Short circuit current questions

Status
Not open for further replies.
O

olc

Senior Member
two quick (I hope) questions:

Assume a fairly high SCCA (100KA) at the meter of a 600A, 120/208V, 3Ph (100KA) .


Can a main fused disconnect with current limiting fuses significantly reduce the required (SCCR) rating of load side circuit breakers?


An existing 600 fused disconnect - sight unseen, would it be safe an prudent to assume the existing fuses are non-current limiting?
 
olc said:
two quick (I hope) questions:

Assume a fairly high SCCA (100KA) at the meter of a 600A, 120/208V, 3Ph (100KA) .


Can a main fused disconnect with current limiting fuses significantly reduce the required (SCCR) rating of load side circuit breakers?


An existing 600 fused disconnect - sight unseen, would it be safe an prudent to assume the existing fuses are non-current limiting?
Click to expand...

It doesnt really matter if they are current limiting or not. What matters is if there is a tested series rated combination with the downstream circuit breakers. Look up the series ratings for the manufacturer and type of circuit breakers you have and see if there is a tested combination that gets you where you need to be.
 
Agree with electrofelon but note that SquareD does show a 22k breaker series rating with a 600 amp CL fuse but emphasizing as he says, the devil is in the detail so you need to check your specific application.
 
In the example, the fused disconnect is a main service disconnect that would feed a 600A MPD (Square D I Line for example) with five feeder circuit breakers.
400, 100, 200, 200, 250
So all five would have to have a series rating with the same 600A fuse? Possible?
 
Last edited:
olc said:
In the example, the fused disconnect is a main service disconnect that would feed a 600A MPD (Square D I Line for example) with five feeder circuit breakers.
400, 100, 200, 200, 250
So all five would have to have a series rating with the same 600A fuse? Possible?
Click to expand...

Yes probable and likely. Also dont forgot to look at your branch breakers to make sure they are sufficiently rated.
 
electrofelon said:
Yes probable and likely. Also dont forgot to look at your branch breakers to make sure they are sufficiently rated.
Click to expand...

My understanding those branch breakers might be series rated with the breaker in the distribution panel, but are not "double series rated" and could not be used in series with another breaker that is already being used in a series rating. Not that they couldn't test this combination but likely they haven't.

That said, unless your branch circuit panel is in immediate vicinity as the distribution panel, there is a good chance that the resistance of the feeder reduces the available fault current at the branch circuit panel to a level that is acceptable for your branch breakers.
 
kwired said:
My understanding those branch breakers might be series rated with the breaker in the distribution panel, but are not "double series rated" and could not be used in series with another breaker that is already being used in a series rating. Not that they couldn't test this combination but likely they haven't.

That said, unless your branch circuit panel is in immediate vicinity as the distribution panel, there is a good chance that the resistance of the feeder reduces the available fault current at the branch circuit panel to a level that is acceptable for your branch breakers.
Click to expand...

Yeah that's why it is good to look at the entire system as a whole. One might focus on the main and the distribution ocpd's and get everything hunky dory but not consider the branch breakers. For example one job I recently had, it worked out well to use a MLO with 6 mains @65k and those series rated with regular 10k branches. It might have been tempting to use a main and use series rated distribution breakers, but then I would have been screwed on the branches as you typically won't find any series ratings from an over 250 amp frame @65k down to regular 10k branches. You really want to avoid having to use 22k "H' branch breakers for cost reasons, unless as you noted there is enough conductor to get down to 10k without needing a series rating.
 
electrofelon said:
Yeah that's why it is good to look at the entire system as a whole. One might focus on the main and the distribution ocpd's and get everything hunky dory but not consider the branch breakers. For example one job I recently had, it worked out well to use a MLO with 6 mains @65k and those series rated with regular 10k branches. It might have been tempting to use a main and use series rated distribution breakers, but then I would have been screwed on the branches as you typically won't find any series ratings from an over 250 amp frame @65k down to regular 10k branches. You really want to avoid having to use 22k "H' branch breakers for cost reasons, unless as you noted there is enough conductor to get down to 10k without needing a series rating.
Click to expand...

And it doesn't take a lot of conductor length to make a big difference, especially for 200 amp or less conductor sizes. Placing that branch circuit panel for the mechanical room right next to the distribution panel with just a offset nipple between is likely not going to be enough conductor length to get below that 10k level unless you want to somehow put enough extra conductor in the cabinet(s) involved to reduce the available current through that length. Running the feeder to the ceiling across the room and back down to a panel there, might be enough though, depends on what starting level and what you have for conductors and even raceway type and motor contributions.
 
Thanks for the replies.
It would be somewhat important to know if the SCCR at the branch panels depends on what we are starting with at the main feeder breaker.

On the other hand the SCCA at the main breaker may have to be significantly lower to make a difference at the branch. I did some rough quick comparison calcs and, for example, starting with 100K at the service, the SCCA at a branch was greater than 10KA. And using a lower SCCA at the main circuit breaker still resulted in >10KA. I think for my example, the only difference would be the SCCR of the main distribution panel (65KA vs 100KA or what ever).
 
I am still not really clear on what exactly you are asking or trying to do. Is this a hypothetical question or a practical question?

Note the amperage/frame size of a circuit breaker is actually a more significant factor in getting a series rating than the difference between the Available SSC and the series rated rating. For example you can commonly get a series rating for a 10K miniature CB to a 200 amp or less (sometimes 225 or 250) breaker that sees an available SCC of 65 or even 100k. But you wont get it off a 400 amp or larger frame breaker. That is why using a MLO with multiple 250/225/200 or less mains can be very advantageous.
 
olc said:
Thanks for the replies.
It would be somewhat important to know if the SCCR at the branch panels depends on what we are starting with at the main feeder breaker.

On the other hand the SCCA at the main breaker may have to be significantly lower to make a difference at the branch. I did some rough quick comparison calcs and, for example, starting with 100K at the service, the SCCA at a branch was greater than 10KA. And using a lower SCCA at the main circuit breaker still resulted in >10KA. I think for my example, the only difference would be the SCCR of the main distribution panel (65KA vs 100KA or what ever).
Click to expand...

SCCR will be less at your branch panels, how much will depend on size and length of conductor to the feeder breaker. With 200 amp and less conductor just going from 10 feet of conductor to 25 feet usually makes a huge difference. Don't forget if you have 100k at service, then go to a distribution panel before going to a branch panel, you drop some just between the service and distribution panel.

100k at service is pretty high - you must have transformer very close to service I'd guess to get that high, especially if it is only 600 amp supply. I just plugged in (well oversized for 600 amps @ 208/120) 500 kVA transformer into fault current calculator and pretty unlikely low impedance (just to make the result high) of 2% and still only come up with about 69 kA of fault current at transformer terminals. Then 10 feet of parallel 350's to service disconnect drops it to 61.5kA, 25 feet drops it to 52.6 kA, and 50 feet drops it to 42.4 kA.

If I plug in values that seem more likely for a 600 amp service - 225 kVA transformer and maybe 4.0% impedance (might be higher than that, so this will yield a high fault current) I only get 15.6kA available at transformer terminals.

Other possibility is someone gave you a worst case scenario of 100kA, but you likely aren't even close to that.
 
The local utility has a downtown service network for which they say the SCCA at the service is 100KA period.
I have asked in the past whether that was at the connection in the manhole (typically out in front of the building somewhere) or at the connection in the building (the service entrance conductors are utility owned and tend to be small), and was told that it is at the connection in the building.


I don't have a problem with the calculations. Just questioning whether a down stream current calc can be reduced by using a current limiting fuse.
 
olc said:
Just questioning whether a down stream current calc can be reduced by using a current limiting fuse.
Click to expand...
The fuse does not limit the fault current downstream for the purposes of selecting breakers on it own. You can only use it in conjunction with a tested combination series listed device
 
olc said:
The local utility has a downtown service network for which they say the SCCA at the service is 100KA period.
I have asked in the past whether that was at the connection in the manhole (typically out in front of the building somewhere) or at the connection in the building (the service entrance conductors are utility owned and tend to be small), and was told that it is at the connection in the building.


I don't have a problem with the calculations. Just questioning whether a down stream current calc can be reduced by using a current limiting fuse.
Click to expand...

Yeah that's too bad you are above 65k, and by the sound of it, no way to get a "better deal" by looking at the actual equipment because it's a network.

Basically it's got to be a tested series rating. IIRC there is a provision to use "engineering supervision" but not sure that would turn out better than a tested series rating.

Probably your best bet, as I said before, is to use the 2-6 rule with 100k breakers that series rated with your branch breakers. I think you can get all the way from 100k down to 10k.
 
olc said:
The local utility has a downtown service network for which they say the SCCA at the service is 100KA period.
I have asked in the past whether that was at the connection in the manhole (typically out in front of the building somewhere) or at the connection in the building (the service entrance conductors are utility owned and tend to be small), and was told that it is at the connection in the building.


I don't have a problem with the calculations. Just questioning whether a down stream current calc can be reduced by using a current limiting fuse.
Click to expand...

That is a CYA figure and absolute worst case scenario and they basically refuse to tell you anything less for their own liability reasons.

Resistance in your supply conductors is going to lower it some, even if it is only 25 feet to the source. And like I mentioned earlier it will take a pretty good sized source to get 100 kA just at the source terminals.

Don't know how to tell you to proceed, very likely actual fault current is much lower than what they will tell you.

Yes it could be a rather large source and feeds multiple clients, but the feeds from it still have resistance and not everyone is going to be so close that the resistance of their supply conductors is negligible.
 
Status
Not open for further replies.
Top