Rational of 240.101 rules on OCP for MV Cables

[Schroeder]

Exact wording not with standing, this section allows for fuses to be rated up to 3 times the FLA of the cable it is protected (6 times for circuit breakers). Does anyone know what the rational is behind this? I.e. if your cable is only rated for 100A why would you choose 300A fuses; Especially when the fuses themselves won’t pick up until 2 x FLA?

Thanks,

Mike

 

[Jraef]

You are confusing "shall not exceed" with "must be at least". You should protect your conductors at the lowest possible level according to their rated current capacity. But sometimes there may be reasons to go higher, for example to allow for inrush current on an inductive load. This section is just giving you the UPPER LIMITS of what you can do. Read the section before it, 240.100, where it says "... that includes but is not limited to considering the appropriate fault studies and time–current coordination analysis of the protective devices and the conductor damage curves."