CB current setting
- Thread starter Neelesh
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: CB current setting
Is the 800A continuous current or non-continuous? What size wire downstream is this circuit breaker protecting?
Is the 800A continuous current or non-continuous? What size wire downstream is this circuit breaker protecting?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: CB current setting
Adding to Ron?s statement, the conductors must be sized for (i.e., have an ampacity of) 100% of the non-continuous load plus 125% of the continuous load (a term meaning it is on for more than3 hours). Once you know that number, you set the breaker to trip at or below the ampacity of the conductors. If the load had been under 800 amps, you could have used the next higher standard trip setting. But above 800 amps, you cannot ?biggie size? the trip point.
Adding to Ron?s statement, the conductors must be sized for (i.e., have an ampacity of) 100% of the non-continuous load plus 125% of the continuous load (a term meaning it is on for more than3 hours). Once you know that number, you set the breaker to trip at or below the ampacity of the conductors. If the load had been under 800 amps, you could have used the next higher standard trip setting. But above 800 amps, you cannot ?biggie size? the trip point.
Re: CB current setting
Set breaker? Is it an "adjustable" (thermal)_trip unit which is most likely a solid state trip or interchangeable trip unit which could either be solid state with a rating plug or a common interchangable thermal magnetic trip? More and more breakers of that size hvae solid state trip units that may also include "adjustable" short time pickup and delay as well as ground fault.
Also, with the older technology themal magnetic trip units breakers of that size will have a "magnetic" pick-up adjustment "adjustable" from 5-10x the trip unit rating.
The previous post stated how the load is calculated and the cable sized. The cable must be able to carry that calculated load. Regardless of the load the breaker is sized to protect the cable. Remember that the calculated load is often that not less that the rated ampacity of the cable.
If you have an "adjustable" (thermal) trip unit the cable must be sized based upon the its maximum setting, ref art 240.6 (B) with the exception (C).
There are no "adjustment" requirements for the magnetic trip setting unless one makes an attempt at coordinating with other breakers. One could also recommend that it be set at the lowest setting so that it would pick up on faults sooner but it could also lead to nuisance tripping with high inrush loads.
Set breaker? Is it an "adjustable" (thermal)_trip unit which is most likely a solid state trip or interchangeable trip unit which could either be solid state with a rating plug or a common interchangable thermal magnetic trip? More and more breakers of that size hvae solid state trip units that may also include "adjustable" short time pickup and delay as well as ground fault.
Also, with the older technology themal magnetic trip units breakers of that size will have a "magnetic" pick-up adjustment "adjustable" from 5-10x the trip unit rating.
The previous post stated how the load is calculated and the cable sized. The cable must be able to carry that calculated load. Regardless of the load the breaker is sized to protect the cable. Remember that the calculated load is often that not less that the rated ampacity of the cable.
If you have an "adjustable" (thermal) trip unit the cable must be sized based upon the its maximum setting, ref art 240.6 (B) with the exception (C).
There are no "adjustment" requirements for the magnetic trip setting unless one makes an attempt at coordinating with other breakers. One could also recommend that it be set at the lowest setting so that it would pick up on faults sooner but it could also lead to nuisance tripping with high inrush loads.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: CB current setting
Neelesh,
You are taking a somewhat alternative method to choose your ratings.
Neelesh,
You are taking a somewhat alternative method to choose your ratings.
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