13.4V initially with engine speed at least 1500 or 13.4V after it has been running and engine is idling? What accessories on?
The alternator amperage output at idle is 55-60% of its rating. As result of the load on the alternator the voltage when idling is reduced proportional to what is turned on in car (the loads). You can't get rid of fuel pump and injection system load but the total of that and the few other things you can't turn off is 6-8 amps. Headlights, a/c with blower motor, radio amplifier when equipped, heated rear glass, seat heaters, and the few other possibilities can give load up to 50 amps. If all those were on and the alternator had been charging 15 minutes the voltage would likely be 12.6-12.8 The battery might even be kicking in a few amps at that point, definitely if cooling fan is running.
The voltage regulator has a temp based feature and the voltage will be higher initially to assure starting drain on battery is replenished and then the voltage drops to mid 13V range to keep from heating battery and losing electrolyte to evaporation.