Oversize Primary Fuse on Transformer

[zimmerDN]

The CEC code allows the use of overcurrent protection device rated at 125% of the rating of a dry type transformer with primary voltage below 750V, overcurrent protection on the secondary side is not required.

Are the transformers built to withstand the load at 125% for long periods of time? Is the 125% primary o/c rating to allow for in-rush?

 

[EEfromOU]

I would expect the 125% is so the transformer can operate at full load without blowing a fuse but inrush could be about ~10 times the full load of the tranformer so I wouldn't consider it is setup to "allow for inrush."

 

[zimmerDN]

What is the reason that the code allows 125% primary o/c protection on a transformer and is it common/best practice to do so without secondary o/c protection?

 

[Skogsgurra]

Is't there any thermal protection to take care of overload? Fuses are usually not at all there for overload protection.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

In Canada and probably the US it's a code issue. Most fused switches and breakers are approved for continuous operation at 80% of the rating. In order for a transformer to deliver full rated current to a continuous load, the fuses or breaker must be 125% oversized.
Rule 8-104 (5),(a)
Most continuous loads are required to be supplied with conductors and overcurrent devices rated or set at 125% of the load current as a code requirement.

Bill
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"Why not the best?"
Jimmy Carter