How much power is enough?

I power my 1000 ft2 layout with two 135 bricks.  I normally only run can motor locos.  I also power almost 50 DZ2500 switches off the track power.  About 500' of mainline, counting 3 passing sidings and the main yard.  The layout is one block as I use TMCC.

Can run two double headed Big Boys, a Challenger pulling a lighted 10 car pax consist,  a 2-6-0 with a lighted short commuter consist, an 0-6-0 switching the yard, and a 2-8-0 local at the same time without problems. 

Track per se doesn't consume power.  How many loco's, lighted cars, accessories, etc.?  That's what eats power and requires large or multiple supplies.

Too Much Power Available is NOT the problem.

The main concern would be sufficent circuit protection to avoid a meltdown of circuits, wiring, motors and accessories.

I have 2 MTH Z4000 transformers and 4 KW transformers in my system. Every circuit is protected with circuit breakers and fuses (where necessary). 

I have individual circuit breaker protection on the 120v power for all my transformers as well as the individual 20V outputs going to my TIUs.

Note: The biggest concern should be the potential for fire if wiring becomes overheated.

I also protect the outputs from each of my 2 MTH TIUs with circuit breakers.  Plus each individual power feed to my layout is protected with circuit breakers.

Even though the manufacturer adds internal circuit protection it is best to add other external protection as a backup.

With great power comes great responsibility. 

Fuse it wisely.

My MTH Z4000 transformers provide a maximum of 400 watts.  As a result the maximum input current would be about 3.3 amperes per transfomer.

A system that can consume up to 3600 watts would probably require about 30 amperes total.  Most "home" circuits are rated at 20 amperes maximum. However depending on the size of wire used it probably would be wise to limit the maximum current to 15 amperes. Some modern homes use 16 gauge copper wire which would allow for slightly higher current loads according to local and national electrical codes.

I'd be somewhat concerned about the load on the household wiring too. That's over 30A. 

A system that can consume up to 3600 watts would probably require about 30 amperes total.  Most "home" circuits are rated at 20 amperes maximum. However depending on the size of wire used it probably would be wise to limit the maximum current to 15 amperes. Some modern homes use 16 gauge copper wire which would allow for slightly higher current loads according to local and national electrical codes.

Huh?  A 20 amp circuit requires 12 gauge copper wire.  If the circuit is only 15 amps then 14 gauge wire is acceptable.  I don't know what electrical code would allow puny 16 gauge wire to be used.

Keep in mind that the design of a circuit either 15 amp or 20 amp requires a de-ration to 80%.  3600 watts would be slightly more than (2) 20 amp circuits properly de-rated.  

15 amps (12 amps)  @ 110 volts    1320 watts.

20 amps (16 amps)  @ 110 volts    1760 watts. X 2 = 3520 watts.

I have one dedicated 20 amp circuit to the train room. That had BETTER be enough....

Gilly

Another thing to keep in mind.

Even if you have 20 PH180 bricks, the possibility that all of them are going to be running anywhere close to maximum power is probably nil, especially on an individual's layout.  You can easily run two or three decent consists on a single PH180, is anyone really going to be running 40-60 at the same time?  That would be a layout the size of the NJ Hi-Railers at least!

I have three PW ZW's for four loops and one PW ZW for switches and accessories = 275 x 4 = 1,100 watts (at best, the reality is somewhat less in output, as I understand it).

I am fine with this.

I am looking into a ZW-L.

Given you said it is a large layout, this does not strike me as a ridiculously large amount of poweror overly large number of power supplies.  More than sufficient, yes, but not ridiculously more.

TOTAL AMOUNT OF POWER: One can do the math pretty easily.  The "hungriest" train I have is the Lionel Santa Fe Anniversary passenger set.  It's A-B-B-A engine set has, I think, four Pullmor motors, and it came with seven lighted passenger cars with incandescent lights, to which I add two more normally.  It will temporarily draw 11 amps when starting (flashing the ZW-L's warning light but dropping below the 10.0 amp limit as the train starts to move before the unit trips) and it routinely pulls 9 amps when up to speed - at around 15-16 volts for about 140 - 150 watts total when running. Even though Gunrunnerjohn's advice (that you'll never be running everything at max, at once) is very sound, you can total up how many trains would run at once and multiply by 150W per train to give you a nice upper limit on what youwould ever likely need.

   Add to that accessories and building/layout lighting.  A large lighted building (incandescent not LED) can consume 20 watts and a layout can have a lot of buildings/accessories. 

   So your friend's layout with 3600W net output could run ten trains at 150 W each and 100 buildings and accessories at 20W each.  That is a lot of power but it is not beyond the limits of what a large layout may need.

TOTAL NUMBER OF BRICKS: people often use two or more power supplies when one would actually do, because they want to deploy them to different locations or track circuits or whatever - there are just lots of reasons.  I have one ZW-L and one CW-80 that run up to four trains and my accessories (a total of 800 watts although I have never come close to using all at once even if I have used up to 150+W per train).  On my Superstreets loops - each has a different supply: 'Streets cars typically run at about 6 watts each and the total there for all seven loops never exceeds perhaps 50 watts total but I have seven separate supplies nonetheless that add up to well over 350W.   Just the way I do it . . .

TWO OR MORE HOUSEHOLD CIRCUITS: Your friend will need at least two and perhaps three normal household circuits to power twenty bricks.  But this, too, is often done.  I run my locos and accessories off of circuit, and my Superstreets power supplies and my workshop tools off another, not just because I need to but because of where I want to plug the various supplies into the wall sockets.  Again, this can be common on a big layout, too.

You need to check your 110 volt wiring fpr Current/Power capacity. You will need to add more wiring capacity to the train room unless you preplanned for this much power. Check code and maybe and with an electrician. Get all outlets used for track power on the same phase so you do not have phase issues later. Put all the room lighting and other stuff (fridge, dehumidifier, Computer and electronics on the other phase of the house to help balance phase load.