One other warning. make sure the EOL relay you get is truly an EOL RELAY. Some "EOL Relays" are solid state, and don't handle the polarity reversal well.
On my first install with internal sounders, I got some EOL relays from a distributor who shall remain nameless. Squirt some canned smoke into a detector, and everything goes off as it's supposed to. And then we get that distinctive smell of burnt electronics. A VERY STRONG SMELL OF BURNT ELECTRONICS. The GC freaked. And when I tried to reset the system, all the smoke zones went into "trouble" status. After peeling the GC off the ceiling and diagnosing the problem, I got on the horn with the distributor. Conversation follows:
"No way! You must have hooked something up incorrectly"
"Yes way! I'm not an idiot! Try it yourself."
Much bitching, moaning, and recriminations.
Distributor rep finally goes out to the stockroom, gets one of the relays, and cuts the plastic off.
"I'll be danged. Let me try hooking it to reversed voltage.....Son of a B****!"
Best bet is to use the same brand EOL relay as your smokes.