Secondary Overcurrent Protection for a Service Transformer

[thomrk]

I'm looking for the official reason on why the secondary overcurrent protection for a service transformer has to be within 10 feet of the transformer. I realize that if the service transformer is outside, Article 240.21(C)(4) allows the secondary overcurrent protection to be any distance away. But what if the transformer is location inside of the building? Is it because the conductors can be accessed between the transformer and disconnect?

I currently have a service transformer that has the secondary overcurrent disconnect 35 feet (conductor length) away from the transformer. I'm trying to answer the question on why the disconnect has to be moved closer. Thanks.

 

[raider1]

What do you mean by "service transformer"?

Is the transformer utility owned or customer owned?

Chris

 

[thomrk]

Update

Update

The utility service is 4160V and feeds service entrance switchgear that is located inside the building. A circuit breaker in the switchgear feeds this transformer with is located besides the switchgear. The transformer in turn feeds the facility. The transformer is owned by the facility.

 

[augie47]

is any portion of the secondary conductors in/under a slab ?

 

[thomrk]

No. The secondary conductors are run overhead in a 3" RGS conduit to the Fused Disconnect Switch.

 

[raider1]

Is it because the conductors can be accessed between the transformer and disconnect?

Sort of, The conductors from the secondary of the transformer are unprotected by an overcurrent device on the secondary side and the NEC is trying to limit the probability that the conductors could be damaged and have a fault.

Chris

 

[iwire]

The primary protection can not be counted on to open if one of the secondary conductors faults to ground or becomes overloaded.

 

[thomrk]

Okay. So what the Code is saying is that an underground cable is better protected and less susceptable to fault conditions than one that is installed in conduit. Therefore, I need to move the secondary disconnect within 10 feet of the transformer.

 

[VUGear]

I thought the reason for the 10' rule was because a transformer secondary is considered a tap.