30 to 40 HP Motor Change

Tim Ganz

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Join Date
Dec 2010
Location
Dallas, Texas
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Making a 30 to 40 HP motor change. Circuit has 60 amp fuse holder now with # 4 THHN wire.

The motor circuit calculator I have says I need an 80 amp dual element time delay fuse but I don't seem to be able to find a touch safe 100 amp fuse holder.

This is similar to what we have now in the 60 amp

http://ep-us.mersen.com/fileadmin/c...olders-US6J-UltraSafe-Class-J-Fuseholders.pdf

Can anyone here direct me to vendor that makes a 100 amp holder like this?
 
I don't think anyone makes that style larger than 60 amp. I suspect that a circuit breaker would be less expensive than fuses and a fuse holder.
 
Tim Ganz said:
Making a 30 to 40 HP motor change. Circuit has 60 amp fuse holder now with # 4 THHN wire.

The motor circuit calculator I have says I need an 80 amp dual element time delay fuse but I don't seem to be able to find a touch safe 100 amp fuse holder.

This is similar to what we have now in the 60 amp

http://ep-us.mersen.com/fileadmin/c...olders-US6J-UltraSafe-Class-J-Fuseholders.pdf

Can anyone here direct me to vendor that makes a 100 amp holder like this?
Click to expand...
If that is your only device, you are misusing it anyway. Those holders are NOT rated to be opened under load, which is the DEFINING criteria for a disconnect switch in a motor circuit. So if you already have another proper disconnect switch in the circuit, then simply replace it with a fused version, there are plenty of them that hold the fuses and maintain touch safety. If you were using these simply as a fuse HOLDER, and there is another disconnect ahead of it that somehow qualifies as the Branch Circuit Disconnect, but you can't change it out for some reason, then there are other types of fuse holders available for 60A fuses, but just may not have the convenient feature for lifting the fuse out like that.
 
Cooper makes a fuse holder with a cover that is "touch safe" from front, sides, and top. The cover is an accessory you purchase seperate. Unfortunately, it doesn't open the circuit when you open the fuse holder, but you should have the circuit locked out anyway. As jraef mentioned these aren't make for making and breaking a load, only holding the fuses. I prefer the style that you linked to though.

http://www.cooperindustries.com/con...ks/blocks/power-distribution-fuse-blocks.html

If you do need disconnect capability AB makes a through door fused disconnect that goes to at least 400A
 
jraef said:
If that is your only device, you are misusing it anyway. Those holders are NOT rated to be opened under load, which is the DEFINING criteria for a disconnect switch in a motor circuit. So if you already have another proper disconnect switch in the circuit, then simply replace it with a fused version, there are plenty of them that hold the fuses and maintain touch safety. If you were using these simply as a fuse HOLDER, and there is another disconnect ahead of it that somehow qualifies as the Branch Circuit Disconnect, but you can't change it out for some reason, then there are other types of fuse holders available for 60A fuses, but just may not have the convenient feature for lifting the fuse out like that.
Click to expand...

The circuit is 480 volt and the main panel disconnect is 100 amp fused which feeds a distribution block then it goes into the 60 amp fuse holder which feeds a C55 contractor and overload.

30 HP will not handle the new product. 40 HP motor is 47 amps so I guess the C55 contractor is ok? The circuit sizing tool I have calls for 80 amp dual element time delay fuses ahead of the contactor which is why I was trying to find a fuse holder with a 100 amp frame.
 
You need to look at the load/life curve of the contactor. There should also be a maximum HP starting rating based on ac-1,3,etc. from there pick your contactor. Then look at the sccr coordination table for the contactor. That will give you the maximum type and size of fusing to meet your requisite sccr. Then take your motor FLA and oversize your fusing according to the fusing table in the over current protection device table in NFPA 79. If the current configuration meets these requirements, fantastic, but don't make a headache for yourself by not sizing everything correctly like it was a new application.
 
When using IEC contactors, I always use the next size larger for motors and resistive (like elements) loads..
 

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