Originally Posted by dschulz
Speccing a new build, with an opportunity to get things right from the start. If planning on using 4 F18s, how many dedicated circuits should I plan for the subs? 1 circuit for all 4 amps? 2? Or 4, with a dedicated circuit for each? This will be a fairly large room (3 rows of seating) and although discotheque volume levels are not needed we do want to be able to play motion pictures comfortably at reference level.
Using some quick math:
If all four subs are taken to the max output of their amplifier (900w each), you'd require 3600 watts, or about 30 amps of current at 120 volts. Allowing a ballpark of 25% for inefficiencies of the amp will bring total power needs to ~4500 watts. Each dedicated 20 amp line will give you about 2400 watts, give or take. So, two 20 amp lines would in reality be more than enough, if you max out your sub amps
Most people who are only going to reference sound levels or less are unlikely to need to max out their 900w sub amps...particularly with four subs and room gain.
Looking at your situation from a slightly different view, if we need to use 3 dB less of power output per sub to achieve the desired SPL, that would cut the max amplifier draw to half, or about 450 watts/sub for 1800 watts total (2250 watts if using 25% amp inefficiencies). If 6 dB less of power output per sub is needed or used, then we are looking at roughly 225 watts/sub or 900 watts total (1125w). If 9 dB less, we are at ~450 watts total (~565 watts w/inefficiencies).
Basically, this is about how loud you want to play and what type of margins you'd like to include. If you want to play it safe, give yourself lots of headroom, maybe considering to go down the BEQ route, then two 20 amp lines. Otherwise, one 15 amp or 20 amp line could be all you'll need.