bonding jumper
Senior Member
Re: Raceway as EGC
Ok Scott, I'm gonna take a stab at it. These equations make the following assumptions, Infinate short circuit available feeding xfmr and 5hp motors are single phase 240V.
After XFMR
37.5KVA / .02% imp = 1875KVA
At PanelBoard Entrance
[200A * 120V(V per leg, each conductor)] / .2% (Voltage drop based on 126A load, 6-5hp motors @ 90% = 28A) = 120000KVA
Rating of 40A 2P breaker is 10KAIC, so:
10KAIC * 240V = 2400KVA
At Fault
#6 = 65A * 120V(Per conductor)/.1(% Voltage drop based on 28A) = 78000KVA
These parts must be combined to get our Fault current rating at fault. They must be used in a recripocal equation as follows,
1 / [(1/1875KVA) + (1/120000KVA) + (1/78000KVA)] = 1803KVA
Since we have available 2400 KVA at the 40A breaker, we have adequate short circuit protection for our fault.
If we swap #6's with #10's we will be dividing by a larger percentage for the branch circuit calculation, thus giving us a smaller KVA rating, thus a smaller short circuit current.
If we increase the size of our xfmr, to 75 kva, or 112.5kva, we will increase the short circuit fault rating. Without doing calculations, I would guess our rating would be too high for our 40A breaker which is rated at 2400KVA.
If we move the xfmr, or use smaller feeders, we again decrease the short circuit rating from the calculated figure because we are increasing our voltage drop percentage.
If the ground fault occurs 100 feet away from the panelboard, we are decreasing our short circuit again cuz of a higher volage drop.
If all the motors were starting we would be decreasing again cuz of a higher amperage being pulled through the conductors, thus a higher voltage drop.
If all the motors shut off, we would have a higher short circuit current because we would have a lower voltage drop due to 0 current being drawn on conductors.
Both ungrounded conductors fault to ground, these calculations should be fore worst case fault, which is line to line fault, haveing both conductors short to ground has the same effect of a line to line fault.
P.S., this is my first time doing short circuit calculations, questions i had, were ratings for KVA I used for conductors. I used 200A for the 3/0 and 65A for the #6. These are from table 310, and I know they are Allowable Ampacities of insulated conductors, and they do not represent the maximum current that these conductors can tollerate or deliver, so I am not sure if my ratings of 200 or 65 are applicable. Lemme know how I screwed up. I'm sure everybody will find a different problem. Thanks.
My methods came from the following web page web page
[ March 29, 2004, 11:59 AM: Message edited by: bonding jumper ]
Ok Scott, I'm gonna take a stab at it. These equations make the following assumptions, Infinate short circuit available feeding xfmr and 5hp motors are single phase 240V.
After XFMR
37.5KVA / .02% imp = 1875KVA
At PanelBoard Entrance
[200A * 120V(V per leg, each conductor)] / .2% (Voltage drop based on 126A load, 6-5hp motors @ 90% = 28A) = 120000KVA
Rating of 40A 2P breaker is 10KAIC, so:
10KAIC * 240V = 2400KVA
At Fault
#6 = 65A * 120V(Per conductor)/.1(% Voltage drop based on 28A) = 78000KVA
These parts must be combined to get our Fault current rating at fault. They must be used in a recripocal equation as follows,
1 / [(1/1875KVA) + (1/120000KVA) + (1/78000KVA)] = 1803KVA
Since we have available 2400 KVA at the 40A breaker, we have adequate short circuit protection for our fault.
If we swap #6's with #10's we will be dividing by a larger percentage for the branch circuit calculation, thus giving us a smaller KVA rating, thus a smaller short circuit current.
If we increase the size of our xfmr, to 75 kva, or 112.5kva, we will increase the short circuit fault rating. Without doing calculations, I would guess our rating would be too high for our 40A breaker which is rated at 2400KVA.
If we move the xfmr, or use smaller feeders, we again decrease the short circuit rating from the calculated figure because we are increasing our voltage drop percentage.
If the ground fault occurs 100 feet away from the panelboard, we are decreasing our short circuit again cuz of a higher volage drop.
If all the motors were starting we would be decreasing again cuz of a higher amperage being pulled through the conductors, thus a higher voltage drop.
If all the motors shut off, we would have a higher short circuit current because we would have a lower voltage drop due to 0 current being drawn on conductors.
Both ungrounded conductors fault to ground, these calculations should be fore worst case fault, which is line to line fault, haveing both conductors short to ground has the same effect of a line to line fault.
P.S., this is my first time doing short circuit calculations, questions i had, were ratings for KVA I used for conductors. I used 200A for the 3/0 and 65A for the #6. These are from table 310, and I know they are Allowable Ampacities of insulated conductors, and they do not represent the maximum current that these conductors can tollerate or deliver, so I am not sure if my ratings of 200 or 65 are applicable. Lemme know how I screwed up. I'm sure everybody will find a different problem. Thanks.
My methods came from the following web page web page
[ March 29, 2004, 11:59 AM: Message edited by: bonding jumper ]