Sizing of control power transformer fuses for degraded voltage conditions

[brainsalad]

Hello,

Suppose in a plant that the voltage at an MCC is below normal due to a degraded switchyard voltage. Fed from the MCC are various motors which use size 1, 2 and 3 starters. With a degraded MCC voltage, the contactors are expected to draw more current due to a degraded source voltage. For 480/120V control power transformers, how can I verify that the fuse (primary or secondary) is sized such that it can tolerate such a condition? Generically, CPT primary or secondary fuses can be sized according to NEC or vendor recommendations, but may need to be sized larger in such cases. Thanks for your thoughts.

 

[waross]

Fuses should be sized to protect the transformer. If the existing fuses are failing, then larger fuses may result in control transformers failing. But go ahead. Remember, when you order larger replacement transformers, to specify control transformers with fuse holders and the appropriate size fuse pre-installed.

Bill
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"Why not the best?"
Jimmy Carter

 

[jghrist]

Are you serving motor load from the CPT? If not, how does the increased motor current at reduced voltage affect the CPT fuses? If you are serving motors from the CPT, then the fuses would be sized to carry motor starting current (which would be lower with reduced voltage) and should not be affected by a small increase in load current.