NEC Code Question

[wbd]

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?

 

[waross]

The possible reason that you suggest is the justification for allowing over-sized breakers to provide short circuit protection for motor feeders and allowing the motor over-load protection to protect against overloading.
I don't have a current copy of the NEC so wait for some more answers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

No, I don't think so. 700 A would be the maximum allowable setting per the NEC. I wouldn't lose much sleep over the 800 A setting, but I don't think it is quite legal.

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.

 

[DanDel]

waross may be correct if the load is a motor. The motor information is needed to determine what cable and protection is required, but it would seem that the 500AT C/B is providing the protection for any motor load anyway, and the 800AT is just there for protection of the cable, which it is not providing.

 

[BJC]

How old is the installation? Once upon a time 700 amp breakers were not common. The next standard size would have been 800 amps.
The reason for two breakers may have been tha the equipment moved or a drawing was revised.

 

[dpc]

NEC has listed 700 A as a standard size for at least 30 years. They are not common, but they have always been available.

If this is the worst NEC violation in the facility, consider yourself a lucky man.

 

[wbd]

New information is available on this situation. The conductor has been confirmed as 1/c 400 mcm XHHW which has a ampacity rating of 380A and there are 2 conductors per phase for a total of 760A and it is considered a dry location.
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.

 

[davidbeach]

Unless you have terminations at both ends rated for use at 90[°]C at both ends you can't say that XHHW has an ampacity of 380A, the best you can get is 335A, or 670A for two in parallel. In a somewhat loose interpretation of the NEC you could install those in parallel and use a 700A breaker. In a reading that conforms more to the spirit of the NEC, you could use those conductors with a 700A breaker if your calculated load is greater than 600A and less than 670A.

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.

 

[DanDel]

Also, the NEC 240.4 (B) has three conditions, one of which is:

(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.