300 KVA Transformer

[Zane Bailey]

I am inspecting a 300 KVA 3 ph transformer suppling a 208 volt Lighting and appliance panel. The main breaker is installed within 10 feet of the transformer. The main breaker is 1200 amp. The feeders are tripple parallel 500 KCMIL. Three per phase and three for the neutral. The full load ampacity would be 300kva/208voltx1.73=833.33 amps. As per Article Table 450-3(b) 1999 NEC. This is the contract we are under. The overcurrent device on the secondary could be set at 125% of the transformers rated current (833.33A x 1.25=1041A.) If the maximum setting is 125% of the rated current of the transformer as per Table 450-3(b). Would the breaker be able to be installed at 1200A because it is the next size standard breaker listed in 240-6 NEC and it is over 800A and the conductors are rated as per 240-3(c) the (ampacity of the breaker size)? (OR) would an ajustalble breaker set at no higher than 1041A have to be installed to meet the requirements of Table 450-3(b) NEC ?

 

[iwire]

Look at the notes under Table 450-3(b)

In the 2005 it's note 1, 'Next standard size breaker'

 

[jtester]

You might not need to protect the low side of the transformer at all if the high side is appropriately protected.

It seems that you haven't protected the 3-500 mcm cu's going to the 1200 amp switchboard though.

Jim T

 

[sandsnow]

To keep that 1200 amp breaker you need to upgrade that 500kcmil to 600kcmil. Under the 2002 NEC.

An adjustable trip (long time pickup) will work with some kind of sealable cover as per the rule in 240.