fuse dropping AIC in existing system

Status
Not open for further replies.
T

Toros

Senior Member
Location
Tujunga, CA
Hi,
I am installing 400A multi-meter residential service with breaker inside rated at 42000A per utility company report .

My question is;
to re-feed the existing house panel (its breakers are @ 10,000 AIC !) mounted 20 feet away from service
do I have to install a disc. switch containing T-fuses @ 100,000 AIC to avoid changing that existing panel
to 42000A for matching the avail. short current??????

Thank you
 
IF I understand 110.22 correctly you would need engineered documentation or manufacturers documentation to support the series rating combination.
Just glancing at SquareD data, it appears with their gear a Class T fuse would not allow direct feed to a 10K breaker
 
Your title is misleading. AIC ratings come from the manufacturer, they are what they are. There is nothing you can do to change an AIC, however there are things you can do to change the Short Circuit Amps (SCA) available, such as adding the fuse in your OP.


You are really asking if it is possible to have a series rating between new upstream fuses and existing downstream circuit breakers.
The answer will be manufacturer dependent.
Also, series ratings have only been around since the mid-80's so age may also be a concern.
 
jim dungar said:
........................................................
Also, series ratings have only been around since the mid-80's so age may also be a concern.
Click to expand...

enlighten me, please.
are we saying a breaker manufacturered prior to the onset of series rating might not be acceptable ?
 
Toros said:
Hi,
I am installing 400A multi-meter residential service with breaker inside rated at 42000A per utility company report .

My question is;
to re-feed the existing house panel (its breakers are @ 10,000 AIC !) mounted 20 feet away from service
do I have to install a disc. switch containing T-fuses @ 100,000 AIC to avoid changing that existing panel
to 42000A for matching the avail. short current??????

Thank you
Click to expand...
Also take into consideration the 20 foot feeder you mentioned - it will have less available fault current on the load end then it has on the supply end. Just a few feet can make a fairly significant difference here. You need to base things on actual available fault current and not just the fact you have a 42kA main. It is 42kA because that is a standard size that is larger then what is available, you may only have 26kA available which puts the load end of your 20 foot feeder much lower then it would if you start calculations at 42kA.
 
augie47 said:
enlighten me, please.
are we saying a breaker manufacturered prior to the onset of series rating might not be acceptable ?
Click to expand...
That is correct.

For example when Square D first introduced the QO breaker, in the mid 50's, it was only rated 5kAIC. I definitely would not trust series ratings on QO breakers without the Visi-trip window.
In the mid-80's UL 'updated' their procedures for SCCR and AIC, it is possible that manufacturers tweaked their equipment designs at the same time.
 
jim dungar said:
That is correct.

For example when Square D first introduced the QO breaker, in the mid 50's, it was only rated 5kAIC. I definitely would not trust series ratings on QO breakers without the Visi-trip window.
In the mid-80's UL 'updated' their procedures for SCCR and AIC, it is possible that manufacturers tweaked their equipment designs at the same time.
Click to expand...

another day...another "learning experience". thanks\
I never thought about that:)
 
Toros
I would like to know more about your electrical service. It is uncommon for a residential service to have a fault current as
high as you indicated. Could you post the utility transformer size and the length of the service. Thanks.
You have another post for a commercial service with the same info as this post. Whats up?
 
Last edited:
bob said:
Toros
I would like to know more about your electrical service. It is uncommon for a residential service to have a fault current as
high as you indicated. Could you post the utility transformer size and the length of the service. Thanks.
You have another post for a commercial service with the same info as this post. Whats up?
Click to expand...

400 amp with multiple meters - maybe a little unusual if it is dwellings only, or is a only single 400 amp service. If there are multiple services fed from a large transformer - and he happened to have one of the shortest service laterals - who knows how high fault level could be.
 
bob said:
Toros
I would like to know more about your electrical service. It is uncommon for a residential service to have a fault current as
high as you indicated. Could you post the utility transformer size and the length of the service. Thanks.
You have another post for a commercial service with the same info as this post. Whats up?
Click to expand...
In LA
if you do not have utility SCE provided documentation on asc
than assume 42000A available at main
plan checker request!!!????
 
That is a poor rule of thumb. 22 ka might be acceptable. Actually it might be as low as 10 ka.
 
On the other hand, this is not really a situation for "rule of thumb" in the first place. If you (with data from POCO) cannot justify a lower value, the AHJ may require you to assume the worst.
A "rule of thumb" here is at best a sanity check for an actual calculation or estimate.
 
Status
Not open for further replies.
Top