OCP for Transformers

[beto]

Can anyone tell me when a primary and secondary overcurrent protection is required. From table 450.3(B)it seems that I can have only primary protection for my 75KVA 480/208V transformer like a 125A fuse and don't have secondary protection. Do you guys see this different? Can you give me a code reference that tells me I need to have secondary protection? I have an inspector that claims I should have secondary protection as well.
Thanks a lot for your help.

 

[ryan_618]

Re: OCP for Transformers

450.6(B) gives an example of when it would be required.

 

[don_resqcapt19]

Re: OCP for Transformers

Even if the transformer itself doesn't require secondary protection, the secondary conductors will.
Don

 

[beto]

Re: OCP for Transformers

Thank you guys!
Your comments helped me a lot.

 

[mclain]

Re: OCP for Transformers

Don
If the transformer secondary conductors were considered a tap and terminated in a OCPD and complied with the requirements of a tap you wouldn't need secondary protection at the transformer would you.

Don McLain

[ January 13, 2004, 10:22 AM: Message edited by: mclain ]

 

[jim dungar]

Re: OCP for Transformers

Secondary overcurrent protection on transformers is always optional.

Secondary lighting and branch circuit panelboards must have protection.

Secondary conductors must have protection unless a "tap rule" is applicable.

Transformer primary protection may be "upsized" (from 125%) only if secondary protection is supplied and it is limited to 125% of the secondary current.

 

[earlydean]

Re: OCP for Transformers

Jim,
What about 240.21(C)? These six opotions are not taps, and they do not require OC protection.

 

[jim dungar]

Re: OCP for Transformers

Earl,

You are correct. I am from the old school and have always generically refered to the Article 240.21 provisions as "tap rules".

 

[templdl]

Re: OCP for Transformers

This is the way I interprete the application:
Art 240.21(C)(1)through (6) the secondary can be protected with a pri. OCPD per (1) Protection by Pri. OC device should the transformer have a 1ph 2w or 3ph, 3w secondary. Ref. to art 450.3 for transformer OCPD sizing.
However, 1ph,3w and 3ph,4w are more common for which art 240.4(F), Secondary conductors, apply.
Note that in either case when secondary protection is provided then it is possible to size the pri. overcurrent protective device up to 250%. The location of that primary device may be at the transformer or located up to 25' upstream from the transformer (25' tap rule), or 25' from where the sec. OCPD is located to where the pri' OCPD is located. That is if the sec. OCPD is located 10' frome the transformer than the pri. OCPD can only be 15' from the transformer.