New transformer and fault currents

[bwyllie]

Upsizing an existing 500kVA transformer to a 750 in an existing building, the available fault current has obvioulsy increased. The main switchboard is being reconditioned with new breakers. According to my calculations the available fault current at the panelbaords in the tenant spaces is approx 22K. The existing main breaker in the panelboards is GE TQD and branch breakers are THQB, both rated 10K. Any suggestions on what I can do to avoid replacing these breakers in the tenat panelboards. Would it be best to replace all the circuit breakers in the main switchboard with new GE breakers that are series rated with TQD and THQB branch breakers, the contractor prefers to use Cutler Hammer breakers.

Thanks.

 

[bob]

You will need to replace the existing breakers regardless of which method you use. The breakers must be rated for the available fault current be they fully rated or series rated. Don't forget to check the adjacent panels. They will also need to be rated for the fault currents at their location.

 

[Hameedulla-Ekhlas]

Calculate the fault current in each node whereever the fault current increases the breaker kA replace the breaker.

By the way, have you used any software for fault calculation?

 

[bwyllie]

No software, some hand calculations, this is only for a 3-story commercial office building.

 

[Hameedulla-Ekhlas]

ok good luck

 

[ron]

The existing GE breakers likely have series ratings with other breakers. So if you are replacing upstream CB's anyway, why not choose them wisely to get series ratings for the downstream tenant panels.