NEC Code Question
NEC Code Question
(OP)
Hello,
I am trying to understand why a system was set up in this particular way at an industrial facility. It may not be correct.
A 480V breaker with a LTPU setting of 800A is connected to a feeder (2/ph 1/C 400 mcm)(not sure yet of insulation) which feeds directly to another breaker about 100 ft away. This second breaker is connected to (2/ph 1/C 250 mcm XHHW)and has a LTPU of 500A. The load is connected to this feeder.
I am trying to understand why the first breaker is set at 800A. If the cable is 75 deg C, the ampacity is 670A, then the 800A setting is too high, except that the overload would be limited by the downstream breaker set at 500A. Lowering the upstream breaker setting to 640A would cause the LTPU to overlap the downstream breaker LTPU setting, thereby resulting in mis-coordination. Setting at 800A eliminates the mis-coordination but is greater than the cable ampacity.
The possible reason I see is that there is no way the 400 mcm cable could be overloaded as the load is limited by the next breaket set a 500A. This uptream breaker is set at 800A to prevent mis-coordination and will adequately protect the cable from a short circuit.
Is this allowed by the code?
I am trying to understand why a system was set up in this particular way at an industrial facility. It may not be correct.
A 480V breaker with a LTPU setting of 800A is connected to a feeder (2/ph 1/C 400 mcm)(not sure yet of insulation) which feeds directly to another breaker about 100 ft away. This second breaker is connected to (2/ph 1/C 250 mcm XHHW)and has a LTPU of 500A. The load is connected to this feeder.
I am trying to understand why the first breaker is set at 800A. If the cable is 75 deg C, the ampacity is 670A, then the 800A setting is too high, except that the overload would be limited by the downstream breaker set at 500A. Lowering the upstream breaker setting to 640A would cause the LTPU to overlap the downstream breaker LTPU setting, thereby resulting in mis-coordination. Setting at 800A eliminates the mis-coordination but is greater than the cable ampacity.
The possible reason I see is that there is no way the 400 mcm cable could be overloaded as the load is limited by the next breaket set a 500A. This uptream breaker is set at 800A to prevent mis-coordination and will adequately protect the cable from a short circuit.
Is this allowed by the code?






RE: NEC Code Question
I don't have a current copy of the NEC so wait for some more answers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: NEC Code Question
If they are both serving the same load, it is not really mis-coordination anyway, so I'm not sure I see a problem with reducing the setting. I'd probably set them both at 500 A.
RE: NEC Code Question
RE: NEC Code Question
The reason for two breakers may have been tha the equipment moved or a drawing was revised.
RE: NEC Code Question
If this is the worst NEC violation in the facility, consider yourself a lucky man.
RE: NEC Code Question
The breaker has settings capable of 720 A or 800 A. Per the NEC, the standard ratings 700 A or 800 A. The breaker is set at 800 A based on the fact that the next higher setting does not correspond to a standard ampere breaker rating, it is not part of a multi-branch circuit and the next higher standard rating does not exceed 800 A.
A colleague and I have a disagreement over whether it is a code violation to set the breaker at 800A LTPU for 400mcm, 2-1/c per phase, XHHW conductor that is feeding another breaker that is set to protect a smaller conductor.
Comments pertaining to code sections and any potentional violations would be appreciated.
FYI - there is a beer riding on this.
RE: NEC Code Question
Gear does exist with 90°C lugs, but it is typically ANSI class switchgear. Your second breaker almost certainly does not have 90°C lugs.
RE: NEC Code Question
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
Since that C/B has an overload trip adjustment (720A or 800A), it may not fit this condition.