Why use WYE in residential apartment buildings

[winnie]

I know that it is common to use two legs of a 208/120V three phase service to create 'single phase' service for apartment buildings.

I am curious as to why this is done.

It would seem to me that the individual residence is better served by a 120/240V single phase service, and that the large collection of residences could be served by three single phase supplies.

If a single 'service' is required, or of 'house loads' such as elevators require 3 phase, then a 'hexaphase' system would fit the bill, providing 120/240V for residences and 208/120V for 3 phase loads. Phase current for the same total kVA would be halved, possibly reducing available fault current.

Just a random musing thinking about @Tainted project.

Jon

 

[don_resqcapt19]

I expect that is decided by the power company to help keep their distribution loads balanced across the phases. I have only seen it on larger apartment buildings, however I have been told that it is sometimes done that way in subdivisions for single family dwellings.

 

[mbrooke]

More efficient and balances the POCO better.

Hexaphase is loads of added complexity for what is typically a none issue (ie 208 on 240 volt elements and motors).

The reduced output power is typically compensated for or not noticed.

230 volt motors are typically rated 197 min to 253 max, so 200 volts on an AC unit is technically within tolerance.

 

[Hv&Lv]

More efficient and balances the POCO better.

??

Balances better?? That is the electricians job

 

[mbrooke]

??

Balances better?? That is the electricians job

Right, after the meter. But not so much on the 12kv side of things.

If a 3 phase feeder is present placing the whole unit or building on one phase make little economical sense.

Me, personally, anything over 150kva tends to go 3 phase.

 

[Hv&Lv]

three phase line to an apartment complex, the electrician should balance the apartments for each phase.

If you try to put all the apartment complex on a single split phase transformer it will lead to imbalance on that phase but that can be easily fixed down the line.

Depending on the size of the complex the service size could be massive from a single split phase XF.

Can’t supply the complex with three different single phase services either.

If you have three different complexes they can be put on three separate split phase transformers so they will have 240/120.

We have most of ours fed with split phase.

 

[Hv&Lv]

If a 3 phase feeder is present placing the whole unit or building on one phase make little economical sense.

That makes no sense. I can place an entire complex on one phase and rebalance if needed by moving other single phase loads. It’s fairly easy to balance at the substation level.

 

[Hv&Lv]

Me, personally, anything over 150kva tends to go 3 phase.

I have an entire motel on a 225kVa split phase XF.
We also have several 167 kVA XFs out there.

you, personally, don’t install anything or design anything, remember?

 

[mbrooke]

I have an entire motel on a 225kVa split phase XF.
We also have several 167 kVA XFs out there.

you, personally, don’t install anything or design anything, remember?

No doubt such exists, but do you really want to balance the rest of the line to take into account half a megawatt of load on phase B? (200% short time overload on that 225kva unit)


I no longer install, but it did give me good insight on the design aspect that anything over 400 amps really should go 3 phase IMO.

{Remember, in a single phase system you have periods of time where zero power is transmitted, where as in 3 phase there is always power transfer at any instant in time.}

 

[Hv&Lv]

No doubt such exists, but do you really want to balance the rest of the line to take into account half a megawatt of load on phase B? (200% short time overload on that 225kva unit)


I no longer install, but it did give me good insight on the design aspect that anything over 400 amps really should go 3 phase IMO.

{Remember, in a single phase system you have periods of time where zero power is transmitted, where as in 3 phase there is always power transfer at any instant in time.}

balancing is something that every utility does. It’s not a problem. Your thinking small. Think about an entire circuit.

(remember, one phase of a three phase system, you have periods of time where zero power is transmitted on that one phase. What are you getting at...)

At the substation level it is always three phase.
Single phase loads aren’t an issue at the substation level

 

[winnie]

Can’t supply the complex with three different single phase services either.

But don't you use 3 single phase transformers to supply a 3 phase service?

If these were 3 mid point grounded transformers then you would have a 6 phase source from which you could pull 120/240 or 208/120 as needed for the individual meters.

Jon

 

[mbrooke]

balancing is something that every utility does. It’s not a problem. Your thinking small. Think about an entire circuit.

(remember, one phase of a three phase system, you have periods of time where zero power is transmitted on that one phase. What are you getting at...)

At the substation level it is always three phase.
Single phase loads aren’t an issue at the substation level

Small, but easy. I crave simplicity. If you have plenty of 225-500kva single phase customers its easy, just rotate each one in successive order. However, if you've only got a few you may have to take a portion of your B phase split phase customers and move them to A and C phase.

Heavily imbalanced distro feeders may be an issue at the substation level, and often so when phase angles, ties and voltage balance is involved.

 

[Hv&Lv]

But don't you use 3 single phase transformers to supply a 3 phase service?

If these were 3 mid point grounded transformers then you would have a 6 phase source from which you could pull 120/240 or 208/120 as needed for the individual meters.

Jon

Yes, but not three different single phase service points on the same building.
Fire department would have a fit...

 

[winnie]

Yes, but not three different single phase service points on the same building.
Fire department would have a fit...

I guess my question should have been 'why isn't '6' phase more common for apartment buildings. Once you split things up at the meter banks, it doesn't matter that there was something uncommon upstream.

Jon

 

[don_resqcapt19]

But don't you use 3 single phase transformers to supply a 3 phase service?

If these were 3 mid point grounded transformers then you would have a 6 phase source from which you could pull 120/240 or 208/120 as needed for the individual meters.

Jon

Can you draw that out for me, I am not getting how that connection would work?

 

[kwired]

You can power approximately same VA with a 250 amp 208/120 gear as you can with 400 amp 120/240 gear, and use one more conductor, though at same time the conductors can be smaller. I don't know the cost difference for transformers here, of course will be load dependent and maybe not sized same from a POCO as it would be for a dry type installed totally to NEC calculations. Maybe you would see 75 KVA transformer for either case I mentioned above in a lot of instances? Pole pigs would be three 25 kVA units for the three phase service.

 

[wwhitney]

Can you draw that out for me, I am not getting how that connection would work?

Not familiar with the standards for drawing transformer coil arrangements, here's a textual description in case that helps:

For a 3 phase primary supply, you have 3 available voltage sources that are 120 degrees apart. Then you use (3) single phase transformers that have center tapped secondaries, one on each of the 3 primary voltage sources, to get (3) 120/240V services that are also 120 degrees apart. Connect all (3) center taps and earth that point. You now have (6) different L-N waveforms, 120V in magnitude, with the pairwise phase shifts comprising all multiples of 60 degrees. Which I guess we are calling hexaphase, although I would still say it is a 3 phase system. [Just as 2-phase has (4) different L-N waveforms, each 90 degrees apart.]

Cheers, Wayne

 

[kwired]

Not familiar with the standards for drawing transformer coil arrangements, here's a textual description in case that helps:

For a 3 phase primary supply, you have 3 available voltage sources that are 120 degrees apart. Then you use (3) single phase transformers that have center tapped secondaries, one on each of the 3 primary voltage sources, to get (3) 120/240V services that are also 120 degrees apart. Connect all (3) center taps and earth that point. You now have (6) different L-N waveforms, 120V in magnitude, with the pairwise phase shifts comprising all multiples of 60 degrees. Which I guess we are calling hexaphase, although I would still say it is a 3 phase system. [Just as 2-phase has (4) different L-N waveforms, each 90 degrees apart.]

Cheers, Wayne

So you have a main switchboard/panelboard with 6 ungrounded buses and a neutral bus?

This going to be more efficient or cost less to install than a conventional three phase gear already cost?

Most the units served by 120/208 single phase feeder are not impacted much by having 208 vs 240. Possibly longer cooking time for some cooking appliances. An oven primarily just longer warm up time, once set point is reached cooking time should still remain same as you are cooking at same temp either way. Dryer may take a little longer, bet nobody notices. Electric heat? Again longer warm up time if starting from a low temp, but to maintain a temp is still not going to be noticeable, still going to take same watt-hours to heat the same space, and as mentioned AC units are usually fine with as low as around 197 volts up to around 250 volts. Water heating is the other main 208-240 volt load, design could be a little more critical with that, larger storage tank or higher watt element are things to consider if marginal sized to begin with.

 

[winnie]

I will draw up later when I am not using a phone, but Wayne has it.

I agree, this 'hexaphase' is a three phase system, in the same way that ordinary US split phase service is a single phase system. In the high phase motor literature, it might be called a 6 hemiphase system to distinguish in both from ordinary 3 phase as well as true 6 phase.

You can get the same VA from a 125A 120V L-N hexaphase system as a 250A 208/120V wye system, but with twice as many (smaller) phase conductors.

Jon

 

[winnie]

So you have a main switchboard/panelboard with 6 ungrounded buses and a neutral bus?

This going to be more efficient or cost less to install than a conventional three phase gear already cost?

Clearly more complicated which increases cost, but 1/2 nominal current and presumably 1/2 fault current which will reduce cost.

Once you are past the main disconnect (6 pole) you divide up into balanced single phase subsets going to your single phase meter stacks, so presumably equal cost after things split to the individual apartments...perhaps some tiny savings on the size of the neutral.

Jon