Transformer Over current protection

[Maple2]

Hi Gurus,

I have heard conflicting opinions on selecting a Transformer OCP device rating. Problem is while CEC 26-26(2b) stipulates conductor size on Secondary side shall have ampacity rating of not less than 125% of rated Secondary current, some in my team are applying same rule to transformers OCP devices, and it seems confusing to me.

Proposal 1: assume a 45kVA 3ph 600v/208v step down transformer
Primary current = 45x1000/ 600 x 1.73 = 43A
125% Pcurrent = 54A, Hence a 60A Overcurrent device on Primary should be chosen.
Secondary current = 45x1000/ 208 x 1.73 = 125A
125% of Full load current = 156A , Panel higher rating = 200A


Proposal 2: assume a 35kVA 3ph 600v/208v step down transformer
Primary current = 35x1000/ 600 x 1.73 = 33A
125% Pcurrent = 42A, Hence a 50A OCP device.
Secondary current = 35x1000/ 208 x 1.73 = 97A
125% of Full load current = 121A , hence a 150A CB Panel on the secondary side should be chosen.


Option 1: Choose the 45kVA, Since the primary side has a 60A fuse already, a 100A CB panel and #2/0 AWG Cu cable on secondary side would suffice
Option 2: Nope! with a 45kVA transformer, CB must be upgraded to match next higher rating of full load secondary current (i.e 150A CB).
Option 3: Choose 35kVA instead, since breaker's 'Mains Rating' is 125A.

Option 1 and 3 makes more sense to me as per 26-254. Your thoughts pls?

 

[waross]

Does this help?

Notes on Rule 26-260
The intent of this requirement is to provide coordination between loads connected to a transformer secondary
and the rating of the transformer circuit (i.e., rating of the transformer overcurrent device and ampacity of
transformer conductors). Compliance with this requirement allows the proper selection of the minimum
acceptable conductor size under the applicable provisions of Rule 26-258 and selection of the transformer
overcurrent device under the applicable requirements of Rules 26-252 to 26-256. Because selection criteria for
the overcurrent devices and conductors is based on the rated (primary or secondary) current of the transformer,
the actual calculated load connected to the transformer may not necessarily fully correlate with the transformer
rated primary and secondary currents, in which case the intent of this requirement could be met by increasing
the rating of the overcurrent device under the relaxation permitted by Rule 26-256(3). This Rule would also have
to be considered when conductor size is permitted to be reduced under the provisions of Rule 26-258(3).

Have you considered selecting a panel with an integral main breaker, 100 Amp, 125 Amp or 150 Amp as required?


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[unclebob]

Using a main breaker on secondary comes with some benefits:

Reduces the arc flash incident energy at 208 V.
Using a simple fuse on the primary side will take care of short circuits, while
the breaker will protect against overloads and ground faults.

 

[Maple2]

I have decided to go with option 1. Thanks