Series Rated Equipment, branch circuit breakers.

[Npstewart]

Helping a friend out with a question he sent my way.

We have a 600A panel with a main rated for 50 kAIC. The fault current per the calculations at the main breaker is 30 kAIC therefore the 50 is sufficient. The electricians submitted on a main rated for 50 kAIC but the circuit breakers are only rated for 22 kAIC and they are claiming this is acceptable because they are series rated. I dont really know of anything in between the main and the circuit breakers that would substantially reduce the fault current down to less then 22 kAIC if we have 30 kAIC at the main. Any ideas on this?

PS. I havent really came across any main breaker rated for 50 kAIC but for purposes of discussion I will just assume this is made.

 

[raider1]

You really need to perform an available fault current calculation at the downstream overcurrent devices. Depending on the size of the feeder conductors and the length of the feeder you may be below 22k.

Now with that said if you are above 22k with the available fault current you must contact the manufacturer to get a series rating between the 2 breakers. Just because the breaker upstream is rated above the available fault current does not means that it is series rated to protect the downstream breakers. The manufactures will have the series rating information.

Chris

 

[Npstewart]

You really need to perform an available fault current calculation at the downstream overcurrent devices.

I guess that is the problem. If there is 30 kAIC at the main, how would you calculate the fault current between the two circuit breakers? The only thing between them is the buss. I dont ever recall seeing a calculation that would do this.

 

[jim dungar]

I guess that is the problem. If there is 30 kAIC at the main, how would you calculate the fault current between the two circuit breakers? The only thing between them is the buss. I dont ever recall seeing a calculation that would do this.

You do not calculate anything. You simply check with the manufacturer for the tested series combination rating.
It all has to do with the internals of each breaker; how fast, or slow, they operate and the arcing resistance inside the breaker.
There is no way to predict which breaker will actually clear the fault, many times the higher rated device never operates; this is why testing is required. A series-combination label must be installed in the equipment.

There is also a limit on the amount of motor load that can be connected (in your case 220Amps), which is why series rating are usually best for what we used to call 'lighting and appliance' panels as opposed to 'power' panels.

 

[JDBrown]

I guess that is the problem. If there is 30 kAIC at the main, how would you calculate the fault current between the two circuit breakers? The only thing between them is the buss. I dont ever recall seeing a calculation that would do this.

I think raider1 was under the impression that your 50kAIC and 22kAIC breakers were in different boards. The effect of the bus itself on available fault current is going to be negligible.

You need to gather all of the information about the main and feeder/branch circuit breakers (model, AIC rating, etc.) and then contact the manufacturer. They should be able to tell you if there's a series rating or not, and what exactly you have to do in order to take advantage of the series rating. If you're not sure what information you need, call up the manufacturer, explain your situation, and ask what information you need to gather in order for them to tell you about the series rating. Anything less than going to the manufacturer is going to be guesswork, and won't bring your board into compliance.

 

[raider1]

I guess that is the problem. If there is 30 kAIC at the main, how would you calculate the fault current between the two circuit breakers? The only thing between them is the buss. I dont ever recall seeing a calculation that would do this.

Ok I misunderstood the location of the downstream overcurrent devices. If they are in the same enclosure and connected to the same buss then you are correct there is very little impedance in the bussing and the available fault current would be at the 30K.

So what you are going to have to do is get with the breaker manufacture and see if they have a tested series rating between the main breaker and the downstream breakers.

Chris

 

[Npstewart]

Thanks everyone.

Im sure that 42 kAIC branch circuit breakers are readily available correct?

 

[Npstewart]

(in your case 220Amps), which is why series rating are usually best for what we used to call 'lighting and appliance' panels as opposed to 'power' panels.

Just curious, where does the 220 amps come from? Is it a fixed percentage of the main breaker? (ie. 36.6%)?

 

[jim dungar]

Just curious, where does the 220 amps come from? Is it a fixed percentage of the main breaker? (ie. 36.6%)?

It comes as a factor (1%) of the lowest AIC breaker in the panel. This is the 'approved' method for dealing with the motors acting like sources of current during faults, see 240.86(C)2.

 

[Npstewart]

Understood. 22000*.01=220. Interesting. Thanks Again.

 

[Jraef]

Thanks everyone.

Im sure that 42 kAIC branch circuit breakers are readily available correct?

You can't make that assumption, no. Every brand is different, and the levels they offer are all different. The ONLY thing that matters is what they have DOCUMENTED, and a Series Rating is DEFINITELY something that is ONLY available with documentation. No guessing or assuming.

 

[topgone]

You can't make that assumption, no. Every brand is different, and the levels they offer are all different. The ONLY thing that matters is what they have DOCUMENTED, and a Series Rating is DEFINITELY something that is ONLY available with documentation. No guessing or assuming.

Well said! Thanks for posting the needed information.