Cougarjack@aol.com wrote:
> 20 amp run current for air conditioner @ 208 volts=4160 watts
Not quite... at 20 amps, the running power would be 7200 watts. The
air conditioner runs on three-phase AC, not the single-phase AC that
we have in most of our homes, and the calculations are a bit
different. The total power is three times the average power per phase,
and when you use the line current and line-to-line voltage, there's a
square root of three thrown into the mix.
208 volts is the line-to-line voltage (or "line voltage") that would
be seen by a delta-connected load. A wye-connected load would see
line-to-neutral voltages (or "phase voltages") of 120 volts. The van
would be supplied with a 4-wire (plus ground) 208 volt three phase
power supply. 208 volt three-phase delta-wound loads like the air
conditioner would be connected to all three phases, and 120 volt
single-phase loads would be evenly distributed (we hope) between the
three phases, with each one connected to one phase and the fourth
neutral wire. The line voltage equals the phase voltage multiplied by
the square root of three.
The 20 amp circuit breaker indicates the maximum line current per
phase of the air conditioner. The breaker would actually be three 20A
breakers ganged together.
The power required by a three-phase delta-connected resistive load is
three times the product of the line current and the phase voltage:
3 * 20 * (208/sqrt(3)) = 7205 watts
or,
3 * 20 * 120 = 7200 watts (with a bit of round-off error)
Now, the air conditioner's dataplate rating is 28.5 amps. I believe
that's the starting surge, since my book indicates that the air
conditioner is connected to a 20 amp thermal (slow-blow, that is)
circuit breaker. I recommend using the starting surge current for
generator sizing, since that surge current is repetitive, and will be
seen every time the thermostat turns on the compressor:
3 * 28.5 * (208/sqrt(3)) = 10268 watts
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