HOW To. . calculate 3ph, transf. for 1ph, only load?

[desim]

good day, I'm needing to design a three phase transformer but I'm not sure I've been given the correct direction on how to go about it. the direction I was given is to assume it was all three phase, but this can't be right.
I'm designing for apartments, which is all 1ph load.
we will be utilizing three phase pad mounted transformers are 208Y/120V (for these services).
let's assume the following: 30 units, 2kva each.
my co-workers tells me the 3ph transformer (if they made this size) should be 60kva. this is where I disagree. I do not have an engineering degree but I've designed tons of three phase commercial projects, and tons of single phase projects (using single phase transformers). this is the 1st I've done an apartment complex.

my question is, if I have 30 units, each using 2kva of single phase load. wouldn't my main, 3 phase panel have 10 feeders on AB, 10 on BC, and 10 CA? where each feeder having a load of 20kva. with this, you would think each leg would only see 40kva? (or maybe it works out to be 60kva div square root of 3) which is 34kva +/-.

I'm sure I'm overthinking this, but I like to know what's what before I put my name on anything.
thanks

 

[bacon4life]

Your co-worker is correct, assuming that the 2 kva per apartment is after all the applicable demand factors.

Apartment buildings usually have two hot legs to each apartment. Normal 120V outlets are connected between neutral and either one of the hot legs. Stoves, water heaters and dryers are connected using both hot legs at 208V.

 

[waross]

Your revenue meter sockets must be suitable for five jaw meters.
In regards to your theory that each feeder sees 40 KVA;
resolve the KVA into amps per phase, consider the phase angles and add the directed vector sums. The answer will be a current that reflects a 60 KVA load. (Thanks David)
See:
Network Meter in a 240/120 V Meter Socket
thread238-428582


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

You're welcome.

 

[Sn00ze]

@bacon4life - Do you mean 240V for both hot legs? 208V is 3-ph mostly seen in industrial applications (motors).

 

[jghrist]

my question is, if I have 30 units, each using 2kva of single phase load. wouldn't my main, 3 phase panel have 10 feeders on AB, 10 on BC, and 10 CA? where each feeder having a load of 20kva. with this, you would think each leg would only see 40kva? (or maybe it works out to be 60kva div square root of 3) which is 34kva +/-.

If you have 10-2 kVA feeders on AB, 10-2 kVA feeders on BC, and 10-2 kVA feeders on CA, why wouldn't you have 10·2 + 10·2 + 10·2 = 60 kVA total? Where does the 40 come from?

 

[desim]

So, lots of replies, thank you everyone.
Starting from the bottom up, how did I get 40kva from 3x (10x2)? Re-read your reply if you look at the AB, BC, and AC, you'll notice each phase is only used twice, not three times.
We do use 240v for 90% of our residential load, but apartment complex are too large to utilize single phase load, and we do not have 120/240v 3 phase pad mounted transformers. Our 208 transformers are used almost everywhere in our system. Our 480v transformers are used in our industrials areas and ago pumps
I'll check out that link provided, thanks for the info

 

[Sn00ze]

@desim,


not all your loads are AB, BC, AC, usually only (as Bacon4life said) "Stoves, water heaters and dryers are connected using both hot legs at" 240V. The rest are going to be 1 phase and a neutral. sizing each leg only helps you with balancing your panel. The overall TX is the one that sees 60kVA.

I think you are getting confused between 3-ph and 1-ph loads & tx. do what waross said

"resolve the KVA into amps per phase, consider the phase angles and add the directed vector sums. The answer will be a current that reflects a 60 KVA load. (Thanks David)"

 

[desim]

@Snooz, I do understand that which is why I'm seeing things not so clear
If we took the 240v out of the equation and only used 120v, that would mean (to me) there's 20kva on AN, 20kva on BC etc which to me could be 20kva across all three phases. Again, I don't have an electrical degree. So if took my original example and the three feeders were three separate 3 phase panels feeding three phase pumps let's say 20hp each (and say 20kva each too). I can clearly see using a 60kva 3-phase transformer. I guess I can't see how 60kva of single phase load ALSO needs a 60kva 3 phase transformer
The same transformer is needed for either 60kva 3-phase, and 60kva 1-phase
Thank you all for your time

 

[Sn00ze]

"I guess I can't see how 60kva of single phase load ALSO needs a 60kva 3 phase transformer"

Current in = current out, but the power needed remains 60kVA. that's why it doesnt matter if your tx is single or 3 phase.

if a single load demands 60kVA, then your whole transformer is used up. in this case 1 winding + your neutral (or 2 phases depending on whether your hypothetical is 120V or 240V) would carry the whole load for the TX.

if a motor is 3 phase then it is already balanced between all 3 phases.

at the end of the day, it doesn't matter, break down your loads into their amps, if you have a 3 phase load in there, then it gets more complicated because you have to consider the vector sum and add them to your single loads. but all together will give you the size of TX you need. after that if you need 3-ph or 1-ph is up to the client. (if there are 3 phase loads i would assume you need 3 phase tx)

Jeez, sorry i dont know if i am explaining it very well.

 

[bacon4life]

@Snooz-Yes, I really did mean 208V. Heating elements operate a bit slower at 208V.

 

[jghrist]

If the load is not balanced among the three phases, you will need a larger transformer because each winding of a 60 kVA transformer is only good for 20 kVA.

 

[stevenal]

The question is why? In my experience 120/208 is used where a three phase load is present, like a swimming pool pump. As noted above, the 208 connected appliances will heat slower and it is sometimes difficult to get the electricians to install the correct meter base. Why not use a 75 kVA single phase transformer? (Yes, 60 is not a common size in either single or three phase).

 

[LionelHutz]

The simple rule is Power in = Power out. So, if the power output is expected to be 60kVA then the power input is 60kVA. The only way to get 60kVA of power input is using a 60kVA transformer.

Now, there is a caveat for 3-phase. The power has to be balanced between the phases. Fortunately, you are balancing the loads between the 3-phases so it should be fine.

You're getting confused because the TX is Y connected. You're adding the power directly instead of using the correct vector sums.

The easiest way is to think of it as L-N loads only. 20kVA on A-N would put 20kVA on 1-coil of the TX. Since the TX is 3-phase there are 3 coils so multiply by 3 and get 60kVA.