Selective Coordination - Thumb Rule?

[charlie b]

Let’s limit this to molded case circuit breakers. You have a main breaker on a branch circuit panel and you have a branch circuit breaker. A calculation would be required, in order to determine the degree to which the two selectively coordinate. It will depend on the make and model of each. I suspect a 400 amp main breaker is more likely to selectively coordinate with a 20 map branch breaker than a 150 amp main breaker and a 30 amp branch breaker. But is there a reasonable rule of thumb (e.g., ratio of breaker ratings)?

I will soon enter “discussions” with an engineering firm over their suggested revision to project requirements. The project has not been awarded to a GC yet, so the make and model of the electrical components cannot be known yet.

 

[MyCleveland]

Do not believe there is a safe rule of thumb on this topic.

Researched a few years back with EATON equipment only and made up a flow chart for what I believe they termed "Total Coordination".

Paths and CBer values vary depending on voltage and AIC ratings.
EX: I can coordinate a 175 with a 150 (208Y-18kAIC)
at 480 I drop to a 225 coordinating with a 50-MAX at 14kAIC

 

[dkidd]

See the first Q&A

https://blog.schneider-electric.com...ently-asked-questions-selective-coordination/

 

[steve66]

Yes, its definately easier to coordinate breakers that have a wider spread between their long term ratings.

But I agree with the previous reply - there are just too many variables to make a rule of thumb. The main problem is the wide range of trip curves for different brands and models of breakers.

And the available fault current also makes a big difference - two curves may only overlap above some certain current. So if the available fault current is less, then the combination is coordinated. If its more, the combination is not coordinated.

 

[topgone]

Let’s limit this to molded case circuit breakers. You have a main breaker on a branch circuit panel and you have a branch circuit breaker. A calculation would be required, in order to determine the degree to which the two selectively coordinate. It will depend on the make and model of each. I suspect a 400 amp main breaker is more likely to selectively coordinate with a 20 map branch breaker than a 150 amp main breaker and a 30 amp branch breaker. But is there a reasonable rule of thumb (e.g., ratio of breaker ratings)?

I will soon enter “discussions” with an engineering firm over their suggested revision to project requirements. The project has not been awarded to a GC yet, so the make and model of the electrical components cannot be known yet.

The prospective current at that location and the time-current characteristics of the two breakers basically determines the coordination or no-coordination of the two! As long as the TCCs don't cross, the other is higher and to the right of the downstream breaker's TCC, you're good. Also, it's best to provide ample separation of trip times of the breakers to account for breaker opening speed.