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Subject: Re: TWL: 15 KW Onan
From: Arild Jensen (elnav@XXX.XXX)
Date: Sun Sep 02 2001 - 10:36:28 EDT
At 11:28 AM 09/02/2001 -0400, Jim Alexander wrote:
>In my search for a suitable vessel I came across yesterday a 44' Yacht with
>a 15KW Onan. This is the single largest Genset I have seen on a boat this
>size. The boat has dual a/c's, electric Fridge and electric range.
>However, I have always been led to believe that it is extremely unwise to
>run an genset under partial load. Since most other boats this size I have
>seen get by with 8KW gen's, isn't this a little large and would it be
>prudent to operate it just for say firing up the stove for a pot of coffee?
>
>Jim
Jim;
Do a load calculation. An electric stove with oven will draw 6Kw when
running full out. ( major dinner party)
Dual A/C - what size? if both are 20,000 BTU and both are hard start
models you can easily figure 2.5 Kw to start each one.
And the fridge, freezer, plus a few other lights wil easily eat up
another kw or so.
When calculating genset capacity you have to size for the maximum peak
surge when motor appliances start up.
Average power consumption is not sufficient, as would be the case for
inverter calculations.
The formula I used if no data was available is a hard start motor uses 80%
of the locked rotor current as a peak start current.
Usually you get the average power consumption plus the locked rotor
currrent on the Mfg plate. This is needed so electricians can
select the correct breaker or fuse.
You didn't mention a hot water tank but that adds another 1.5 Kw You have
to assume the air conditioner will start up after the stove is turned on
full so the A/C load is on top of the full stove rating and if the cook is
busy, chances are something is being washed up and the water heater is
going to cycle on. And of course during meal preparation the fridge door
is opening repeatedly. So the fridge will also cycle on.
You will damage the motors if you run them on low voltage so to be safe I
would calculate a genset size that can sustain full output voltage under
worst case conditions.
Even without specific numbers I come up with 12 Kw so 15Kw is not
unreasonable.
Many people skimp and this also causes premature failuer since the
genset is overloaded.
The difference is, its the appliance motors that burn out and need
replacement.
As for doing a pot of coffee only, well that is what inverters are for.
( smile)
Cheers
Arild
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