Load Calculation Backup Generator

 

I'm thinking about buying an 8000W, 240V, portable, backup generator to
connect to my house when the power goes out. This type of generator,
obviously, is designed to be used with extension cords that would be
routed through an open window or door, for example. However, I would
like to connect it directly to the service entrance on the back of my
house. I am aware that this type of connection requires a "transfer
switch".

My question is, do I have to worry about how much of a load I have on
each phase of the generator's output? Or, do I only have to worry about
total wattage?

To clarify, let's take an extreme (and unrealistic) example. Let's say
I have a house that draws 6000W on one phase and 1000W on the other
phase and I connect to the service entrance via the generator's  240V,
30A receptacle. So, in this situation I would be drawing about 50 Amps
on phase 1 and 8 Amps on phase 2, for a total of 7000W.

Just as an example, there is an 8000 W generator at:

http://tinyurl.com/plg7l

There is a schematic (PDF, page 2) for this generator at:

http://tinyurl.com/ra6kr

This schematic shows a "120/240V 30Amp" socket connector protected by a
"30Amp 2P" circuit breaker (CB1). So, I guess I've answered my own
question. It looks to me like a current greater than 30A on either pole
will trip the breaker.

So, it's back to the drawing board and here's where I'm definitely
getting in over my head.  Is it possible to parallel the windings
somehow so that the two legs are in phase?

If you could do that then the total output could be connected to both
bus bars in the circuit breaker box. Now in the (hypothetical)
situation where one bus bar draws 6000W and the other draws 1000W,
there wouldn't be a problem.

The disadvantage would be that you couldn't operate any 240V
appliances. However, with these relatively small generators, you
probably wouldn't want to do that anyway. A clothes dryer probably
draws 5000-6000 watts all by itself. You would lose the option to
operate one or two burners on your electric range, but you could use
your microwave oven or your toaster oven instead.

 



     You are really good at answering your own questions!  You are beginning to
understand the tradeoffs involved in a standby power system.  If you are willing
to do a little intelligent load management, and perhaps give up a few of your
largest loads entirely, you will do fine.

  Now how are you going to fuel this monster?

Vaughn

 

On Wed, 10 May 2006 00:00:19 GMT, "Vaughn Simon"


Exactly how our friends in Punta Gorda Florida (Charlie) referred to
it. "who is going to feed the monster"?
It is not an insignificant amount of fuel either. They were filling up
cans every day.

 



The thing to remember is that your Genset when in 120/240Vac mode can
only supply 1/2 the total load amperage to each leg.  So if you have
a 6Kw genset then only 3Kw per leg can be supplied without overcurrenting
that sator winding.  For figuring sake, you can probubly allow an extra
10% to that if you have good airflow thru the generator core, but even
that maybe iffy.  If you were to use a Transfer switch, have a 120Vac
only mode on the genend, and wire the two 120Vac legs together on the
Genset side of the transfer switch, then you could use all 6Kw @120Vac,
but none of your 240Vac Loads would operate.  Most of the 240Vac Loads
wouldn't really be nessesary in Emergency Powered situations anyway.

Bruce in alaska
--
add a <2> before @

 



     I don't talk about it much because few people would understand, so
therefore many would assume I was doing something dangerous, or immoral, or
unpatriotic... but that is exactly the way my CCK has been wired since the
beginning.   It is one of the ways that we survive just fine on 4KW (or even
less when the Onan fails us).

Vaughn