120/208V allows you to use 3 phase, 4-wire system as the main risers and main source, which is much more economical ($/kw - installation cost and even space) than a single phase system of 120/240V.
Rbulsara and Davidbeach thank you for the response.
So how is it more economical?......Can you also expand on the following:
1. 120/208 v 4w 3p can be utilized as the main riser??....
2. Main source??
3. $/kw - installation cost and even space??
I would really appreciate if a more detail explanation is given for the three items above. All of the information you can share will be highly appreciated.
I see the point from the designers view. However, the majority of electrical equipment is intended to be utilized in a 120/240 volt system. For instance, if a 230 volt rated motor is installed in a 120/208 volt system, it will likely work for a short period of time but not as long as if it would of been energized by a 120/240 volt system. Similarly, it can be concluded that there are many other home appliances which were not design to be used in a 120/208 volt system. Consequently, it appears to me that economics is a benefit from the designers point of view, but the end user of the 120/208 volt system has to pick up all of the costs later on. Indeed, there are motors rated for 200 volt systems and other special home appliances that can be purchased at a higher cost to be utilized in the 120/208 volt system. So depending on what side you find yourself, it can be argued that one individual saves monies by passing on costs to the end user.
The choice for residential would normally arise in apartment buildings. Here, the builder would be buying most of the 200 volt or 240 volt equipment such as HVAC, water heaters, ranges, etc. as well as the electrical service equipment. He can decide which is more economical overall.
Re "So how is it more economical?"
Let's say you put a 20A 80% breaker on each available leg.
Case 1) 120/240 single phase, 3-wire (L1, L2, N):
120v x 80% x 20a x 2 hots = 3840va
3840va / 3 wires = max 1280va delivered per installed wire
Case 2) 208Y/120 three-phase, 4-wire (L1, L2, L3, N):
120v x 80% x 20a x 3 hots = 5760va
5760 / 4 wires = max 1440va delivered per installed wire
3-phase power can deliver more power per pound of installed copper, making 3-phase more economical. Think of the neutral wire as not "doing much" (keep in mind the neutral current is zero when perfectly balanced) -- so increasing the ratio of hot wires to neutral wires will increase the cost efficiency. 120/240 1ph is more economical than 120v 1ph; 208Y/120 3ph is more economical than 120/240 1ph.
Re "120/208 v 4w 3p can be utilized as the main riser??...."
In a high-rise, quite possibly not. Depends on just how high you're rising. In a very tall building, you'd most likely go with a medium-voltage riser with 120v transformers every few floors. In a mid-rise, you might use a 480-volt riser. In a 4-story high rise, maybe you could get away with a 120v riser. You need to review voltage drop and estimated cost of the various distribution systems to make that choice.
2. Main source?? & 3. $/kw - installation cost and even space??
Not quite sure what you're getting at here....
In my experience, 230v motors are quite commonly run on a 208v without any problem at all. Also, keep in mind there are usually very few motors running above 120v in most high-rise condos (perhaps an air conditioner fan, usually not much more than that). Fans don't have very high starting torque requirements (compared to, say, pumps). You could also bump up the secondary tap on your 208 transformer to perhaps 213v, that's well within the 230 x 90% = 207v minimum voltage requirement of a 230v motor. If you're really worried about motor life though, then just make sure your AC units are specified with 208v or 208-230/460v motors.
" the majority of electrical equipment is intended to be utilized in a 120/240 volt system"
That's not true. I have done several condos ( when work was scarse and we wanted to keep or crews working.)
You can get 208 Volt electric ranges, cloths dryers, water heaters, air conditioners, heaters and everything you need to equip a very nice condo.
We ran 2,000 amp bus duct up the building and installed plug in swiches at ever third floor and meter banks at every floor.
Load calcs per the code are a bear. Each unit has a 208/120 volt 3 wire panel. You have to mind your neutral currents.
