transformer overcurrent protection

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texnwis

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I have a 25 kva 480 single phase primary to a 240 single phase secondary transformer to run a 70a load. what do you suggest I size my primary and secondary fuses to.
 
size per 450.3(B) to protect the transformer and 240.4 for the conductors.
the correct "sizes" would be determined by your conductor size and if the system is two-wire to two wire, or two-wire to three wire {see 240.4(F)}
transformer. The sizes may also be influenced by the type of load (motor load, etc.) In summary, there are a lot of variables which have not been
addressed.
 
texnwis said:
I have a 25 kva 480 single phase primary to a 240 single phase secondary transformer to run a 70a load. what do you suggest I size my primary and secondary fuses to.
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I always thought that the primary side was was always sized by the nameplate rating.

25k/480=52.08 amps

52 amps x 1.25=65 amps

OCPD next size up 70 amps

Feeders.....65 amps is #6

I have never been sure if you are allowed to size the primary OCP by the secondary load.

I m sure someone will correct me on this if I am wrong.
 
ksmith846 said:
I always thought that the primary side was was always sized by the nameplate rating.

I have never been sure if you are allowed to size the primary OCP by the secondary load.
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You are correct.

The secondary load has no affect on sizing the primary protection. However, the primary protection can influence the sizing of the secondary device (i.e. exceeding the 125% of primary FLA, 450.(B) requires a secondary device be provided.)
 
When you have a 2 wire to 2 wire transformer, primary current directly corresponds to secondary current, so you can use primary OCPD to provide secondary protection.

You are _permitted_ to size the primary OCPD at less than the maximum given in 450; depending upon the situation this _might_ result in the primary OCPD tripping on inrush current, however in my experience with _small_ transformers (25KVA and the like) this has not been a problem even with significantly undersized OCPD, though with an unloaded secondary.

-Jon
 
brian john said:
Two excelent explanations (Part I and Part II).

http://www.electricalknowledge.com/forum/forum.asp?FORUM_ID=11
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That was a very good article. I have also noticed that Transformer rules
are largely violated in the field. If I had a penny for every undersized
system bonding jumper or lack of , or improperly grounded or bonded
transformer I've seen . Well, I wouldn't be here, I'd be drunk on some island
in the Caribbean right now. :grin:

I teach my guys the theory first before I show them the proper methods
of installing a transformer. Why, we bond the steel,cold water pipes, earth,
etc. After learning the "Why's" my guys start getting why following
250.66 is so important.
 
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