How to choose a Circuit Breaker with the correct AMP rating for protection of a transformer.

As an electrical engineer / electrical technician you will come across this scenario a from time to time. In my case, the transformer has already been installed, and this transformer fed a block of offices and a small fabrication shop. As the company expanded they decided to add a machine shop with a new machinery like  Lathe machine, Milling machine, drill presses, machines for electroplating and the installed transformer could no longer feed the added load.
The a new transformer had already been procured before management asked the question of what size of Circuit breaker would be used to protect this new Power Transformer from overload and short circuit.
There a lot of factors to be considered when sizing a circuit breaker but the four major ones that should be on your mind are
  1.        The ambient temperature of the environment where the breaker is to be installed.
  2.      Continuous running current when all connected loads are switched on (Maximum load current)
  3.       Presumed short circuit current.
  4.       Characteristics of connected load i.e. Inductive, Resistive etc.


To correctly size the Circuit breaker to be used on the transformer secondary we should know the Full load current (Isec) and the Short Circuit current(Isc of the transformer). And we need the data below to correctly calculate the Full load and short circuit current of the transformer.
·         KVA Rating of the transformer (kVa)
·         Secondary Voltage of the Transformer (Vsec)
·         % Impedance(Z) of the transformer (to calculate short-circuit current).

Additionally, all these information can be gotten from the nameplate of the Transformer. For this scenario
kVA = 500
Vsec = 440
%Z = 5
I sec = KVA*1000/sqrt3 * Vsec
I sec = 500 * 1000 / 1.73 * 440
I sec = 500,000/761.2 = 656.86A
I sc = I sec / %z
I sc = 656.86 / (5/100)
I sc = 656.86 * 100/5
I sc = 13,120A

Circuit breaker required = I sec * Safety factor
Circuit breaker required = 656.86 * 1.25 = 821.1A

So as Per NEC 240.6, the next standard ampere rating is 1000A and the lower band is 800A so we select a MCCB rated at 800A with an adjustable Magnetic trip setting within 13,120A.

We selected the 800A breaker as it is still well above the transformer FLA of 656.86A and selecting 1000A would be using a sledgehammer to kill a fly. 


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