Relay contacts all have a current rating. This rating is specified by the manufacturer, and it tells you how much current the contacts can safely turn on and off. If you use a relay to control more than its rated current, it will likely go to an early grave.
Relays that switch more than their rated current often end up with their contacts welded together. They appear to be 'on' all the time.Note that contact ratings are for resistive loads.
You must take extra precautions if your relay is switching a circuit with incandescent lamps or inductive loads such as solenoids or another relay coil.
To help your relays live a long happy life, have them switch incandescent lamps that normally consume 1/8th of the contact current rating or less. That's right, 12.5% or less. For example, if a relay is rated for 0.5 amps, only switch an incandescent lamp rated at 62 mA (0.062 Amps) or less.
To maintain UL registration, no rating can be exceeded. A contact rated at 12VDC, 100A cannot be used in a 24VDC, 50A application.
If the rating is 12VDC@100A, I would consider this to be the maximum voltage which could be applied to the relay contact and still expect it to reliably extinguish the arc produced on opening of the contact. To double the voltage would greatly reduce the contact life, even if you are only running half the current.
The contact spacing is the main factor limiting your voltage rating to 12VDC. The contact area will limit the current rating.
Relays have a break rating specified in Watts for DC (apparently 1200 W in your example), but also an absolute max. voltage.
It is always recommended to read the mfg. specs. The major issue at DC is breaking the circuit (quenching the arc - unlike AC, where there is a zero voltage point every half cycle). You may also want to consider the inductive time constant (L/R) of the circuit being switched vs. relay specs, and add a free wheeling diode (in reverse polarity, of course) across the contacts.
Contacts specs (current and voltage) should be looked at as separate specifications and not as a composite power spec. Also if you check the spec on most relays they have different max current specs for AC Vs DC current.
This is because AC can be more easily switched off as the AC voltage zero crossings help keep contact arcing down, where as DC is much harder on contacts at the same ratings. Not a good idea to utilize relay contacts at or near their rated maximum as contact life will be shortened. I would not go above 50% of ratings if reliability is important.
Relay contacts are available in a variety of metals and alloys, sizes andhttp://relays.te.com/appnotes/app_pdfs/13c3236.pdf
styles. There is no such thing as a universal contact. The relay user
should select contact materials, ratings, and styles to meet, as precisely
as possible, the requirements of a particular application. Failure to do so
can result in contact problems and even early contact failure.