mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
NEC Article 240 section IX deals with Overcurrent Protection over 600 Volts Nominal.
The specific area of contradiction with the rest of the NEC overcurrent protection practices deals with 240.101, which permits overcurrent prtotection of a 600V+ feeder at 300% ampacity with a fuse, and 600% for a breaker or electronic fuse.
There is a qualification to 240.100 in 240.101, which throws it back on an "engineer" to verify that the conductor is analyzed on a Time-Current-Curve (TCC) to assure that the conductor damage curve is protected.
But it's always surprised me that this section is there, and that it's so radically different than the "under 600V" conductor protection philosophy.
My rationale is that it falls into a category similar to Motor Short-Circuit protection versus Overload protection, in that the low voltage loads will provide protection against overloads at the 600V+ level. And this exception facilitates coordination with devices at the higher voltage. And, a TCC of the system will show key damage points to protect against. But, if enough low-voltage circuits were added, you could overload the 600+V feeder.
I'm curious to hear opinions from some other folks on this forum as to their rationalization of the Overcurrent Protection Over 600 Volts, Nominal section.
Thanks,
JM
The specific area of contradiction with the rest of the NEC overcurrent protection practices deals with 240.101, which permits overcurrent prtotection of a 600V+ feeder at 300% ampacity with a fuse, and 600% for a breaker or electronic fuse.
There is a qualification to 240.100 in 240.101, which throws it back on an "engineer" to verify that the conductor is analyzed on a Time-Current-Curve (TCC) to assure that the conductor damage curve is protected.
But it's always surprised me that this section is there, and that it's so radically different than the "under 600V" conductor protection philosophy.
My rationale is that it falls into a category similar to Motor Short-Circuit protection versus Overload protection, in that the low voltage loads will provide protection against overloads at the 600V+ level. And this exception facilitates coordination with devices at the higher voltage. And, a TCC of the system will show key damage points to protect against. But, if enough low-voltage circuits were added, you could overload the 600+V feeder.
I'm curious to hear opinions from some other folks on this forum as to their rationalization of the Overcurrent Protection Over 600 Volts, Nominal section.
Thanks,
JM
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