LV relation between t/f rating and load demand

[saad778]

I have been given a LV SLD of a housing scheme to study. In one of the drawings the transformer size is 750kVA, the incoming from the transformer connects to a busbar which further branches out into multiple MCCB'S. Now the load on the busbar comes out to be(MENTIONED IN THE DRAWING is 382.4kW(D) and 476.47kW(C), PLEASE TELL me how can i arrive at a value of 750kVA based on these loads

 

[ron]

What does C and D mean? Is it demand and / or continuous?

 

[saad778]

What does C and D mean? Is it demand and / or continuous?

I believe its demand and connected load

 

[ron]

I believe its demand and connected load

Do they have calculations that show how they came to the demand value or the 750kVA service size? Is this in the US? Are the NEC 220 calcs provided?

As you point out, there is no obvious connection.

 

[Ingenieur]

Perhaps there is no connection?
they calculated the connected and demand loads
assumed a pf
and selected a xfmr
eg, a 500 kva would be good for 450 kw at 0.9'pf
went with a 750

what voltage and phase?

 

[topgone]

Perhaps there is no connection?
they calculated the connected and demand loads
assumed a pf
and selected a xfmr
eg, a 500 kva would be good for 450 kw at 0.9'pf
went with a 750

what voltage and phase?

See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.:blink:

 

[Ingenieur]

See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.:blink:

in Pakistan?
with 32 A cb's?
I think 380/220 or something 400?

 

[saad778]

in Pakistan?
with 32 A cb's?
I think 380/220 or something 400?

Voltage level is 440v here in Pakistan. IEC is followed mostly. Yes, another SLD shows the connected loads which adds up to the mentioned load on the bus bar. but what worries me is how a 750kVA T/F is selected.

750,000/(root(3)*440) comes out to be 984A and for a 25% Overcurrent 1230A is the calculated value. I guess this is how they came up with the value.
I have to design a LV network for the housing scheme, and this is the only source i have to study and make my own.:?:?

 

[saad778]

Perhaps there is no connection?
they calculated the connected and demand loads
assumed a pf
and selected a xfmr
eg, a 500 kva would be good for 450 kw at 0.9'pf
went with a 750

what voltage and phase?

Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value

 

[Ingenieur]

Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value

Just an example
if you were designing for a connected load of 476 kw
assume a nominal pf of say 0.85 worse case
kva = 476/0.85 = 560 kva
if using demand 382/0.85 = 450 kva
in my opinion a 500 kva would suffice
most actual loads run nowhere near the connected or demand calculated values
but with more data I may select a 750 to be conservative and not worry about it
in the overall project cost (large housing complex) the overall cost dfference is small
efficiency may be lower but since it may run coler it will last longer

but consider, sometimes there is no exact mathematical way to determine things like this
it is engineering judgement
based on
standard equipment ratings
delivery times
expansion
etc

 

[Jraef]

...and/or the desire to be capable of some future expansion without needing to change out the transformer, cable, main bus etc., etc. when the time comes. There is a price to pay for that besides just the initial cost of that larger transformer in that there will be slightly higher losses in the larger transformer, but that difference would likely be insignificant compared to a change-out at a later date.

 

[topgone]

Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value

You have to factor in the diversity factor and the power factor.

Transformer size = connected load(in kW)/PF X DF X 1.30
where: 30% fudge number to take care of future loads(your choice)

IMO, your 750kVA trafo and CB selection is a result of a power factor of 0.8, a diversity factor of 0.7!
See the math:
connected load = 476.47 kW
transformer size = (476.47/0.8) X 0.7 X 1.3 = 541.98kVA. The nearest size would be 750kVA!

Sizing of the secondary protection cb also depends whether the system is supervised or not (NEC 450.3). 1200A if not supervised and 2000A, if supervised(supervised = only qualified personnel monitor and service the transformer installation is ensured).