well the way it works is for any
equipment that is to be energized for a period greater than 6 hours shall be rated @ 120% of the maximum ampacity... 6 hours or greater of operation is considered a continuous duty cycle, so this is done to protect the thermal element inside the breaker/fuse. not only can it cause premature tripping or nuiscance tripping, but it can also fuse/weld the breaker shut and cause a failure to trip in a short circuit condition., if you run it @ its maximum rated load with a continuous duty cycle, especially over extended periods of time. the danger is not on the first run, but maybe on the 5th.... by rating your breakers @ 80%, you prevent that.
with a 100 amp breaker, just because the numbers are simple to demonstrate-
100 amp breaker = 100% load = 100 amp
80% load = 80 amps.
80 amps x 120% = 100 amps
you can see you the 120% works now. by rating your breakers @ 80%, your rating your equipment for 120% ampacity, as required by code
multiplying the max amperage by .8 on a breaker, gives you the exact number of amps you can safely run. or watts. the equation works either way. i always use amps myself just because thats just easier for me, ive been doing it long enough i tend to have all the wattages memorized anyway...
so to size a breaker for a grow op-
multiply the max wattage of all connected equipment by 120%. then convert the wattage into amperage by dividing by the voltage.
heres an example: (im ust inventing the wattages, bear with me if there not dead on)
3 600 hid lights. max input power = 660w per ballast.
2 fans 45w each
2 pumps, 75w each
timer/contacter 20w
660 x 3
45 x 2
75 x 2
+ 20
2240w
x 120%
2688w
2688w/240v= 11.2 amps @ 240v. you
always round up to the nearest size so for our examples sake it would be a 15 amp breaker.
to find what you can run on an existing breaker-
multiply the amperage by the voltage by 80%
20 amp breaker, @ 120v... how many lamps can i run on it??
20 x 120 x 80% = 1920w of lighting goodness for your baby girls.
1920w x 120% = 19.2 (always round up, remember?? so its 20 amps.)
and thats how it works. for any power equation.