# Thread: LV relation between t/f rating and load demand

1. Junior Member
Join Date
Sep 2017
Location
Karachi, Sindh, Pakistan
Posts
4

## LV relation between t/f rating and load demand

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

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

3. Junior Member
Join Date
Sep 2017
Location
Karachi, Sindh, Pakistan
Posts
4
Originally Posted by ron
What does C and D mean? Is it demand and / or continuous?
I believe its demand and connected load
Last edited by jumper; 09-07-17 at 05:38 PM. Reason: fix quote

4. Originally Posted by saad778
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.

5. 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?
Last edited by Ingenieur; 09-07-17 at 05:21 PM.

6. Senior Member
Join Date
Nov 2007
Posts
817
Originally Posted by 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?
See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.

7. Originally Posted by topgone
See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.
in Pakistan?
with 32 A cb's?
I think 380/220 or something 400?

8. Junior Member
Join Date
Sep 2017
Location
Karachi, Sindh, Pakistan
Posts
4
[QUOTE=Ingenieur;1857319]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.

9. Junior Member
Join Date
Sep 2017
Location
Karachi, Sindh, Pakistan
Posts
4
Originally Posted by 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?
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

10. Originally Posted by saad778
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
Last edited by Ingenieur; 09-08-17 at 10:19 AM.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•