Transformer protection 1

[thinker]

Please help to resolve the argument on correct fuse sizing.
We have a 575 kVA transformer with 480 VAC primary. Our sub
contractor suggested to use 800A fuses for primary protection. NEC requires a protective device sized at 125%
of rated current (which is in our case 692x1.25=865A).
Local power people require to comply with 125% and install
next higher size of the fuse 900A.This is quite expensive
since next size frame of disconnect and fuse would be needed.We argue that NEC presents 125% as a maximum rating.
Would it still be legal to protect a primary of XFMR with
692FLA using 800A fuse?

 

[GordS]

I would agree that the 125% is a maximum rating. Just be aware that a smaller fuse may give problems with nuisance failures upon energization.

 

[tulum]

I am no expert...however, when you are wrong in this forum, someone will let you know!

If the manufacturer says that a 800A fuse will do the job, get them to choose the fuse and put it in writing while you are at it.

125% is a maximum rating (I think it may go as high as 300% if there is appropriate secondary protection... I could be wrong).

However, with respect to being legal; 800A is a "tighter" rating and of course is legal to use.

Keep in mind;

1. The chosen fuse must handle the inrush of the xfmr. By the manufacturer saying it can, hopefully it can.

2. Depends on how you want to coordinate this fuse with other fuses in the system. Different size fuses will give you different zones of protection.

3. etc.

Regards,
Tulum

 

[mpparent]

Hey thinker,

If I'm not mistaken the code DOES NOT allow the next higher device for transformer protection, hence the 1.25x being a MAXIMUM for selection.

Mike

 

[thinker]

Mpparent, please refer to NEC-2002, Table 450.3(B), Note 1
regarding next higher size.
At the same time: how to estimate the inrush (magnetizing)
current peak for this transformer to compare it with the fuse time-current curve?

 

[dpc]

A general rule of thumb for transformer magnetizing inrush is 8 to 12 times full load amps (self-cooled) for 0.1 seconds. I'd start with 12 times. If you can make that, then 10 times is generally adequate.

 

[alehman]

My experience has been that 125% is only marginally adequate to handle the inrush. It depends of course on the specific fuse curve and upstream system.

NEC permits primary OC protection >125% only if secondary protection <=125%.

 

[jbartos]

Question: What type of protection is available on the transformer primary upstream feeder or next upstream? If within the NEC allowed 250% of the transformer primary current, then the 125% of the secondary transformer current will be required only.

 

[busbar]

Direct your communications with ‘local power people’ based on [as applicable] Article 450, IEEE C57.109 and fuse-manufacturers’ published application data, specific experience you have and, hopefully, your coordination study. Consider also that for awhile the transfomer may not see nameplate loading, but how is it going to operate with reasonable load growth?