Specing 3 phase transformer

[TBotNik]

All,

Looked all over the internet and can not find what I'm looking for to correctly calculate my amperage for this xfmr and the 120/240 breaker panel main breaker rating, so I spec everything right for the electricians. What I have is a new computer room with 300 computers @ 120/240 VAC 6.5 continuous amps. I additionally have 7 data switches @ 120VAC 2 continuous amps, an HVAC blower unit @ 240VAC 3 phase 15 amps and per conversation with the client need 20-30% expansion for other laptops, fans, monitors, etc that will be utilized in the room.

What I'm missing in my formula is the distribution factor across the 3 phase. I'm aware of the 1.75 multiplier, but not sure that is the right way to calculate my formula. I also looked in the NEC and find no table or calculation for this, unless I'm not knowing where it is and recently completed the course, and this was not covered there either.

Anyway my total load in Watts is 384,960 or 385KW,

If I use the logic on balancing this load across 3 legs of LPL=TL/3 (load per leg = total/3) I get 1080 amps per leg, but using LPL=TL/1.75 I get 1842. I've ordered the complete computer spec sheets as this just does not seem right and thinking what the company sent me is startup amps, not continuous. Anway a breaker panel with a 2,000 amp breaker just does not sound right and the step-down XFMR to support it also sounds bizare, but need input on if I'm doing these right.

With current calculations I need the fused switch in the 480VAC 3phase MDP of either 400 or 700 amps, so all is critical here.

Thanks in Advance for all your help!

Cheers!

TBNK

 

[charlie b]

We need to back up a few steps here. If you need both 120 volts single phase and 240 volts single phase out of the same transformer, then you are looking at a “high leg delta.” See the attached image. I have not done a design that includes one of these beasts, so I don’t know the math that would tell you how much current you can get at each voltage level. But you can see that the 120V loads can only come from one third of the transformer. Someone else will have to answer the rest of your question.

But let me ask whether you really need 240 volts, or will 208 volts work? A step down transformer from 480V to 120/208V is far more common than a high leg delta. That math is easier. If you have 385 KW, the current on each leg will be (385 x 1000) / (208 x 1.732), or 1068 amps. The factor of 1.732 is the square root of 3, and is likely the source of your 1.75 factor.

 

[augie47]

My "off the top of my head" thought, considering the "high-leg" and your specs for 120/240 equipment, that you would be better off using one three phase transformer for the three phase loads and single phase transformers for the single phase loads. For balancing you might use more than one single phase transformer.

 

[ron]

I am in agreement with Charlie.

Work with 120/208V and your life will be easier, since it is more common in the 3 phase commercial world.

 

[Besoeker]

We need to back up a few steps here. If you need both 120 volts single phase and 240 volts single phase out of the same transformer, then you are looking at a “high leg delta.” See the attached image. I have not done a design that includes one of these beasts, so I don’t know the math that would tell you how much current you can get at each voltage level. But you can see that the 120V loads can only come from one third of the transformer. Someone else will have to answer the rest of your question.

But let me ask whether you really need 240 volts, or will 208 volts work? A step down transformer from 480V to 120/208V is far more common than a high leg delta. That math is easier. If you have 385 KW, the current on each leg will be (385 x 1000) / (208 x 1.732), or 1068 amps. The factor of 1.732 is the square root of 3, and is likely the source of your 1.75 factor.

Might be a challenge to balance the load equally on the windings.

 

[Ingenieur]

too many questions
where did the total load of 3 x 1080 = 3240 A come from? kw/120?
why a 2000 mcb if the load is 3000?
how does the 480 figure in? a 480:??? strep down xfmr?
where did the 1.75 come from? 1.732? sqrt 3 ?
what does 120/240 at 6 A?
does you calc include the 30% growth?
3 ph 240 ahu sounds off and waaaaaay too small for >300 kw load

as others have
break the loads dow
240 ??? kva
120 ??? kva

will the 240 work on 208
multiple xfmrs may be the way to go but need more info

500 kva 120/208/3 is only 1400 A

it will be simpler 120/208
are you an electrical engineer?

 

[topgone]

Might be a challenge to balance the load equally on the windings.

Agree. Wye-connected supplies will always be unbalanced when the loads are phase to neutral.

 

[Jraef]

Most of the 240/120 3 phase 4 wire transformers I've seen will tell you that the 120V loading is limited to 5% of the transformer capacity. That's because tapping 120V from it is inherently unbalanced and there is no way to balance it. Here is how it's worded in an old textbook I have:

This Delta secondary connection has only one winding (S3) with a neutral conductor. The mid-point of winding S3 is tapped which gives the XI and X3 to neutral a voltage reading of 120 volts. In a three=phased system, winding S3 is the workhorse; it has to carry all the 120V lighting and appliance loads plus one-third of all the three-phased loads. (The 120V loads must not exceed 5% of the nameplate KVA, and the total of the nameplate KVA must be derated by 30%). Winding S1 and S2 cannot carry any 120 volt loads as there is no neutral connection to these windings. Windings S1 and S2 can only carry one-third of the three-phase loads each, and the 240 volt single-phase loads.
*Caution: A240 volt Delta connected transformer with a 120 volt neutral tap creates a condition called “high leg” As indicated in the above diagram, the voltage between Phase B (X2) and the neutral tap will be 208 volts; therefore, no 120 volt single-phase loads can be connected between X2 and the neutral tap.

That's another reason why 208Y120V is a better choice if you have a lot of 120V loads.

 

[Ingenieur]

Agree. Wye-connected supplies will always be unbalanced when the loads are phase to neutral.

if you have a y xfmr connect to a y load with neut
ignore harmonics
loads are ~ equal, pf are close, +/25% or so
all loads are 1 ph (line to neut)
i neut will ~ 0

even if line
a = 10 a
b = 1 a
c = 0 a
n = 9.6
worse unbalance case n < largest load current
worst case a loaded, b = c = 0, a current = neut current
correct???

 

[Besoeker]

Agree. Wye-connected supplies will always be unbalanced when the loads are phase to neutral.

Yes, I agree. But what I thought Charlie posted was a high leg delta. With the centre tap on winding being used to produce the neutral I would have thought that would have made unbalance more probable.

Not something I have experience of here or elsewhere in the world so I am not in a position to advise the OP what to do.
We probably would have gone for two transformers.

 

[Ingenieur]

assume 99 devices at 1000 w at 120 vac
use 100 kva xfmr 120/208

3 ph current = 100000/(1.732 208) = 277 A per line

assume 99/3 = 33 per phase-neut
i per phase = 33 x 1000 / 120 = 275 A per each ph-neut

single phase loads can be balanced

 

[kwired]

"120/240 VAC 6.5 continuous amps."

We need clarification on what that load actually is, is it 120, 240 or combination of both - if combination how much is 120 and how much is 240, then we need to ask if the 240 load is suitable to operate at 208, and if that changes the current draw?

 

[TBotNik]

Further Calculations

Further Calculations

We need to back up a few steps here. If you need both 120 volts single phase and 240 volts single phase out of the same transformer, then you are looking at a “high leg delta.” See the attached image. I have not done a design that includes one of these beasts, so I don’t know the math that would tell you how much current you can get at each voltage level. But you can see that the 120V loads can only come from one third of the transformer. Someone else will have to answer the rest of your question.

But let me ask whether you really need 240 volts, or will 208 volts work? A step down transformer from 480V to 120/208V is far more common than a high leg delta. That math is easier. If you have 385 KW, the current on each leg will be (385 x 1000) / (208 x 1.732), or 1068 amps. The factor of 1.732 is the square root of 3, and is likely the source of your 1.75 factor.

View attachment 19106

charlie b,

Further calculations and information shows all computers @ 6.5 amps @ 220 VAC. Then there is an HVAC blower 15 amp 3 phase and 7 data switches @ 2amps 120VAC. Datasheet from the computers show 1375 watts @220VAC, which is 6.25 amps, but guessing it's the additional power factor putting them @ 6.5.

Anyway total load is 523500 Watts which calculates to over 1,300 amps using the 1.75 multiplier. Current MDP only has max of 600amps left as it is rating 1,200 amps @ 480 VAC.

I'm thinking they go over the MDPs capacity, so actually need to order new service from Oncor, our service provider, here in the DFW area, and then they can have the XFMR they install calculated to the need and knowing them this will expand so should spec 2,000 amp service.

Additionally, with commercial breaker panels maxing at 44 breaker spaces, using 220vac 40amp breakers, supporting 4 computers per ckt, this calculates out to 4 breaker panels needed. That includes space for the 3 phase blower breaker the 7 120vac switches and misc 120 breakers totaling 10 spaces.

I just need to fully understand the load per leg thing, so I distribute this right, in the specs, and make sure I have everything cover in this spec, so the electricians can just go by my punch list. If I'm doing everything right the 4 panels main breaker should be 500 amps.

Cheers!

TBNK

 

[TBotNik]

Runs High

Runs High

Furthermore, for this location, the substation just across the highway, the voltage run 5-15% high, due to short distance and thus close to zero resistance in the lines. If I spec 220 here will get 230-240 when I actually test the voltage.

Time of day, when testing shows the variants, depending on loads caused by surrounding facilities.

Cheers!

TBNK

 

[TBotNik]

Current MDP

Current MDP

Oh, the current MDP is pulling just under 800 amps @ 480 vac 3 phase, not fully balanced.

That's where I know it has 400 amps left.

Cheers!

TBNK

 

[TBotNik]

Found it

Found it

All,

OK, finally found it at:

http://myelectrical.com/notes/entryid/8/three-phase-power-simple-calculations

There it shows that single phase or total power to 3 phase is TP/3, thus distributing the power even across each leg of the 3 phase, so now I can actually calculate this 3 phase load correctly!

Thanks all for you help!

Oh! That puts amps per leg @ 794 or 800 amp 220 vac 3 phase service or 364 amp 480 3 phase. This then let's us still use the existing MDP with new 400 amp fused disconnect!

Cheers!

TBNK

 

[Ingenieur]

I come up with 476 kva

a 3 phase 240 with a center tap would work
98% of the load is 240/1 and can be balance at 100 units / phase
the 120 is moot
the 3 ph is moot

any other loads? lighting, etc.

what is the 1.75?
assuming a 500 kva
3 phase I = 1200 A
use 3 x 400 panels w/3 ckt mdp
assuming 750 kva for expansion
3 ph I = 1800 A
use 3 x 600 or 5 x 400 w/6 ckt MPD

 

[kwired]

We need to back up a few steps here. If you need both 120 volts single phase and 240 volts single phase out of the same transformer, then you are looking at a “high leg delta.” See the attached image. I have not done a design that includes one of these beasts, so I don’t know the math that would tell you how much current you can get at each voltage level. But you can see that the 120V loads can only come from one third of the transformer. Someone else will have to answer the rest of your question.

But let me ask whether you really need 240 volts, or will 208 volts work? A step down transformer from 480V to 120/208V is far more common than a high leg delta. That math is easier. If you have 385 KW, the current on each leg will be (385 x 1000) / (208 x 1.732), or 1068 amps. The factor of 1.732 is the square root of 3, and is likely the source of your 1.75 factor.

View attachment 19106

I think restrictions on how much neutral can be loaded only come with single core type units. If you have three separate single phase transformers connected in a three phase bank especially with wye connected primary - that 120/240 unit is connected exactly same way it would be if it were being used for supplying single phase only, same applies when there is just two units connected for open delta secondary.

Many utilities have open delta systems that supply multiple customers - some only as single phase services and nobody ever gets concerned about to much neutral load, they do things as if there were only a single phase transformer as the supply.

 

[TBotNik]

Delta vs Y

Delta vs Y

charlie b,

You are showing a Delta, but ideally the XFMR output, especially where it is 120/240 or 120/208 is a Y not Delta, so load can always be balanced.

Cheers!

TBNK

 

[Ingenieur]

charlie b,

You are showing a Delta, but ideally the XFMR output, especially where it is 120/240 or 120/208 is a Y not Delta, so load can always be balanced.

Cheers!

TBNK

a center tap delta 240 with the tap producing 120 is the std way to get 120/240/3
a 240 wye will produce 138 l-n