New Medium Voltage Install

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gabany

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23KV-2300V delta delta transformer, 5MVA.
The secondary amperage rating of the transformer is 1200+ amps. I want to install fuses (no breakers) on the secondary. Ideally, I would want to come into a set of switches (each 400A) and distribute the feeds. Does the 1200A rating of the 2300V switchgear buss prohibit this? Fusing the Primary of the transformer 125A.
 

charlie b

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Staff member
Location
Lockport, IL
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Retired Electrical Engineer
Re: New Medium Voltage Install

You can have up to six sets of fuses. The total of their ratings cannot exceed 125% of the secondary current. 125% of 1255 amps is 1568 amps. So you could have three 400 amp feeds and up to three other feeds that total less than 368 amps. But the "up to six" sets of fuses would have to be grouped in one location. See Note 2 to Table 450.3(B).
 

gabany

Member
Re: New Medium Voltage Install

I am looking at the 2002 NEC, which Table 450.3(A) which allows 250% (over 600V). Am I correct in reading that 6 - 400A feeds would be OK? Does the actual buss of the switchgear, however, need to be rated at or above the secondary available current of the transformer (1255A)?
 

gabany

Member
Re: New Medium Voltage Install

In fact, let me expand on my rational of thinking. Article 240.4(f) (2002NEC) allows the secondary conductors of a transformer to be protected by the primary in a delta-delta arrangement, providing the primary is protected accordingly. By fusing my transformer at 125A (primary), I meet the req'ts for no secondary conductor protection, providing secondary conductors are rated at 1200A. However, does this include the actual 1200A rating of the switchgear?
Thanks!
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: New Medium Voltage Install

Sorry. I missed the part about "over 600 volts." But I think you are mixing your rules.

240.4(F) allows you to have "primary only" protection, but only if you meet Table 450.3(A). That table permits "primary only" protection only for supervised locations. So if you want to take advantage of 240.4(F), you would have to put the transformer in a supervised location. Is that part of your plan?

You said that you wanted to install 6 separate feeds, each rated at 400 amps. That is 2400 amps, not 1200. So I don't understand what you are planning to do.

But let me try to answer what I think to be your real question. The switchboard would have to be rated for every bit of the current that could flow in all of the incoming lines. If you have three feeders, and each is fused at 400 amps, then you can use a 1200 amp switchboard. If you have four feeders, and each is fused at 400 amps, then you would have to "biggie size" to a 1600 amp switchboard. But the fact that your transformer can put out 1255 amps (as its rated current) would not require you to have a switchboard rated for over 1200 amps.
 

gabany

Member
Re: New Medium Voltage Install

Thanks for the reply.
Yes the transformer is in a supervised location.
I want to install 6 fused disc. switches - rated at 400A each (Actual Fuse sizes TBD). Art. 450 states the sum total of these secondary fuses cannot exceed 250% of rated secondary current.
This is similar to a 100A loadcenter having 24 20amp breakers. Not every load will be maxed out 100% of the time. I am not concerned about the loading of the transformer, that has been addressed.
My only concern is the fact that there will be 6 fused disconnects for secondary protection, and the fact the secondary switchgear is rated at 1200A
You are right, I am applying rules 240 and 450, and I just want to make sure I am not missing anything.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: New Medium Voltage Install

I don't think the 1200A switchgear is a problem. Refer to 408.13 and 408.16(B) of the 2002 NEC. Also note that 408 doesn't have a specific section for over 600V to overide these two rules.

It seems to me like you are OK (if the load is under 1200A, like I think you said). But are you sure this is under the NEC? This isn't part of a Power Company, is it?

STeve

[ September 02, 2005, 05:27 PM: Message edited by: steve66 ]
 

gabany

Member
Re: New Medium Voltage Install

Thanks, I never thought to look there. 408.16(D) "Exception" is exactly what I needed.

This is not Utility, it is distribution within our mill.
 
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