Let's say my transformer is sized correctly for the load it serves and the transformer primary OCPD is sized to meet the 450.3(B) maximum requirement, but I still get nuisance trips due to high inrush current. Is there NEC code that allows me to go up one standard size in that situation? Thanks.
Transformer OCPD primary sizing
- Thread starter unsaint33
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jumper
Senior Member
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Not that I know of.
ptonsparky
Tom
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- EC - retired
OP.
You have primary protection of 250% with 125% on secondary and it still trips?
Can you apply note 3?
You have primary protection of 250% with 125% on secondary and it still trips?
Can you apply note 3?
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Any chance that it is a magnetically assymetric (one winding closer to the core) construction and it is being used in the opposite direction from what was intended in the design? That can increase the startup current by a factor of 4 or more.
This is why the NEC now prohibits a nominal step up or step down transformer being used in the opposite direction unless the manufacturer explicitly states that such operation is OK.
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