A while ago, a question was published about the NEC requirements for OL protection of MV conductors.
Here I am copying a part of this question.
Trying to get a handle on how to interpret 1 KV+ NEC requirements for overcurrent protection.
"First, 215.3 seems pretty clear when it states, "Feeders shall be protected against overcurrent in accordance with the provisions of Part I of Article 240. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall
not be less than the noncontinuous load plus 125 percent of the continuous load.". That seems pretty cut and dry. But then exception #2 reads, "Feeders over 1000 volts, nominal, shall comply with Part IX of Article 240". So the very first immediate question here is do feeders over 1,000 Volts need to comply with BOTH Part I and Part IX? In other words, is Part I the general rule and Part IX provides modifications to Part I, or do I apply Part IX exclusively and simply ignore Part I?
Second question. Article 240.100(C) requires: "(C) Conductor Protection. The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short circuit conditions." Yet just above it in 240.101(A) just below it, a fuse continuous rating cannot exceed 300% of the ampacity and the long term trip setting of a breaker cannot exceed 600% of the ampacity.
This latter section is the source of my confusion in general. Because if I use 300% rated fuses or 600% rated breaker relays, then clearly although there is some short circuit protection, there clearly isn't any "overload" protection at all. This is nothing like the "100% of noncontinuous load + 125% of continuous load" rule arising out of Article 215.3 which other than the laundry list of exceptions in 240.4(A) through (G) is both very easy to understand seems very logical."
I would like to answer this question very specifically.
Yes, NEC allows MV cables to be protected with fuses at 300% and Relays-CB combinations at 600% of the FLA of the cables. This rule is very different from that of the LV cables which require much closer protection at 100% of the rating of the cable.
The fact that the NEC allows for such a high rating for overload protection in MV cables, does not mean that these cables are not protected by OL currents.
You can verify this by plotting the intermediate characteristic curves (see IEEE Buff Book section 9.5.2). After plotting these curves in a TCC with the OCPDs curves set at these higher ratings (300% and 600%) you will see that, in general, they are indeed very well protected against overloads.
I understand that is counterintuitive, but that is the way it works.
Here I am copying a part of this question.
Trying to get a handle on how to interpret 1 KV+ NEC requirements for overcurrent protection.
"First, 215.3 seems pretty clear when it states, "Feeders shall be protected against overcurrent in accordance with the provisions of Part I of Article 240. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall
not be less than the noncontinuous load plus 125 percent of the continuous load.". That seems pretty cut and dry. But then exception #2 reads, "Feeders over 1000 volts, nominal, shall comply with Part IX of Article 240". So the very first immediate question here is do feeders over 1,000 Volts need to comply with BOTH Part I and Part IX? In other words, is Part I the general rule and Part IX provides modifications to Part I, or do I apply Part IX exclusively and simply ignore Part I?
Second question. Article 240.100(C) requires: "(C) Conductor Protection. The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short circuit conditions." Yet just above it in 240.101(A) just below it, a fuse continuous rating cannot exceed 300% of the ampacity and the long term trip setting of a breaker cannot exceed 600% of the ampacity.
This latter section is the source of my confusion in general. Because if I use 300% rated fuses or 600% rated breaker relays, then clearly although there is some short circuit protection, there clearly isn't any "overload" protection at all. This is nothing like the "100% of noncontinuous load + 125% of continuous load" rule arising out of Article 215.3 which other than the laundry list of exceptions in 240.4(A) through (G) is both very easy to understand seems very logical."
I would like to answer this question very specifically.
Yes, NEC allows MV cables to be protected with fuses at 300% and Relays-CB combinations at 600% of the FLA of the cables. This rule is very different from that of the LV cables which require much closer protection at 100% of the rating of the cable.
The fact that the NEC allows for such a high rating for overload protection in MV cables, does not mean that these cables are not protected by OL currents.
You can verify this by plotting the intermediate characteristic curves (see IEEE Buff Book section 9.5.2). After plotting these curves in a TCC with the OCPDs curves set at these higher ratings (300% and 600%) you will see that, in general, they are indeed very well protected against overloads.
I understand that is counterintuitive, but that is the way it works.