NEC Breakers (MCB, MCCB) - trip time

[NickParker]

What is the trip time for the breakers (MCB, MCCB) manufactured in accordance with NEC/NEMA?

In IEC land, MCBs are guaranteed not to trip at below 112% of their rating and permitted up to 1.45 times rated current before the breaker trips.

Is there any kind of rule similar to these in Americas?

 

[dpc]

Covered in UL 489. Last time I checked: Enclosed MCCBs are rated to carry 100% of their rating for up to three hours, in 25 deg C, if cabled in accordance with conductor size as listed in UL 489. If breaker is 100% rated (not common), it will carry 100% of its rating continuously. If current is over 100%, it has to be assumed the breaker will eventually trip. Also, UL 489 specifies a range of acceptable trip times at 300% current for different size MCCBs. This is sensitive to ambient temperature and wire size in thermal-magnetic breakers.

 

[che12345]

Dear Mr. NickParker (Electrical)
"... What is the trip time for the breakers (MCB, MCCB) manufactured in accordance with NEC/NEMA? In IEC land, MCBs are guaranteed ... 1.45 times rated current before the breaker trips. Is there any kind of rule similar to these in Americas?..."
1. Yes. In US, the most widely reference are UL489, UL 1077 ...etc.
2. NEC is concerned on the [installation requirements]; not on the ["product" requirements].
3. UL 489 and UL 1077 requirements are poles apart from that of IEC. They defers in definition, ratings, safety requirements, tripping characteristics, testing methods etc, etc. This result to strictly speaking, they are NOT interchangeable.
4. Refer to UL 489, UL 1077 publications for full detail.
Che Kuan Yau (Singapore)




dear

 

[dpc]

As che12345 points out, UL 489 covers molded case circuit breakers, while UL 1077 covers MCBs. UL doesn't even call MCBs "circuit breakers" but rather "supplemental protectors".