Transformer Secondary Conductor Sizing

[nightfox1925]

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?

 

[alehman]

If you need overcurrent protection for the switchgear bus, it must be sized for that purpose. Normally that would be no larger than the bus rating. If the same device is to serve both functions, it must be sized for the smaller value.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[rbulsara]

If the main breaker is rated 100% , you can use 100% rated cables.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Do you have the Canadian Electrical Code?
See Rules:
14-100
(b)
(c) if the bus feeds a single overcurrent device.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nightfox1925]

What I am trying to get is for example:

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?

 

[dpc]

required ampacity will be 1.25 x 1804.22 = 2255.30A

Based on what requirement?

 

[waross]

What disconnects or protection do you have on the equipment. Do you have a 480 Volt breaker or are you relying on the primary protection of the transformer to protect the secondary conductors?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VTer]

look at 215.2(A) of the NEC. If you want a 2000A bus then you need a 100% rated MCB. I also would suggest 5 sets of 600MCM for 2000A.

"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

 

[waross]

I think that the underlying question may be;
"Is the bus considered part of a connected device or an extension of the conductor."


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

NEC requires feeder be sized based on the load. The 125% requirement only applies for continuous loads.

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.

 

[nightfox1925]

Waross,

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.

 

[REDDOG]

Doesn’t a 100 % rated over current devise have to be supplied with 75 degree C cable Ampacity not 90 degree C Ampacity.
Overcurrent devises above 100 amperes are Listed for use with 75 degree C cable Ampacity.
NEC 110.14(1) (b)

 

[dpc]

Yes, I believe all 100% rated device require 90 deg C insulated conductors but applied at the 75 deg ampacity.

At least for UL listed breakers.

 

[nightfox1925]

I was trying to look for the same in CEC but couldn't find it. In case I missed something, does the CEC code have similar requirement?

 

[NAZ55]

EC&M had a good article on this in their Feb, 2008 issue.
Here's the link

http://ecmweb.com/mag/electric_transformer_secondary_conductors/

 

[dpc]

I believe it may be a UL/manufacturer requirement.