Interrupting Current and max available fault current calculations

[francis.key19]

Hello,

I am trying my hand at estimating max available fault current at a GE 480VAC panel (with several TEY CBs (60A, 30A, 100A)). I will try to describe this situation.

The incoming service transformer is a 12.47KV prim, 480VAC primary, 2500KVA, 5.52%Z. This service transformer secondary is feeding a GE spectra series switchboard with a 4000A main. The feed from the transformer to the main is approximately 55', 12 conductor per phase, 600mcm per conductor. From the switchboard, we use a 400A CB to feed a 480VAC panel. This panel has several GE TEY breakers all of which is rated for 14KA interrupting current. The cable length from the 400A CB to the 480VAC panel is approximately 277'. We ran 2 conductors per phase of 3/0 cu THHN. Given the maximum primary fault current (3.41primary), I computed the max available short circuit current at the 480VAC panel to be around 14.2KA not included the motor contribution downstream of this panel.

My question is if this number is correct (i.e 14.2KA) and the CBs in this panelboard is only rated for 14KA, is this an issue? I tried to get a better measurement for cable length but the number just does not work out. By the way, the cable is rated for 40C ambient and 90C for conductor. Is there an issue with my calculations? There are 6 other panels that are about the same distance and conductors and their fault currents are all a little larger than 14.2KA.

Thank you in advance.

Francis.

 

[kwired]

Here is what I come up with using a spreadsheet that Mike Holt has on his free stuff page, with some values being assumed, like same size neutral as ungrounded conductors, metal raceway for the feeder, copper in nonmetallic raceway for service conductors.

 

[francis.key19]

Here is what I come up with using a spreadsheet that Mike Holt has on his free stuff page, with some values being assumed, like same size neutral as ungrounded conductors, metal raceway for the feeder, copper in nonmetallic raceway for service conductors.

Thank you for pointing out the spreadsheet and the sample calcs. There is no neutral from the service transformer to inside the building. This is basically an ungrounded system. Regardless, the max fault current looks to be larger than the IC of the breakers. Does this mean I have to change all of the breakers to larger IC rating? or is there another option that is less disruptive? IC rating of the existing breakers is 14KA at 480VAC.

 

[mayanees]

I used SKM and calculated a little over 14038 amps at the end of your 277 foot cable. I assumed metal conduit.
I think you need to scrutinize the parameters of the installation to make sure it is accurately reflected in the calculation.
The numbers are so close that it may only take an extra 10 feet of cable to pass the 14kA-rated breakers, or you may pick up some impedance in the primary or secondary cables that reduces the 480V panel short circuit current to less than 14 kA.

Another option would be to use series-rated breakers, provided you meet the criteria for applying them (240.86). But if you're an industrial facility with motor loads, that may not work.

If you need a PE to certify the calcs and pass judgment on the ratings PM me. I'm actually doing a Power Study job in Frederick, MD this week.

John M

 

[jim dungar]

I used SKM and calculated a little over 14038 amps at the end of your 277 foot cable. I assumed metal conduit.
I think you need to scrutinize the parameters of the installation to make sure it is accurately reflected in the calculation.
The numbers are so close that it may only take an extra 10 feet of cable to pass the 14kA-rated breakers, or you may pick up some impedance in the primary or secondary cables that reduces the 480V panel short circuit current to less than 14 kA.

Another option would be to use series-rated breakers, provided you meet the criteria for applying them (240.86). But if you're an industrial facility with motor loads, that may not work.

If you need a PE to certify the calcs and pass judgment on the ratings PM me. I'm actually doing a Power Study job in Frederick, MD this week.

One thing that is missing from many of the quick Short Circuit calculators is the X/R ratio of the circuit versus the X/R ratio used to test the circuit breakers to get their UL Listing. Almost all power study software, like SKM, calculates this ratio, however not everyone applies it the same way. The end result is a multiplier used either to 'increase' the calculated SC or 'decrease' the device's rated AIC.

I find that there will often be misapplication issues if there is not at least a 20% margin between a 'quick SC' and a published AIC.

 

[Julius Right]

It is a problem, indeed.
First of all the transformer short-circuit voltage of 5.75% could be less-a 15% tolerance may be expected. The 12*3 600 MCM cables reactance depends on the distance between the cables and the resistance could be increased by skin and proximity effects [same as 2*3/0 cables].
NEC art.110-9 states:
110.9 Interrupting Rating. ?Equipment intended to interrupt current at fault levels shall have an interrupting rating not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment.?
However IEEE-141/1993 ch.4.3.2 Type of short circuit:
?Analytical studies indicate that sustained arcing short-circuit currents, in per unit of bolted fault values, may be typically as low as:
a) 0.89 at 480 V and 0.12 at 208 V for three-phase arcing.?
In this case the short-circuit current will be approx.13 kA.
The ratio X/R of the circuit may reduce the current at the interruption moment, also.

 

[francis.key19]

It is a problem, indeed.
First of all the transformer short-circuit voltage of 5.75% could be less-a 15% tolerance may be expected. The 12*3 600 MCM cables reactance depends on the distance between the cables and the resistance could be increased by skin and proximity effects [same as 2*3/0 cables].
NEC art.110-9 states:
110.9 Interrupting Rating. ?Equipment intended to interrupt current at fault levels shall have an interrupting rating not less than the nominal circuit voltage and the current that is available at the line terminals of the equipment.?
However IEEE-141/1993 ch.4.3.2 Type of short circuit:
?Analytical studies indicate that sustained arcing short-circuit currents, in per unit of bolted fault values, may be typically as low as:
a) 0.89 at 480 V and 0.12 at 208 V for three-phase arcing.?
In this case the short-circuit current will be approx.13 kA.
The ratio X/R of the circuit may reduce the current at the interruption moment, also.

Look like the 2*3/0 cables are installed in an 8" covered wireway with many other conductors of various sizes. This installation method would change the X/R of the circuit. Is there a way to calculate what the X/R ratio is for this method of wiring. Inside this covered tray, there are 12 other conductors.
Thank you.

 

[jim dungar]

However IEEE-141/1993 ch.4.3.2 Type of short circuit:
?Analytical studies indicate that sustained arcing short-circuit currents, in per unit of bolted fault values, may be typically as low as:
a) 0.89 at 480 V and 0.12 at 208 V for three-phase arcing.?

I know of no major company or AHJ that routinely performs or accepts a Device Evaluation, for compliance with NEC 110.9, that was not based on Bolted Fault Current values, as opposed to arcing ones.
Bolted faults are usually the result of a installation error, and it is very easy to determine the maximum amount of potential current.
While arcing faults are more common in systems that have been running, their variability makes it hard to predict the potential current.

 

[kingpb]

14.1kA, 14.2kA, even if it was 13.5kA, is all too close to the rated. There are limits of accuracy in calculations and therefore equipment modifications or upgrades need to take place. There will be some motor contribution, albeit small, that will increase the number further.

There is not enough "fine tuning" that can be done to make your install work as it is.

 

[mayanees]

14.1kA, 14.2kA, even if it was 13.5kA, is all too close to the rated. There are limits of accuracy in calculations and therefore equipment modifications or upgrades need to take place. There will be some motor contribution, albeit small, that will increase the number further.

There is not enough "fine tuning" that can be done to make your install work as it is.

.. and that's one engineer's opinion. I have a completely different philosophy.
I perform an objective analysis that takes into account any parameters that could enable this existing equipment to meet the fault current requirements of the system. If my detailed analysis determines 13,999 worst-case amps available, and the X/R rating for which the equipment was tested is equal to or greater than the actual X/R of the system, then I'm approving the equipment for continued service.
I was trained in the DuPont system where engineering involves determination of the minimum essential ratings to meet system requirements, and if I could determine that this gear was exposed to less than 14 kA of fault current, then I'm confident that the system is safe, and I've saved my client money. Minimum essential design.

And in the real world, there's very little chance that the equipment will experience a three-phase bolted fault of 100% contribution, but if it does, and I've performed due diligence in my analysis, it's going to stay together.

 

[jim dungar]

If my detailed analysis determines 13,999 worst-case amps available, and the X/R rating for which the equipment was tested is equal to or greater than the actual X/R of the system, then I'm approving the equipment for continued service.

Personally I have no problem with this. The key is to consider all of the factors, not just a quick SC calculator

 

[kwired]

Originally Posted by mayanees
If my detailed analysis determines 13,999 worst-case amps available, and the X/R rating for which the equipment was tested is equal to or greater than the actual X/R of the system, then I'm approving the equipment for continued service.

Personally I have no problem with this. The key is to consider all of the factors, not just a quick SC calculator

I would agree that his "detailed analysis" is likely fairly accurate, compared to using a quick calculator, if the quick calculator is anywhere near the limits, further investigation is probably a good idea, if the quick calculator is well under the limits - why spend any more time on analysis?

 

[bob]

Hello,

We ran 2 conductors per phase of 3/0 cu THHN. Given the maximum primary fault current (3.41primary), I computed the max available short circuit current at the 480VAC panel to be around 14.2KA not included the motor contribution downstream of this panel.

My question is if this number is correct (i.e 14.2KA) and the CBs in this panelboard is only rated for 14KA,
Thank you in advance.

Francis.

please correct the given primary max fault current. if what you show is correct(3.41 amps) you have a serious problem.
You won't have any secondary fault amperage

 

[jim dungar]

... if the quick calculator is well under the limits - why spend any more time on analysis?

That is one of the factors you should be considering.
I have previously posted, that the threshold should be about 20% anything closer needs additional analysis.

 

[francis.key19]

please correct the given primary max fault current. if what you show is correct(3.41 amps) you have a serious problem.
You won't have any secondary fault amperage

I meant 3.41KA. This was given to me by the power company with the 5.62%Z. I also used a demo version of Etap to see if the calculation is correct and so far it is about 14.3KA including motor contribution. I checked the length and method of installation against the numbers and all is consistent. Someone had mentioned about series rated in the context of motors, can someone explain what would be the issue with series rated.? I am not sure the solution to this, but the installation seems to not have any margin of safety here. What are some potential solutions? Thank you in advance.

FK

 

[mayanees]

series ratings are covered by NEC 240.87.. or .86.
It limits the amount of motor contribution to a % of the sc rating of the lower-rated device. i.e. 1% of 10 kA = max 100 amps of motor load.