Fused and non-fused disconnects

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T74

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When does a disconnect need to be fused?

I wired a 3 phase 10.5KW electric oven. Does the disco need fuses?
 
No, as long as the circuit has over current protection, the local disconnect does not need fusing
 
T74 said:
When does a disconnect need to be fused?

I wired a 3 phase 10.5KW electric oven. Does the disco need fuses?
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Generally speaking, a disconnecting means never needs to be fused. However, there are requirements where a disconnect is not fused, OCPD must be installed immediately adjacent thereto, such as service disconnecting means.

Where the disconnect only serves as a local disconnect for servicing equipment, it is not required to be fused. However, if the supply circuit is rated greater than permitted for the equipment, a fused disconnect would be desired so as to install lesser rated OCPD. Another situation may be where the disconnect is supplied by a feeder tap.
 
ron said:
It needs to be fused if the short circuit current available at the switch is >5kA. Regular safety switches have a low SCCR.
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As I recall, the SCCR does factors into required fusing, not as a directly per NEC, but rather a consequence of equipment selection.
 
Short simple answer is if the load side conductors require a different level of overcurrent protection than what is already on the line side conductors - then you will need the proper overcurrent protection incorporated in the switch or immediately adjacent to the switch (and this can include circuit breakers as well), the reasons for the need for a change in overcurrent protection are somewhat unlimited and there is no other single answer fits all solution here.
 
Smart $ said:
As I recall, the SCCR does factors into required fusing, not as a directly per NEC, but rather a consequence of equipment selection.
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NEC 110.10 requires 'non-protective' equipment to be used within its ratings (SCCR) based on available fault current.
 
Smart $ said:
Exactly... but that's not a direct requirement to fuse the disconnect. It's a consequence of equipment rating.
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It is a direct requirement to properly apply the disconnect, you are correct that the NEC does not directly say how to do it. Another case, proving the NEC is not a design manual.
 
I install a non-fused disconnect for the oven. On the inspection the inspector said it should be fused. I questioned him on it and he told me not to worry about it.

For my own sake, I wanted to follow up on it. It sounds like the consensus is that non-fused is acceptable.
 
T74 said:
I install a non-fused disconnect for the oven. On the inspection the inspector said it should be fused. I questioned him on it and he told me not to worry about it.

For my own sake, I wanted to follow up on it. It sounds like the consensus is that non-fused is acceptable.
Click to expand...

If the nameplate specified that fuses should protect it, as in (maximum fuse size) then it needs to be protected by fuses.
But I have not seen that on a nameplate in a long time.
 
T74 said:
I install a non-fused disconnect for the oven. On the inspection the inspector said it should be fused. I questioned him on it and he told me not to worry about it.

For my own sake, I wanted to follow up on it. It sounds like the consensus is that non-fused is acceptable.
Click to expand...
If required overcurrent is already provided upstream then overcurrent protection is not required at any disconnecting means located at equipment.

If available fault current is too high, that is a different issue, and kind of doesn't change anything anyway unless proper fuses are used.
 
I see fused disconnects installed primarily for 3 reasons:
1. The equipment design calls for "Fuse protection" (as opposed to "fuse/breaker" or
"over-current"
2. To address high fault-current issues.
3. Where at the time of installation the exact size/type of over-current protection is
not know and the E/C rather select fuses than change out breakers once the
equipment is installed (happens a lot on HVAC units which historically call for
"odd" sizes)
 
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