80% rated CB at reduced current instead of 100% rated CB

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elec_eng

Senior Member
I have a 800A CB Plug In unit from 4,000A busway for 800A continuous load. 800A CB specified was 100% rated CB so the feeder is sized for (3) sets of 300KCM.

The equipment vendor is proposing we use 1,000A CB with 80% rated, instead. Sounded like they have several 1,000A CB with 80% rated. They are proposing to set the trip at 800A so we don't have to change the feeder size. They said they tested this CB with 800A and didn't exceed the thermal rating.

Cost and physical dimension aside, I have two questions:

1. Code standpoint - Is this code compliant, specifically 215.2 and 215.3.
2. Not clear how 100% rated CB is tested but if 1,000A CB was tested with 800A load and pass the thermal testing, does it make this CB 100% rated at 800A?
 
elec_eng said:
Not clear how 100% rated CB is tested but if 1,000A CB was tested with 800A load and pass the thermal testing, does it make this CB 100% rated at 800A?
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All breakers are tested at 100% in open air.
The problem is breaker and conductors need to be installed inside of enclosures, which means they could overheat if loaded to the maximum. So normal breakers and conductors used at 100% loading need to be oversized to allow for the poor heat disapation, the NEC requires a 125% multiplier.
However if enough ventilation is provided, byte manufacturer, a breaker can be listed for 100% loading.

Dialing the trip unit rating from 1000A down to 800A does not change it from a being an 80% to a 100% device,
 
elec_eng said:
The equipment vendor is proposing we use 1,000A CB with 80% rated, instead. Sounded like they have several 1,000A CB with 80% rated. They are proposing to set the trip at 800A so we don't have to change the feeder size
Click to expand...
How does using a 800 non-continuous trip work with an 800 amp continuous load?
 
jim dungar said:
However if enough ventilation is provided, byte manufacturer, a breaker can be listed for 100% loading.

Dialing the trip unit rating from 1000A down to 800A does not change it from a being an 80% to a 100% device,
Click to expand...

Does this mean if the manufacturer provides the enough ventilation for this 1,000A breaker for 800A load such that there will be no overheating, then this breaker can be 100% rated for 800A? That's what this manufacturer is saying.
 
elec_eng said:
Does this mean if the manufacturer provides the enough ventilation for this 1,000A breaker for 800A load such that there will be no overheating, then this breaker can be 100% rated for 800A? That's what this manufacturer is saying.
Click to expand...
Yes, kind of. We would not usually say it is 100% rated, we would say it can carry an 800A load continuously.

The 80% 1000A breaker can always carry 800A continuously when it's longtime setting is 1000A. If you dial the breaker down to 800A you might experience unexpected tripping on an 800A continuous load. You need to look at the breaker Time Current Curve and compare it to your load profile to see if it's setting will do what you want.
 
jim dungar said:
If you dial the breaker down to 800A you might experience unexpected tripping on an 800A continuous load.
Click to expand...
What sort of internal mechanism would such a breaker with a dial for the long time setting use to determine whether to trip on long time? If it's electronic, I would have thought it would be programmed to never trip for a continuous loading at exactly its long time setting.

Or are electronic trip curves sometimes programmed to mimic the full behavior of thermal trip mechanisms, including the undesirable feature of the possibility of tripping on a continuous load equal to the long time setting? That would seem strange to me, to program in undesirable behavior, just to match the behavior of thermal trip mechanisms.

Thanks, Wayne
 
wwhitney said:
...are electronic trip curves sometimes programmed to mimic the full behavior of thermal trip mechanisms...
Click to expand...
Yes, they mimic the behaviour of thermal elements, often right down to the cooling off period.

Overcurrent relays may or may not mimic thermal devices.
 
jim dungar said:
Yes, they mimic the behaviour of thermal elements, often right down to the cooling off period.
Click to expand...
Thanks for diving into the details.

Do they also mimic the dependency on ambient temperature? E.g. by using a thermistor to monitor temperature?

Because if the electronic model is based only on values from say instrumenting a thermal breaker in open air at 40C, under which conditions any breaker is not to trip at 100% of rated current, I would think the result would be behavior that never trips at 100% of rated current.

Cheers, Wayne
 
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