Fuses as OCPD, Section 240

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gilmomoney

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Hi There,

An electrician I'm working with recently said that fuse sizes must be size limited to 80% of their current rating. I've been looking in section 240, but I can't find a specific article that might call this out and I've often sized fused circuits at 100% of the current requirements.

I know that CBs (unless fully rated) are limited to 80%, but if someone can point me to an article that applies to fuse current limitations, I would appreciate it.

Thanks in advance for the help.

Mike
 
For general protection, fuses and circuit breakers need to be applied the same.

The UL requirements for breakers and fuse, mounted in enclosures and with cables installed, both have the same "80%" rating issue.
 
Fuses and circuit breakers are very similar as overcurrent devices in the sense that they generally protect the circuit from overloads, overcurrent and faults.

Another common feature is the percentage of current in general a fuse/circuit breaker - in percentage is applied for.


The 80% & 125% requirements are not driven by the type of protection (fuses/CBersP), as much as by the application the OCPD are installed in.

Such as motors, A/C, heating and lighting. The type of load is what I am trying to get at.
 
Pierre C Belarge said:
Fuses and circuit breakers are very similar as overcurrent devices in the sense that they generally protect the circuit from overloads, overcurrent and faults.

Another common feature is the percentage of current in general a fuse/circuit breaker - in percentage is applied for.


The 80% & 125% requirements are not driven by the type of protection (fuses/CBersP), as much as by the application the OCPD are installed in.

Such as motors, A/C, heating and lighting. The type of load is what I am trying to get at.
Click to expand...

Fuses are thermal devices that depend on the actual melting of the conductive element. Breaker's long time element is also based on thermal characteristics, but they are not melting elements, but a thermal module that changes physical characteristics with temperature and actualte the mechanical elements of the contact parting device. The later, therefore, operates at a much lower temperature than the fuse's melting temperature. As the result when fuses are operated close to their rating they will thermally 'age' and eventually open without warrant.

In short - no pun unintended - fuses are excellent short citcuit protective devices and lousy overload devices.
 
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