100% Breakers - Size of conductors

[osaengineers]

Hi there to all,
My question is as follows:
If I have a 800ampers 100% rated breaker, Can I connect 2 sets of 500KCMs to it (total ampere rating for the feed of cable would be 760ampere using that 500KCM is good for 380ampere.). Please let me know where in the code it addresses it

I think so, since it says you can go to the next size breaker which is a 800A

Thanks for the Help

 

[roger]

240.4 Protection of Conductors

(B) Devices Rated 800 Amperes or Less The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

(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).

(3) The next higher standard rating selected does not exceed 800 amperes.

240.6 Standard Ampere Ratings

(A) Fuses and Fixed-Trip Circuit Breakers The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

Roger

 

[charlie b]

There is a detail that you left out. What is the calculated load? You can do what you describe, so long as the calculated load is under 760 amps. But if the calculated load is between 760 and 800, or if there is no calculation and you have to presume a load of 800, then you need to have conductors with an ampacity of 800.

 

[templdl]

My exact thoughts Charlie.
Often questions are more than what they appear to be.
1) Why a 100% rated breaker in the first place?
a)It simply can't be installed or mounted in any enclosure unless it suitable for use with a 100% rated breaker.
b) To apply a 100% at 100% the cable sizing comes first which involves calculation the load a 100% continuous (not 125% as with 80% rated breakers) plus 100% noncontinuous. Cable sizing is based of 75degC but the cable must be rated at 90degC. By only having to use 100% of the continuous load it provides an opportunity to use a smaller cable than would be used otherwise. With smaller cable comes a smaller breaker to protect that smaller cable which decreases cost. Sometimes it may affect the enclosure size or panelboard selected. All must be evaluated to determine if it is practical.
It is also misunderstood that a 100% rated breaker is better than an 80% rated breaker which is wrong. Both are identical except for the label which goes back to UL testing the breaker for a 100% application which includes correct cable application an the enclosure.
I can't count the times where a 100% rated breaker was specified in an MCC but the MCC was not listed for 100%. Thus, the breaker had to be applied as would be a standard 80% rated breaker anyway.