Power Block Controller

You can definitely do what you suggest with the BPCs.  That's how I have my layout set up.  And you are correct that with the sidings turned "off" by the BPCs, there's no additional power draw on the TPC 400 and 180 watt bricks.   It's not the amount of track that affects the power draw -- it's what is on the track and actually on and operating (a motor, electronics in an engine, lights in a car, or whatever).

I can't speak directly to the use of the SC-2 as I don't have any.  But I've seen previous posts where someone asked about using an SC-2 to switch track power on and off, and the conclusion was that the SC-2 is rated to carry sufficient amperage for that to work.

I think an ASC (Accessory/switch Controller) would be a better choice for powering the sidings. 

Two BPCs would certainly be overkill.

BPCs give you the ability to select a block of track to be conventional or TMCC.  An ASC can be used to turn power on & off to the blocks if staying in one operating mode.  There is no sense in using BPCs if you are using one TPC. 

Geezer

The lionel manuals for both the BPC and the ASC are on line. You can also read up on the TPC connections you may need.  You should read them.  The ASC is basically an on/off switch for turnouts and accessories but can also be used for block control, OTC control - almost anything you want to control on/off - expensive toggle. If you use the ASC/SC1 or SC2, the TPC doesn't come into play - no place to connect it to the ASC/SC.

It isn't clear what you  use the TPC to do on your layout. IMHO, if the TPC plays a vital role - like running conventional - then consider the BPC. There are ports to connect the TPCs. A BPC can only control 4 blocks, so two would be needed and they can be pricey.

You most likely operate the turnouts with the hand-held remote and the use of a toggle might seem archaic. BUT, 5 toggles would be substantively cheaper, especially in the vain of how you describe your use of the sidings.

If all you want to do is turn power off or on to your sidings I would agree

with the others on the ASC. It controls 8 devices and the the power ratings

are the same as the BPC. (output voltage 24 volts, output aperage 20 amps)

I made the expensive mistake of buying two BPCs to do what one ACS could do

Also I find the commands used to operate the ACS is less confusing.

But if you want to do something more exotic then may be the BPC is better

choice.

Joe

 A SC2 would work with your setup as I have a similar setup. I have 2 180 powerhouses hooked to aTPC400 and using a SC2 to turn power on and off on blocks. It Works great. I use the power pack from the SC2 to power the SC-2. The setup also powers the mainline really works well. Plenty of power and easy control of the sidings.