iOS transformer fuse sizing

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Fred Shenkman

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Hello all,

This is for elevator application where iso transformer directly supplies a KEB pwm a/c drive.

Looking for input and advise on sizing primary side and properly fusing same. Following is example to use. 85kva 460/460 3 phase.

So using kva=V*I*1.732/1000 would give me 106.69A.

Typically would use THHN #2 or #1 depending on run length?

What would be the rule of thumb for picking proper fuse size?

Side note,I realize that this is not really a influence to the questions but the elec insp used this in his violations, Gearless PM motor being used with following data: 174V, 105A, 32HP.

He commented that that if time delay fuse in mainline it should be 106x175% or for non time delay 106x300%. Thats crazy!
 
Since your transformer is just for isolation in essence you're looking for the OCPD size ahead of the drive?

Welcome to the Forum. :)
 
Im trying to figure rule of thumb for main line fuses that directly feed transformer. So a 85kva fla is 106A correct. I would think a time delay of 125A, next fuse size up, would be used.

Or is there a certain % over that needs to be used. Fuse is to protect the wire and equipment?
 
Okay. I'll try to rephrase. Typically transformer is fused so that secondary side is not. I believe when fused at 125% that is allowed. Also is there anything that says that primary can not be fused lower than fla. ie...100A in this case

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Fred Shenkman said:
Okay. I'll try to rephrase. Typically transformer is fused so that secondary side is not. I believe when fused at 125% that is allowed. Also is there anything that says that primary can not be fused lower than fla. ie...100A in this case

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Nothing but the chances of a nuisance trip when energized.
 
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