# Re: PC55s on a few different ballasts

```Scott wrote:

If you want to know how many watts a ballast is gong to drive through
your bulbs, look at the amp rating on the ballast and multiply by 120.
While that won't give you the true value, it will be close (probably
within 20% in most cases) and much closer than assuming that your bulbs
will be driven at the wattage printed on the bulbs.

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This is not true. Since the voltage that developes across the lamp is
dependant on the lamp length and lamp diameter and gas fill, the wattage
drawn by the lamps can be much lower than the rated amperage of the
ballast * voltage. Case in point is the 112 watt 4 x 32 ballast driving
2 24" T12s. The total wattage drawn by the ballast is only 45 watts.
This is only about 22.5 watts per lamp and they are 20 watt lamps. There
is going to be at least 3 watts in ballast losses too. This is in the
ballpark for what I would have expected though. 2 265 mA circuits per
lamp = 466 mA Since the operating current for those lamps is supposed to
be 430 mA then they are being overdriven by 466/430 mA which equals to
8%. It would be quite a different story if they were 17 watt lamps. Then
the lamps would be overdriven 75% and they whole ballast should draw
about 60 watts. If they were 4' lamps the ballast should draw about 112
watts. In fact the the ballasts draws .80 amps which on my house voltage
is about 94 watts or 47 watts per lamp. This is only about 50%
overdriven and not 75% overdriven. Partly due to the fact that my
household voltage is not 120 volts but probbaly also because the lamps
were old and probably don't draw as much current as they used to. Even
so there seems to be something that mitigates the current through the
lamp or the voltage through the lamp to slightly below what is expected.
The lamps are damned bright though and measure with my digital camera as
being 50% brighter than lamps that are only driven at normal currents.

Wayne

```