At 09:58 AM 7/9/2006, you wrote:
>I had a local electrical contractor come out to estimate the cost of backup
>power for the house, shack and barn. Each has a 200 amp breaker panel. I
>have one 500 gal propane tank for the house and another for the barn about
>100 yards away.
>
>For all three they proposed the Generac 45KW Quietsource unit running off
>the propane tanks. It has a 2.4 liter inline 4 cylinder engine. HP is not
>specified. The unit measures about 6X4X3 feet.
The usual rule of thumb is HP=2*kVA. (A HP is actually about 750W, but
it's hardly a 100% efficient conversion from idealized brake HP on the
motor to watts out of the generator. the rule of thumb basically assumes
67% efficiency)
Another handy rule of thumb is that the BSFC (brake fuel specific) is 1/2
pound of fuel per horsepower per hour. This one is handy for sizing fuel
tanks, etc. gasoline and diesel are both about 6 lb/gallon.
>I called and asked why so much capacity is specified. The answer is that
>the proposal is their standard for a business installation which requires a
>rating of 2X max current to avoid power dips. On average the load is 25
>percent of max.
Not only that, but for some installations, the regulatory requirement is
that the generator be sized to accomodate the calculated "book" load (the
same way your service drop and panel is sized), not actual demand.
The underlying assumption is that while YOU might be intelligent and know
how to manage the load to match the generator output, the next occupant, or
someone else in your facility, might not. If you size it to the same
standard as the regular utility feed, then it's literally a no-brainer.
As with all regulatory requirements, there are ways to get waivers and
exemptions, but, usually, because that exemption process is labor intensive
(and with expensive people like attorneys and engineers involved), it's
cheaper to follow the recommendation.
Jim
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