but at that point you are nearing the limits of the device.

The current rating is the amount of current the device will pass without heating up too much. It has to do with the thickness of the metal, the material, plating etc.

The current carrying capacity doesn't increase with a lower voltage

@cdb101 ,
A suggestion for solving questions like this is to ask yourself what would happen if you take the question to an extreme. In this case what if the voltage were 0.1V, then, if you idea were correct, the connector would need to carry 45000A. I hope it is obvious that this can't happen, invalidating the idea.

You can take this to the furthest extreme and realise if the voltage were 0V then the required current would be infinite, which is absurd.

You can apply this technique to quite a lot of problems.

Note that switch ratings are very different to wiring/terminal block ratings,
if this were a switch the DC ratings would be very different from AC.

With a non-switched piece of wire or screw terminal the limits are thermal
(max rms current) and breakdown (max voltage).

Be conservative - if you have 14A to handle, use a 25A or 30A part rather
than 15A if feasible - much less self-heating, longer life.