Fused or Non-Fused, That is the Question
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mayanees
Senior Member
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- Westminster, MD
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- Electrical Engineer and Master Electrician
It's okay to use an unfused disconnect when the avaliable fault current is less than 10,000 amps. That's my understanding of the maximum short-circuit rating for an unfused disconnect.
Fuses with higher interrupt ratings will allow the switch to be rated at that higher level.
From experience, I did a design for an industrial plant that included all fused disconnects. The owner value-engineered the system to use all unfused disconnects, and looked to us for approval of that change. I think that out of the ~ 75 motor disconnects used on the project, only about 15-20 were able to be changed to unfused where the fault current at the motor was less than 10k amps.
Of course that requires analysis for every application to determine the available fault current.
John M.
Fuses with higher interrupt ratings will allow the switch to be rated at that higher level.
From experience, I did a design for an industrial plant that included all fused disconnects. The owner value-engineered the system to use all unfused disconnects, and looked to us for approval of that change. I think that out of the ~ 75 motor disconnects used on the project, only about 15-20 were able to be changed to unfused where the fault current at the motor was less than 10k amps.
Of course that requires analysis for every application to determine the available fault current.
John M.
RICK NAPIER
Senior Member
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- New Jersey
What is this in reference to? An appliance? Feeder circuits? Mains?
mayanees
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- Westminster, MD
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- Electrical Engineer and Master Electrician
HAH!
My reference was to motor disconnects! That's what I thought the question was directed at, but as I look back at the OP, it didn't say!
John M
If you are looking at packaged HVAC equipment, the nameplate or the cutsheet should say either "Maximum Overcurrent Protection" or "Maximum Fuse Size".
If it says Maximum Fuse Size, you have to provide fuses, so a fused disconnect is probably required.
If it says maximum overcurrent protection, you can use a HACR breaker in the panel feeder, and omit the fuses. Assuming the fault current allows, as John M. mentioned.
If it says Maximum Fuse Size, you have to provide fuses, so a fused disconnect is probably required.
If it says maximum overcurrent protection, you can use a HACR breaker in the panel feeder, and omit the fuses. Assuming the fault current allows, as John M. mentioned.
My experience on RTUs has been that fusible is the best route based on the following:
(a) As mentioned by Steve66, some units require fusible protection per their listing.
(b) The actual required protection once the unit is installed is often different from the original specs and, if the disconnect, is NF, a breaker change may be costly or a delay (based on the units often requiring "odd" breaker sizes)
(c) The 10k rating per mayanees' post
(a) As mentioned by Steve66, some units require fusible protection per their listing.
(b) The actual required protection once the unit is installed is often different from the original specs and, if the disconnect, is NF, a breaker change may be costly or a delay (based on the units often requiring "odd" breaker sizes)
(c) The 10k rating per mayanees' post
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