inverse breakers

33554 Views 5 Replies 6 Participants Last post by bee77, Mar 17, 2011

john salyards

Discussion Starter · Feb 16, 2008

seems to be alot of questions about these specialty bbreakers where can i find them in the code book

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RePhase277

· #2 · Feb 16, 2008

john salyards said:
seems to be alot of questions about these specialty bbreakers where can i find them in the code book

They are defined in article 100. They are mentioned in the motor over current table. Other overcurrent protection articles can be found in 240. But overall, the code doesn't say much about them. They would be specified in a set of plans, more than a code article.

But if you don't know, an inverse-time breaker is one whose trip time is inversely proportional to the amount of overcurrent. That is, the larger the overcurrent, the faster it trips. As opposed to instantaneous-trip.

You want inverse-time on circuits that expect to have a slight overload for a short period of time, like motor circuits.

InPhase277

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brian john

· #3 · Feb 16, 2008

All molded case circuit breakers are INVERSE TIME circuit breakers (OK 99% of them). All this means is the higher the current the faster the CB trips.

Circuit breakers utilized in typical branch circuit wiring are thermal magnetic devices. That is thermal is the long time/overload portion, higher the current the faster the circuit breaker operates for over loads.

For thermal Such as a 20 am CB may carry 300% (60 amp) for 30 seconds
200% (40 amps) for 90 seconds
110% (22 amps) for days
All times are estimates/guesses as I do not have time curves available at this time, (there are in the office).

For Instantaneous Instantaneous occurs at multiples above 400% typically and operates on the same principle, the higher the current the faster the CB trips.

Inverse only CBs are normally part of a motor control stater or motor control center where the thermal (long time/overload) protects the motor against overloads, the inverse time is for fault protection and is sized to permit the motor to start (starting currents are typically 4-6 times). Inverse CBs have settings from 4-12 times normally that is for a 20 amp CB 80 to 240 amps.

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Esakoh

· #4 · Oct 6, 2009

What is the smallest-size fixturewire permitted in the NEC is ........AWG?

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John Valdes

· #5 · Oct 6, 2009

brian john said:
All molded case circuit breakers are INVERSE TIME circuit breakers (OK 99% of them). All this means is the higher the current the faster the CB trips.

Circuit breakers utilized in typical branch circuit wiring are thermal magnetic devices. That is thermal is the long time/overload portion, higher the current the faster the circuit breaker operates for over loads.

For thermal Such as a 20 am CB may carry 300% (60 amp) for 30 seconds
200% (40 amps) for 90 seconds
110% (22 amps) for days
All times are estimates/guesses as I do not have time curves available at this time, (there are in the office).

For Instantaneous Instantaneous occurs at multiples above 400% typically and operates on the same principle, the higher the current the faster the CB trips.

Inverse only CBs are normally part of a motor control stater or motor control center where the thermal (long time/overload) protects the motor against overloads, the inverse time is for fault protection and is sized to permit the motor to start (starting currents are typically 4-6 times). Inverse CBs have settings from 4-12 times normally that is for a 20 amp CB 80 to 240 amps.

Yep, I have Table 430-152 Inverse Time Breaker marked "regular breaker". So as to not be confused.

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bee77

· #6 · Mar 17, 2011

found these explanation from google. Clear & usefull,
Thanks !!

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