I haven't used a 12 V trigger either, but I'll give you my reasoning so far.
The trigger is providing a source voltage/current, and that spec says it will provide 12 V and UP TO 100 ma. Meaning, it can't drive a load that requires more than 100 ma. The relay coil is nothing more than a resistor (more accurately it's an inductor, but it has some static resistance), and that resistance determines the current at a given voltage. The relay spec is saying it will require NO MORE than 18 ma to drive the relay. So the source CAN provide more current than the relay needs, but the relay dictates how much current is actually being drawn. I interpret these specs as being needing more supply current (100 ma) than the relay requires (18 ma), so we're good there.
Next, the NC refers to a normally closed contact, which means the contact is closed when the relay is in it's shelf state (not energized), and you must energize the coil to cause the contact to open. This relay has a couple different ratings. It's rated for 10 A for resistive loads, but inductive loads (like our fan motor) are different. It gives the 1/6 hp for the NC (normally closed) and 1/3 hp for the NO (normally open) contacts. I'll be using the NC contacts because I want to turn the fan on when my trigger goes to 12 V.
This next part is where it gets a little hairy. My fan is rated in Watts, specifically, 132 W. Watts and hp are two different measurements of the same quantity, sort of like miles and meters. Converting 132 W to hp gives me 0.17 hp. So I should be within the rating for the NO contacts, but not necessarily for the NC contacts. That should be fine, but it still made me a little hesitant since there are a lot of unknowns in how each component is rated. So I decided to add another power relay to control my fan. This relay is rated for 2 hp on the NO contacts
which should be fine. BTW, I'm going with this relay for the trigger
. It's just in a different box and is slightly cheaper. Here's the schematic for what I'm doing (the squiggly line is the relay coil).
As far as the ground goes, I'm assuming the trigger is a 12 V DC signal, so no ground is needed. There should be two wires coming from the trigger which will supply the 12 VDC and the reference leg. Those two wires get connected across the relay coil. For the thermostat, that's actually a 24 VAC signal, and would be connected to the G and C terminals on the T-stat, and again the other end connects to the relay coil. However, I think you've talked me out of worrying with the T-stat connection (although I did order a relay for it).
Assuming this is just a standard electromechanical relay, the "polarity" across the coil shouldn't matter. It truly is just a coil of wire, and passing a current through it generates the desired result. Here's the schematic for the relay if that helps. I think we'll be dealing with wire in the 18 AWG range, so a wire nut for splices to the relay pigtail.
And lastly, if you decide you need some relays, DON'T buy them from Grainger. You can find them much cheaper elsewhere.
Did I answer everything?
EDIT: Looks like TMcG beat me to it, but after all this typing, I'm going to leave this anyway Edited by J_P_A - 5/8/13 at 12:05pm