Bill writes:
Ecnerwal wrote:
- you'll need
a lot of fixtures to warrant #12 and a 20 amp 240V circuit, .vs. #14 and
a 15 amp circuit.) At half an amp or so per 4-lamp F32T8 ballast at
240V, 24 ballasts (circuit derated to 80% for being on a long time) and
96 tubes (384 feet, if you like) on one circuit and 14 ga wire. For that
matter I'm danged if I know why folks are talking 12Ga wire on a
lighting-only circuit that might, at most, be 16 two-tube fixtures, and
probably won't even be that. Unless the ballasts are terrible (awful
power factor, inefficient) that should normally be well within the
capacity of a single 15 amp 120V circuit - and if being split in twain
so that they don't all go out at once (which, if it is really a lighting
only circuit, is far from likely in my experience, but I respect it as a
design goal) it's even more blatantly inside the reach of a 15 amp 120V
circuit to run half or 2/3's of that load on 14Ga wire.

I do overkill, but I try to avoid _stupid_ levels of overkill when it
costs me serious money. 20 amp breakers and/or 12ga wire to feed 2-lamp
4-foot fixtures in this quantity seems rather stupid, even for me - and
I have most of 1000 feet of 12-2NM I happened to buy back when it cost
$118.

Please see if my math makes sense:

Amps/fixture = (32w + 32w)/120v .6.

There is some loss in the ballast. The ballast will have a rating
plate that documents the maximum load in kva. The graingers catalog
should list kva ratings for most fixtures. Looks like the typical
shop light with F34T12 lamps is 40 fixture watts per ea.

scott