High impedance wires

Tainted said:
Can't use all QS because I have loads that are less than 100A :(

Might be a stupid idea but do you think I can have 800A Bussman JJN and JJS fuses in series and then it will series rate with both HQP and HQS breakers?
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Series rating are not "stackable". You cannot use the results from one series rating as the input to a different one.
 
Large fuses (>600A) take a very high amount of fault current in order to reach their current limiting range, this is why they are not very good for series ratings.
Large breakers typically begin to operate at relatively lowers levels making them easier to use with series ratings.

As far as coordination, the most likely need for coordination will be at downstream equipment where the fault current is lower, rather than at the load side terminal of the breakers in your meter/service equipment.

Honestly, if I had more than 40kA of fault current flowing in a non critical installation, I would want something to stop it fast in order to limit the damage to equipment and thus reduce the 'time to repair'.
 
jim dungar said:
Series rating are not "stackable". You cannot use the results from one series rating as the input to a different one.
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Tainted said:
True, I didn't realize my idea stacked them until you pointed it out lol
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Right you cant use the result from one series rating as the input for another one, but I dont believe that is what Tainted was proposing. He was proposing using two fully rated OCPDs, each of which provides a series rating with a downstream device. I believe that would work. Eaton says you can insert any fully rated device between a series combination. So this is what we would have:

A->B->C->D or E where A,B,C are fully rated. B series rates with D, and C rates with E

Am I seeing this correctly?
 
Tainted said:
Can't use all QS because I have loads that are less than 100A :(

Might be a stupid idea but do you think I can have 800A Bussman JJN and JJS fuses in series and then it will series rate with both HQP and HQS breakers?
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Im sure you thought of this, but what size and how many of those feeds are under 100A? can you just bump them, or a portion of those feeders up to 100A?
 
electrofelon said:
Right you cant use the result from one series rating as the input for another one, but I dont believe that is what Tainted was proposing. He was proposing using two fully rated OCPDs, each of which provides a series rating with a downstream device. I believe that would work. Eaton says you can insert any fully rated device between a series combination. So this is what we would have:

A->B->C->D or E where A,B,C are fully rated. B series rates with D, and C rates with E

Am I seeing this correctly?
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Yes exactly what I was proposing, I thought I was looking at it wrong when Jim pointed out about stacking. I know you cannot stack but this is different scenario. Honestly I did come up with a good idea but it seems so sketchy to do it I'd rather not risk it until it's fully tested. And if I the engineer spec it, there will be contractors who would think I am crazy for doing it this way lol. It's just so out of the norm that I've never seen someone do it like this before.

electrofelon said:
Im sure you thought of this, but what size and how many of those feeds are under 100A? can you just bump them, or a portion of those feeders up to 100A?
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Each meter bank has like around 13-15 that are under 100A. We are putting in the meter banks and refeeding apartments. New meter bank will be used for apartment upgrades as needed. If I upgrade the breaker I would have to upgrade the feeder downstream of the breaker which we don't want
 
What about looking at a different manufacturer, maybe they have a product offering that meets your needs? I'm sure it is always going to vary based on the situation, but I have found siemens to not have very good series ratings in several areas (one is very few triple ratings, and the other is no tested combinations for greater than 250 frames to 10K branches).
 
electrofelon said:
What about looking at a different manufacturer, maybe they have a product offering that meets your needs? I'm sure it is always going to vary based on the situation, but I have found siemens to not have very good series ratings in several areas (one is very few triple ratings, and the other is no tested combinations for greater than 250 frames to 10K branches).
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I already invested so much on the design and I am married to Siemens, I'm considering just running the full 40 feet. My short circuit calculation was based on worst case scenario, I might do an exact calc, maybe it'll reduce it further
 
Knightryder12 said:
Wow, you must have some really crappy utility transformers where you are. I have only designed one project that had over 100K AFC at the secondary bushing of a utility transformer.
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If anything, I would say a high AFC would mean BETTER transformers due to lower impedance, probably less losses, and a stiffer supply and better voltage regulation. Also I believe the OP is in NYC so this could be a network distribution system.

This is the highest AFC I ever came across, 500 KVA @ 1.8 Z = still only 77k with infinite primary. POCO did say it was 115k
 

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electrofelon said:
If anything, I would say a high AFC would mean BETTER transformers due to lower impedance, probably less losses, and a stiffer supply and better voltage regulation. Also I believe the OP is in NYC so this could be a network distribution system.

This is the highest AFC I ever came across, 500 KVA @ 1.8 Z = still only 77k with infinite primary. POCO did say it was 115k
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I was under the understanding that it would be a less quality built transformer that would have that much fault on or at the secondary bushings. But you could be right I am not sure.
Had a POCO rep tell me it was due to crappy built transformer they were getting.
 
I've continued to gnaw at the idea of adding inductance to drop the fault current.

I think coils of wire designed as air core inductors could plausibly do the trick. My back of the envelope suggests that a 12" diameter 3 turn coil installed in each phase would do the trick. That gives about 2.4 uH, or an impedance of 0.001 ohm. To go down this route you'd probably need an engineer to sign off on the fault current calculation, forming the coils would be no trivial feat (I'd probably get them manufactured in copper bus bar rather than trying to form 750kcmil wire myself), and you'd need to be careful about heat in the enclosure.

But you could plausibly fit these in a space little larger than the wireway or conduit that you already have to use.

-Jon
 
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