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anbm

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1. Do they make 100% rated thermal magnetic breakers. If they are available, what are their size range?



2. With 80% rated breaker, for example: 200A CB, does the breaker actual tripped at 160A (80% of 200A)? Or can be it overload some period before it is tripped?

Thanks!
 
Do they make 100% rated thermal magnetic breakers.
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Yes, they make breakers that are rated to run at 100% of their rating.

With 80% rated breaker, for example: 200A CB, does the breaker actual tripped at 160A (80% of 200A)? Or can be it overload some period before it is tripped?
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A typical inverse time breaker will not trip when loaded to 100% or even a little above that. With an inverse time circuit breaker the amount of time it takes before the breaker trips is inversely proportional to the amount of current that flows through the breaker.

So for example with a 200 amp breaker, the breaker would not trip if it were loaded to 200 amps. If the breaker were loaded to 250 amps it would trip but it would take some time for it to trip. But if you load the 200 amp breaker to say 800 amps it would trip very quickly.

Chris
 
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raider1 said:
Yes, they make breakers that are rated to run at 100% of their rating.



A typical inverse time breaker will not trip when loaded to 100% or even a little above that. With an inverese time circuit breaker the amount of time it takes before the breaker trips is porportional to the amout of current that flows throught the breaker.

So for example with a 200 amp breaker, the breaker would not trip if it were loaded to 200 amps. If the breaker were loaded to 250 amps it would trip but it would take some time for it to trip. But if you load the 200 amp breaker to say 800 amps it would trip very quickly.

Chris
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You meant to say, "inversely proportional to the amount of current" did you not?
 
090106-1601 EST

For a typical current vs time trip curve for Sq-D QO breakers see p25 of
http://ecatalog.squared.com/pubs/Ci...rs/QO-QOB Circuit Breakers/0730CT9801R108.pdf

Very hard to locate this from the Sq-D main page.

Theoretically you should be able to operate at 99.9% of the breaker rating for this curve. However, there are variances for different reasons that mean you should never design your system such that the load is this close to the long time trip point. 80% of the breaker rating may not be a bad choice, but lower might be required sometimes, for example Texas.

Note: 25 deg C is 32 + 9*25/5 = 77 deg F.

.
 
anbm said:
1. Do they make 100% rated thermal magnetic breakers. If they are available, what are their size range?
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Up until recently, most manufacturers did not offer 100% rated breakers below 400A. Lately (within the last few years) there are several who offer them down as low as 125A.



2. With 80% rated breaker, for example: 200A CB, does the breaker actual tripped at 160A (80% of 200A)? Or can be it overload some period before it is tripped?
Click to expand...
Answered adequately by others.

But what you may be missing out on is the issue of WHY breakers are only rated for 80% as standard. The reason is, you cannot load up the cables any higher than that anyway. Here is an example;

  • You have a load that says it will draw 390A.
  • Your cable must be sized for 125% of that load, so your minimum cable rating will be 488A.
  • You use cable rated for 520A because that is the nearest size over 488.
  • You protect the cable with a 500A breaker.
  • You can only load the breaker to 400A, but that still fits your 390A load requirements.
 
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