In article , Bill
wrote:
Please see if my math makes sense:

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

So a 15 Amp circuit(derated 80% to 12) with 14ga wire may support
at least 12/.6 = 20 fixtures.

Should I be considering a "ballast surge" or anything like that (like
they have when an electric motor is first started)?

Bill

Well, I looked at some actual 120V input numbers for a "high ballast
factor" (more light out, more current in than "standard" or "low"
ballast factor) 2-lamp F32T8 ballast, and got 18 fixtures (0.64 amps)
for 12 amps (15 derated 80% for continuous use) - actually 18.75, but
there are no 3/4 fixtures on the menu.

Starting surge, if any, won't be enough to matter - that's NOT
continuous for one thing, and a 15 amp breaker will happily supply 30
amps for a longer period that you might expect (far longer than any
start-up surge in your ballasts. Wire sizes and code are quite familiar
with those details and take them into account.) A modern ballast
probably has little, if any, anyway.

Now, that is an energy-star efficient ballast which also has good power
factor - and the "standard" ballast factor version (0.46 amps) will run
26 ballasts on 12 amps. A different brand of standard ballast (0.49
amps) will run 24 ballasts. I'd prefer a real example for a terrible
ballast, but the cheap shop lights are a few miles from the computer -
IIRC some of them might be as much as 0.75 amps for a two tube fixture -
they don't actually use more power, but the power factor is terrible, so
they draw a lot more amps.

As such, I think you are fairly safe running your 15-16 fixtures on a 15
amp circuit - but as always, check the actual numbers before you spend
your money on fixtures. The ones with terrible power factors draw a lot
more current, even if they are running the same wattage.

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