Transformer over 1000V Primary

[dougjl]

According to table 450.3(A)the primary protection using fuses in a supervised location allows maximum setting to be 250% of FLA.
That part is clear. My question is when sizing the conductors 125% of full load amps does not seem like the cable will be properly protected.
The primary of the transformer being used has FLA of 120A. This means I can use a 300A fuse. I am not seeing anywhere in code that calls for the cable to up-sized as well. Should the cable be sized at 125% or at 300%? I must be missing something.
Thanks

 

[jim dungar]

MV fuses are sized and ptotected differently than LV fuses. Cables are often protected using their damage curves.

 

[electrofelon]

450 is addressing ONLY the transformer protection. It is not saying anything about conductor protection. Your "normal" rules for sizing the ocpd protecting the primary conductors would apply.

 

[don_resqcapt19]

The conductors to the primary must be protected at or below their ampacity. One reason that few engineers specify 250% primary protection.

 

[wwhitney]

The conductors to the primary must be protected at or below their ampacity.

Is that true for over 1000V (see thread title)? I don't see it in Part IX of Article 240, but I have basically no familiarity with over 1000V.

Cheers, Wayne

 

[electrofelon]

Is that true for over 1000V (see thread title)? I don't see it in Part IX of Article 240, but I have basically no familiarity with over 1000V.

Cheers, Wayne

Well the conductor protection for MV is much more non specific. IIRC there is more general wording such as just coordinating protection so conductors will not be damaged, and settings can typically be way above the conductor ampacity. But, 450 is still just about transformer protection. One would have to look at article 240 for the conductor protection.

 

[don_resqcapt19]

Is that true for over 1000V (see thread title)? I don't see it in Part IX of Article 240, but I have basically no familiarity with over 1000V.

Cheers, Wayne

like you I really don't know about over 1000V and in general neither does the NEC

 

[wwhitney]

like you I really don't know about over 1000V and in general neither does the NEC

OK, well 2017 NEC 240.101(A) says:

"Rating or Setting of Overcurrent Protective Devices. The continuous ampere rating of a fuse shall not exceed three times the ampacity of the conductors. The long-time trip element setting of a breaker or the minimum trip setting of an electronically actuated fuse shall not exceed six times the ampacity of the conductor. For fire pumps, conductors shall be permitted to be protected for overcurrent in accordance with 695.4(B)(2)."

I gather the rules for OCPD sizing are different for over 1000V, and in particular may allow the nominal OCPD rating to exceed the conductor ampacity regularly. Probably there are some other factors that control OCPD sizing, and the above is just a further maximum.

Cheers, Wayne

 

[jim dungar]

Probably there are some other factors that control OCPD sizing ....

A big one is the long time rating of a fuse.
The 'number' in a LV fuse tells you its long time/continuous ampere rating.
The 'number' in MV fuse tells you what curve to look at.

 

[electrofelon]

Probably there are some other factors that control OCPD sizing, and the above is just a further maximum.

Cheers, Wayne

Look at 240.100(C) (which was what I was recalling/paraphrasing in my previous post):

(C) Conductor Protection. The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short-circuit conditions.

I found it interesting that per 240.100(A), the protection must be at the point of supply, but under engineering supervision, can be in another location.

 

[dougjl]

Thank you all for the information. I now have some more references to look at. Sounds like the best course will be to model the system and see it the cable is damaged during a fault and go from there. Again thanks for the interesting ideas and thoughts.