O/L Protection of Cables

F

FaradayFF

Senior Member
Location
California
Hi folks,
I was thinking about something - usually we size OCPD rating to be lower or equal to feeder ampacity. However, I can see a scenario where OCPD having higher Amp rating than feeder ampacity where a separate overload device would be sized to provide O/L protection on the said feeder.
Is this allowed per NEC?
Thanks,
EE
 
FaradayFF said:
However, I can see a scenario where OCPD having higher Amp rating than feeder ampacity where a separate overload device would be sized to provide O/L protection on the said feeder.
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Can you explain this scenario? Curious as to why would you use an overload device instead of an OCPD to protect the feeder conductors?
 
infinity said:
Can you explain this scenario? Curious as to why would you use an overload device instead of an OCPD to protect the feeder conductors?
Click to expand...
This is an existing installation where there is a 100Amp CB supplying a 50kW load. The conductors are #4 AWG and there is an overload device part of the starter in the MCC bucket feeding the load. The overload is configured to provide O/L protection for the load and as such is also protecting the cable. Since #4 AWG is good for 85Amp, the cable ampacity is below the breaker rating, and that's why I was wondering if the intent was for the O/L device to provide the required overload protection for the cables. The breaker, O/L device and the starter are all located in the same MCC bucket.
Thanks,
EE
 
Voltage? The KW isn't useful without that. I guess we'll assume the load is a motor since an O/L is mentioned. Might also assume copper THHN at 75deg and not derated for any reason.

Please don't make us guess about the setup, give us all the info the first time.
 
FaradayFF said:
This is an existing installation where there is a 100Amp CB supplying a 50kW load. The conductors are #4 AWG and there is an overload device part of the starter in the MCC bucket feeding the load. The overload is configured to provide O/L protection for the load and as such is also protecting the cable. Since #4 AWG is good for 85Amp, the cable ampacity is below the breaker rating, and that's why I was wondering if the intent was for the O/L device to provide the required overload protection for the cables. The breaker, O/L device and the starter are all located in the same MCC bucket.
Thanks,
EE
Click to expand...
A starter is only intended to run motor loads. If the load is not a motor, the 100 amp OCPD must be replaced with a 90 amp one to be code compliant.

If the load is a motor, the OCPD for the circuit that supplies the starter, only provides short-circuit and ground-fault protection for the conductors that supply the starter. The overload device in the starter provides overload protection for the branch circuit, the motor circuit conductors and for the motor.
 
FaradayFF said:
This is an existing installation where there is a 100Amp CB supplying a 50kW load. The conductors are #4 AWG and there is an overload device part of the starter in the MCC bucket feeding the load. The overload is configured to provide O/L protection for the load and as such is also protecting the cable. Since #4 AWG is good for 85Amp, the cable ampacity is below the breaker rating, and that's why I was wondering if the intent was for the O/L device to provide the required overload protection for the cables. The breaker, O/L device and the starter are all located in the same MCC bucket.
Thanks,
EE
Click to expand...
This exact situation is what Article 430 is all about, so long as the load is a MOTOR. If it isn't a motor, then Article 430 no longer applies and you go back to whatever rules apply to what it is. If it is listed in Table 240.3, you use that appropriate section. If not, then you default to Article 240.4
 
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