Power consumption questions

When you plug an external multi-channel amp in to an audio-video receiver for increasing power output and overall performance, I'm assuming the power supply in the AVR shuts off automatically. Is that correct or does it stay on and work in conjunction with the new separate amp for driving the loudspeakers? Or does it get bypassed but stays on and used for powering the preamp processor?

If it does in fact get bypassed for driving the speakers, how could I then calculate how much of the rated power normally consumed by the AVR would be reduced (in watts) if just used as a preamp processor? I'm talking about Rotel equipment here and this information is not given in the owners manuals or specifications anywhere. In my case, the AVR has a rated power consumption of 450 watts and the multi-channel amp 800 watts.

The reason I'm asking this is that I only have a 15 amp 120VAC circuit available for my system and with the potential addition of this mulit-channel amp, I could be coming close to maxing out considering the other appliances that would be in use on that same circuit. Thanks in advance for the replies.
The power consumed by the AVR when used as a preamp would be around what it consumes when sitting idling, with or without speakers connected. That is just a few watts, certainly nothing to be concerned about that would impact your 15A circuit.
I think your Rotel AVR consumes more than just a few watts for your intended application. Its user manual published the power consumptions for standby, idle, and max. However, from my experience, these published specs tend not to be accurate. For example, Rotel's website states that the 1057AVR consumes 6 watts during standby. However, its user manual states 36 watts for standby and 78 watts for idle.

I also think you may have too many equipment on that 15A circuit, if they all plug in the same circuit. It looks that you have a TV, 2 subwoofers, an AVR, a muti-channel amp, etc. If you have a cable or satellite box, it consumes more than just a few watts of power too....
78 watts or even 100 watts if the AVR idle figures are understated, is still less than 1 amp, hardly worth worrying about. The other equipment may be another matter, and I would be concerned about any non-audio devices on the same circuit. Nevertheless, in real world use, a multichannel system does not draw that much power at normal listening levels.

My HT system is on a single 20A circuit Components on that circuit include 100W x5 AVR, 210W x 2 amp, 150W 12" sub, LCD TV, DVD player, 2 cable boxes, all plugged into a Belkin PF-60 line conditioner. That system draws 2 amps at idle. While watching movies at normal listening levels, it draws between 3-4 amps. Comparison of the current readout of the Belkin unit against that of my RMS reading Fluke clap-on meter confirmed the readout to be accurate. Similarly, my 2-channel system, with 600W x 2 amp @ 4 ohms, draws around 3 amps at normal listening levels.

As a newlywed many years ago, I started out in an apartment with only two circuits, 20A in the kitchen, and 15A for the rest of the place. I made do just fine with my system on that one circuit and amps in the 200 to 330 watt range.

Consider the purchase of a 'Kill-a-Watt'...an external, plug in power meter. It will give you Volts / Amps / Watts and other data, including hours and Watthours.

So, instead of reading back plates of equipment, you will be able to measure stuff and see. Even a hi power amp won't draw much current if not pressed. If pressed and the current capacity of the circuit is insufficient, you may see a voltage drop or sag.

Another measure is 'Power Factor', which for most gear is very near 1.0, meaning that current and voltage neither lead or lag one another. This is good.

The cost? I found 'em at Fry's, a local HUGE SoCal retailer for all of 20$. Other power measuring equipment has up to and including, a USB port to connect to your confuser. They are pricey and to me, vast overkill.

Let us know what you MEASURE.
I like this: http://www.p3international.com/products/special/P4400/P4400-CE.html
Yes....that's the killawatt...
I had only seen it 'internet', but when I spotted it at the local retailer, purchased it immediately. Pretty useful gadget and a great way to see just how much electricity you DO use.
Thanks everyone. Great input. Looks like overall I can expect not to have problems and blowing the circuit breaker. I can not change the other devices that are powered in this same circuit (lights, TV, exercise machine, etc). But the draw should be well under the 15 amps. I just don't want to invest in this amp only to find out it's not going to work out. I'll look into this meter too.
Follow up. I have the RMB-1075 now and so far so good. No problems with power but I do see the lights flicker for just a millisecond when I turn on the amp at times. Thanks again for all of your input and help.