How to calculate the required Short Circuit Current Rating of main disconnect?
I know I should figure this out myself, and ask an engineer, but maybe there is some kind soul here that will help an elderly gentleman...
So that I am better informed when I talk to the engineer!
The long version is this:
* My shop used to be a computer mainframe building in the 50's. It used a lot of power. Punch cards still in the dropped ceiling. Now there is just me and the plumber. We have one heat pump and couple lights/computers that's it.
* The service is 12kv to a disconnect then 4160 that drops to xfmrs above my shop and then continues to another "temporary building" from the 50's.
* The 4160v ==> 120/208 wye to my 400A 3p main fuse.
We had to open the 12kv switch a few times during the last year.
The last time it was because they are building here and we had to de-energize so that drill rigs could get required clearance.
==> [NOTE: We have some electrical engineers here because of the building. And I will talk with them. I just want to review the process before I consult]
So for the third time this year we got a generator to supply both buildings and I taped up the 500mcms and bolted the generator leads to the 1950's lugs.
High pressure, little time, you know the drill.
So, this time, one of the studs securing the lugs snapped off on me when I torqued it down.
It's an old FP 400A fused disconnect.
500mcm line & load after the skinny service drop
So I got no heat pump I'm just single phased right now off a #10.http://forums.mikeholt.com/images/smilies/rolleyes.gif
:roll:
Hey, it's nice weather!
Now, sure, the safest and most expensive way would be to replace the switch with new SquareD 400A disconnect.
Next I could gut the old FP can or at least figure a way to press a new stud on the existing board.
But when the generator was loaded there was only 50AMP to the two buildings when all the AC was running during the hottest part of the year.
If this building clamps 30A with the ac pressing I would be surprised.
So the load is not really the issue.
Now the 12kv disconnect protects the 4160 xfmrs but there is no disconnect after that.
So each building and each disconnect has to deal with the output from the 4160 xfmrs.
They are 30 feet in the air and I do not have a boom.
I can't get to the fuse blocks at the 4160v xfmrs but you know they are usually so small... 400A would be 20A.
Little wires in/big wires out.
I have to get the ampacity of the fuses at the 12kv switch and the Kva rating of the xfmrs. And I will...
But for now... I'd rather look what I have in inventory and make a plan of attack to see how much this will cost in materials.
Cause the old FP switch is just... FP!
It looks like the 400A fuses protect the building from the xfmrs above.
Reducing them to 100A would be even better. If they could withstand a meltdown of the xfmrs above.
The can I have in inventory is a SquareD D323N
on the label it say there is a RFK10 kit that will protect up to 100,000 RMS symmetrical amperes up to 240V.
If this were to be acceptable, then the only remaining problem that I can think of before I go to the engineer would be how to get the 500mcm onto the lugs.
Tap rule doesn't help. I don't see enough room to mount a bigger lug anyway.
So then the question is why can't I put the 500mcm and bolt it to some #1/0 that will fit the lugs?
Will the wire burn up before the fuse opens? No.
Any brief thoughts?
rhttp://forums.mikeholt.com/images/smilies/mad.gif
emarks?
Thanks,
--Paul
I know I should figure this out myself, and ask an engineer, but maybe there is some kind soul here that will help an elderly gentleman...
So that I am better informed when I talk to the engineer!
The long version is this:
* My shop used to be a computer mainframe building in the 50's. It used a lot of power. Punch cards still in the dropped ceiling. Now there is just me and the plumber. We have one heat pump and couple lights/computers that's it.
* The service is 12kv to a disconnect then 4160 that drops to xfmrs above my shop and then continues to another "temporary building" from the 50's.
* The 4160v ==> 120/208 wye to my 400A 3p main fuse.
We had to open the 12kv switch a few times during the last year.
The last time it was because they are building here and we had to de-energize so that drill rigs could get required clearance.
==> [NOTE: We have some electrical engineers here because of the building. And I will talk with them. I just want to review the process before I consult]
So for the third time this year we got a generator to supply both buildings and I taped up the 500mcms and bolted the generator leads to the 1950's lugs.
High pressure, little time, you know the drill.
So, this time, one of the studs securing the lugs snapped off on me when I torqued it down.
It's an old FP 400A fused disconnect.
500mcm line & load after the skinny service drop
So I got no heat pump I'm just single phased right now off a #10.http://forums.mikeholt.com/images/smilies/rolleyes.gif
:roll:
Hey, it's nice weather!
Now, sure, the safest and most expensive way would be to replace the switch with new SquareD 400A disconnect.
Next I could gut the old FP can or at least figure a way to press a new stud on the existing board.
But when the generator was loaded there was only 50AMP to the two buildings when all the AC was running during the hottest part of the year.
If this building clamps 30A with the ac pressing I would be surprised.
So the load is not really the issue.
Now the 12kv disconnect protects the 4160 xfmrs but there is no disconnect after that.
So each building and each disconnect has to deal with the output from the 4160 xfmrs.
They are 30 feet in the air and I do not have a boom.
I can't get to the fuse blocks at the 4160v xfmrs but you know they are usually so small... 400A would be 20A.
Little wires in/big wires out.
I have to get the ampacity of the fuses at the 12kv switch and the Kva rating of the xfmrs. And I will...
But for now... I'd rather look what I have in inventory and make a plan of attack to see how much this will cost in materials.
Cause the old FP switch is just... FP!
It looks like the 400A fuses protect the building from the xfmrs above.
Reducing them to 100A would be even better. If they could withstand a meltdown of the xfmrs above.
The can I have in inventory is a SquareD D323N
on the label it say there is a RFK10 kit that will protect up to 100,000 RMS symmetrical amperes up to 240V.
If this were to be acceptable, then the only remaining problem that I can think of before I go to the engineer would be how to get the 500mcm onto the lugs.
Tap rule doesn't help. I don't see enough room to mount a bigger lug anyway.
So then the question is why can't I put the 500mcm and bolt it to some #1/0 that will fit the lugs?
Will the wire burn up before the fuse opens? No.
Any brief thoughts?
rhttp://forums.mikeholt.com/images/smilies/mad.gif
emarks?Thanks,
--Paul
