45kVA 3 phase transfomer

Status
Not open for further replies.
Sparky2791

Sparky2791

Senior Member
Location
Northeast, PA
Occupation
Electrical Design
Just wondering is it safe to say each leg of a panel supplied from a 45kVA , 120/240V Delta - 3 phase secondary step down transfomer is being fed with 15 kVA?

I was asked this question and was unsure if that is correct.


Thanks!
 
JoeStillman said:
It's only fed with 15 kVA per leg if that's the load connected to each leg. The loads on a high-leg delta panel are almost never balanced on all three legs.
Click to expand...
Yes of course, Phase B is usually under loaded due to high leg. I suppose how it should have been asked .........is each leg is capable of 15kVA at 100% loading? 3 /45 = 15kVA I think I was over thinking the question because responding to your post it is obvious.

I ask because my colleague saw a 45kVA 3 phase transfomer with secondary's into a wire trough and (1) 240V single phase disconnect with 150A fuses tapping phase A & B and a second 120/240 single phase disconnect with 60A fuses tapping phase A & C. Doing the math for the 150A fused disconnect... 30kVA (15 per leg) 30,000/240V = 125A x 1.25 = 156A so really a 175A fuse would be permitted but a 150A is better. as for the second disconnect Phase A seems it could be overloaded since it is tapped for both disconnects.(where the unbalanced load come in) I know this has to do with actual loads connected to phase A but fusing makes overloading a possibility. The good thing is the 60A disconnect feeds a small load center with very little spear circuit space so not much more load can be added to the load center which ultimately could add more to Phase A depending on how connected in the load center.
 
LibertyEngineering said:
Yes of course, Phase B is usually under loaded due to high leg. I suppose how it should have been asked .........is each leg is capable of 15kVA at 100% loading? 3 /45 = 15kVA I think I was over thinking the question because responding to your post it is obvious.

I ask because my colleague saw a 45kVA 3 phase transfomer with secondary's into a wire trough and (1) 240V single phase disconnect with 150A fuses tapping phase A & B and a second 120/240 single phase disconnect with 60A fuses tapping phase A & C. Doing the math for the 150A fused disconnect... 30kVA (15 per leg) 30,000/240V = 125A x 1.25 = 156A so really a 175A fuse would be permitted but a 150A is better. as for the second disconnect Phase A seems it could be overloaded since it is tapped for both disconnects.(where the unbalanced load come in) I know this has to do with actual loads connected to phase A but fusing makes overloading a possibility. The good thing is the 60A disconnect feeds a small load center with very little spear circuit space so not much more load can be added to the load center which ultimately could add more to Phase A depending on how connected in the load center.
Click to expand...
Tapping in the wireway sounds like a violation of 240.21. Whether the secondary fuses are oversized depends on the size of the primary protection of the transformer.

Also, the secondary current calculation would be 15kVA*sqrt(3)/240V, not 30kVA/240V
 
In general a single core three phase 45kVA is effectively 3 single 15kVA transformers. However, 240/120V delta transformers are not always general units, open delta banks can be made from different sized units.
It is not uncommon for a single core transformer single phase 120/240V 3w output to be limited to 5% of the three phase rating, although some manufacturers do not have this restriction (maybe they use some special construction like 5 legged cores).

In any case the total transformer output is a combination of the single phase and three phase loadings.
 
LibertyEngineering said:
Just wondering is it safe to say each leg of a panel supplied from a 45kVA , 120/240V Delta - 3 phase secondary step down transfomer is being fed with 15 kVA?

I was asked this question and was unsure if that is correct.


Thanks!
Click to expand...
I guess it depends on what the questioner actually means. There is no requirement that the load be equally split between the phases, as long as no phase is overloaded.
 
david luchini said:
Tapping in the wireway sounds like a violation of 240.21. Whether the secondary fuses are oversized depends on the size of the primary protection of the transformer.

Also, the secondary current calculation would be 15kVA*sqrt(3)/240V, not 30kVA/240V
Click to expand...
At first I would think 240.21 (B) (1) would apply but than I read 240.21(B) (1) (3) and see it needs to terminate in a panelboard, switchboard, switchgear or control device. Disconnect switch is not listed, interesting! Is this why you say its a violation?

So I would think one would use 240.21 (C) (2) for the tap however this is a bit confusing. 240.21 (C) (2) (2) includes disconnecting means in this requirement so a disconnect switch would apply, however 240.21 (C) (2) (3) does not include disconnect switches????
 
LibertyEngineering said:
At first I would think 240.21 (B) (1) would apply but than I read 240.21(B) (1) (3) and see it needs to terminate in a panelboard, switchboard, switchgear or control device. Disconnect switch is not listed, interesting! Is this why you say its a violation?

So I would think one would use 240.21 (C) (2) for the tap however this is a bit confusing. 240.21 (C) (2) (2) includes disconnecting means in this requirement so a disconnect switch would apply, however 240.21 (C) (2) (3) does not include disconnect switches????
Click to expand...
I was thinking more along the lines of the first part of 240.21 Location in Circuit. "Conductors supplied under the provisions of 240.21(A) thru (H) shall not supply another conductor except through and overcurrent protective device..."

The transformer secondary conductors must meet the requirements of 240.21(C). The requirements of the first paragraph of 240.21 don't allow those conductors to be tapped.
 
david luchini said:
I was thinking more along the lines of the first part of 240.21 Location in Circuit. "Conductors supplied under the provisions of 240.21(A) thru (H) shall not supply another conductor except through and overcurrent protective device..."

The transformer secondary conductors must meet the requirements of 240.21(C). The requirements of the first paragraph of 240.21 don't allow those conductors to be tapped.
Click to expand...
But the 'other conductors' are supplied through an overcurrent protective device, The disconnect switches tapping the secondary conductors have fuses?
 
david luchini said:
The tap conductor from the wireway to the disconnect switch is not supplied through an overcurrent protective device.
Click to expand...
I see - I guess the way I was reading it is the conductors may supply another set of conductors through an overcurrent protection meanng the tap conductor being supplied are required to have the OCP but the way it actually is intended is the conductors being tapped (feeder)must first be supplied through overcurrent. Than you may tap them according to the rules of the code.

So since 240.21(C) allows the connection of multiple feeders to be connected directly to a transfomer secondary without overcurrent protection when in compliance with C1,C2 or C3 that is the only way to go. Just need to be careful to comply with one of the 3 when making the connection. Thanks for the education!
 
Status
Not open for further replies.
Top