Transformer Secondary Conductor Sizing
Transformer Secondary Conductor Sizing
(OP)
Hi all,
I am aware that NEC requires 125% over-rating factor maintained on top of cable deration. I have a transformer with a secondary full load current. I'm sizing it to be equal to 125%FLA/de-rating factors...then select the cable size.
However, if these cables are connected to a switchgear...is the 125% rule still valid if the switchgear busbars are rated for 100% current rating? If the Code says transformer secondary conductors...does it mean that the switchgear bus bar is also included?
I am aware that NEC requires 125% over-rating factor maintained on top of cable deration. I have a transformer with a secondary full load current. I'm sizing it to be equal to 125%FLA/de-rating factors...then select the cable size.
However, if these cables are connected to a switchgear...is the 125% rule still valid if the switchgear busbars are rated for 100% current rating? If the Code says transformer secondary conductors...does it mean that the switchgear bus bar is also included?






RE: Transformer Secondary Conductor Sizing
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: Transformer Secondary Conductor Sizing
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer Secondary Conductor Sizing
See Rules:
14-100
(b)
(c) if the bus feeds a single overcurrent device.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Secondary Conductor Sizing
I have a 13.8-0.48kV, Delta-Wye (solidly grounded) 1.5MVA power transformer feeding a downstream 480V switchgear.
The secondary full load is 1804.22A. Assuming no deration, the calculated required ampacity will be 1.25 x 1804.22 = 2255.30A. In CEC Table 2, using a 90 Deg C conductor, 750MCM at 5 conductors/phase will suffice = 500A x 5 = 2500A. This is to comply with Code.
However, with this FLA, can 2000A busbar rating be specified on the 480V switchgear (since the busbar is 100% current rated)?
What if I'll use 4 x 750MCM conductors = 4 x 500A = 2000A and ensure that the overcurrent device is set to pickup at 2000A, can I deviate from the code?
RE: Transformer Secondary Conductor Sizing
Based on what requirement?
RE: Transformer Secondary Conductor Sizing
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Secondary Conductor Sizing
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Transformer Secondary Conductor Sizing
"Is the bus considered part of a connected device or an extension of the conductor."
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Secondary Conductor Sizing
There is nothing wrong with having a standard 2000 A breaker on a 2000 A bus. It does not have to be 100% rated, and almost never is. It just means that the sum of the non-continuous and 125% of the continuous load cannot exceed 2000 A.
In Canada, I have no idea.
RE: Transformer Secondary Conductor Sizing
There is primary circuit breaker and a secondary circuit breaker. However, the secondary cirucit breaker is in the switchgear interconnected by cables.
My question, dealt more with the fact that if the cables have a 125% factor, does it necessarily mean that the switchgear bus bars have the same 125% factor with respect to continuous load.
I am looking on a scenario where the equivalent ampacity is based on a 125% factor and the bus bars on a 100% rated factor.
RE: Transformer Secondary Conductor Sizing
Overcurrent devises above 100 amperes are Listed for use with 75 degree C cable Ampacity.
NEC 110.14(1) (b)
RE: Transformer Secondary Conductor Sizing
At least for UL listed breakers.
RE: Transformer Secondary Conductor Sizing
RE: Transformer Secondary Conductor Sizing
Here's the link
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RE: Transformer Secondary Conductor Sizing