Table 450.3(B)

[NEC User]

Per table 450.3B primary protection of a transformer is always required. Note 2 indicates secondary protection is sometimes required.
My 2 questions are as follows:
When is secondary protection for a transformer required?
Why is the primary protection allowed to be increased from 125% to 250% if you are including secondary protection?

I'm installing a 10KVA Single Phase Transformer in between 2 panels. On the secondary side, the transformer feeds a main circuit breaker panel. I am sizing the main circuit breaker per the transformer secondary protection.
Primary: 480V
Secondary: 240V

The configuration is as
Per table 450.3(B) I'm sizing the breakers as follows:
Primary: (2.5)(10000/480)=52 A Max. I am going to use a 50A breaker.
Secondary: (1.25)(10000/240)=52 A Max. I am going to use a 50A breaker. (As the main circuit breaker of the panel).

 

[GoldDigger]

If the secondary is protected, then under most conditions the primary protection will be providing primarily short circuit and ground fault protection while the secondary breakers will be providing protection against long term low level overload.
The only overload protection needed on the primary side would be in case of a high resistance fault in the transformer itself. And that will overheat the transformer at a current far lower than the design copper and iron loss limit, so even primary protection right at the transformer limit would do no good for that particular fault.

Tapatalk!

 

[NEC User]

Do I get to choose when I only want to use primary protection at 125% of the transformer? Or is there a section of the code the requires using secondary protection of transformers for certain installations?

 

[GoldDigger]

Sometimes.
Yes.

Tapatalk!

 

[infinity]

Look at 240.21:

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/FeederSecondaryTapRules~20020326.htm

 

[david luchini]

Do I get to choose when I only want to use primary protection at 125% of the transformer? Or is there a section of the code the requires using secondary protection of transformers for certain installations?

You get to chose when you want to use Primary Only Protection for the transformer.

As Rob noted, using primary only protection for the transformer does not allow you to ignore the Secondary Conductor protection requirements.