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Sizing Overcurrent Protection for Transformers

Monday, April 25th, 2011

A customer emailed us recently, asking if there was a way to change the default overcurrent protection size for transformers in Design Master Electrical. They like to size their OCP’s bigger than our defaults.

After trading some emails, we discovered that they were sizing for 125% of the kVA of the transformer, while our software was sizing for 100%. You know what? Sizing for 125% is the better answer. Our software has always sized for 100%, but I can’t give you a justification for that number. Transformers are rated to work at their full load, while conductors and breakers are typical rated for 80% of their load. Sizing the OCP (and associated conductors) for 125% of the load accounts for this.

We’ve updated Design Master Electrical to size transformer overcurrent protection to 125% of the kVA. This change will be included whenever we release the Electrical 8.0.2 patch.

(The NEC reference for this is Table 450.3(B). The maximum rating for overcurrent protection for a transformer larger than 9A is 125%. Smaller transformers can actually be sized larger: 167% for < 9A and 300% for < 2A.)

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