KAIC rating on a transformer

[redwiggler]

I have a temporary 1000 kVA "medium volt" transformer (25K/480/277) that is feeding a new construction job site. It has an impedance rating of 3.81% and according to the EOR he has based the calculated fault current at 31,570 amps assuming an infinite buss. The transformer feeds a 1200 amp disconnect that is fused at 700 amps supplying an 800 amp, 3-phase 480/277 volt panel used for the temporary on a new building under construction.

The problem is the 800 amp temp panel that was installed has a 14kaic rating and according to the Engineer it needs to be a 42kaic rated panel to handle the 31,570 fault current. I was somewhat surprised to see an available fault current study done on a temporary installation, but I see new stuff everyday.

My question is... can the 1200 amp disconnect be fused properly to protect the 800 amp panel with the 14kaic rating? Or does the panel need to be swapped out for one that is rated at 42kaic? How far down the line does this rating go (kaic)? We also have construction trailer that has a 100 amp panel inside the trailer that I am certain doesn't have a kaic rating that high that is being fed from a step down transformer coming out of the 800 amp temporary construction panel. How does the kaic rating affect that panel?

 

[kwired]

Transformer doesn't have an "interrupt" rating. It does have an available fault current.

Keep in mind this available fault current is what is available at the transformer terminals, add some conductor and it does lessen because of the resistance of the conductors is current limiting. So to know what fault current is available at your 800 amp panel you need to know what is between it and the source and do some calculations. Other items further downstream will have less available current because of more conductor between them and the 800 amp panel.

Your 800 amp panel is probably series rated with many different fuse types, but probably only series rated with a limited number of circuit breakers - and most likely only breakers by the same manufacturer, should you still be high on available fault current after doing those calculations. If you are close, finding a way to add a little more conductor will cost less then another fused disconnect or high AIC breaker.

 

[kwired]

Re-read your post, at what point in the system is the 31,570 A available?

If at the transformer secondary then it will be less at your equipment, how much depends on what is between the two.

I also would guess your 800 amp panel is likely series rated if you have RK-5, fuses ahead of it, or at very least class J fuses.

 

[jim dungar]

How does the kaic rating affect that panel?

The correct terms are important.

AIC is a term reserved for fuses and switches. It stands for Amps Interrupting Capacity. AIC is the amount of fault current that the device can 'clear' without failing (i.e. expolding). The number of amps is very high so we typically abbreviate by using kAIC just like we do wire sizing (e.g. kcmil).
SCA is a typical abbreviation uses for the amount of amps that could flow through a fault. The AIC rating of a protective device must be greater than the amount of Short Ciruit Amps that is available. The SCA is determined by calculations involving the impedance (length and size) of the conductor and any transformers in the circuit.
Short Circuit Current Rating (SCCR) is related term used with non-protective devices (e.g. terminal blocks and non-fused equipment).

For decades, the NEC sections 110.9 and 110.10 have required these values to be considered.
To emphasize the importance of the sections, over the years the NEC added sections like 110.24 and 409.22 which require equipment to be labeled in the field.

So, in your case:
They followed the NEC by determining the amount of available fault current by considering the utility and the service transformer. They reported this value as roughly 32kA. This means that your protective devices, and panelboard, needs to have an AIC rating higher than this amount. Your engineer has specified 42kAIC equipment.

All equipment downstream from this service entrance should have a calculation performed to determine the SCA at that point. It is likely that the conductors size and length may reduce the amount of short circuit (fault) current allowing you to employ devices with lower AIC values. This process needs to be done at each panel location. Transformers have the greatest impact for reducing fault current. For very conservative results you can estimate your conductor lengths on the extremely short side, maybe something like 9'.

 

[redwiggler]

Transformer doesn't have an "interrupt" rating. It does have an available fault current.

Keep in mind this available fault current is what is available at the transformer terminals, add some conductor and it does lessen because of the resistance of the conductors is current limiting. So to know what fault current is available at your 800 amp panel you need to know what is between it and the source and do some calculations. Other items further downstream will have less available current because of more conductor between them and the 800 amp panel.

Your 800 amp panel is probably series rated with many different fuse types, but probably only series rated with a limited number of circuit breakers - and most likely only breakers by the same manufacturer, should you still be high on available fault current after doing those calculations. If you are close, finding a way to add a little more conductor will cost less then another fused disconnect or high AIC breaker.

From what I understand the Transformer has an available fault current of 31,570 amps at the Transformer Terminals

 

[electrofelon]

From what I understand the Transformer has an available fault current of 31,570 amps at the Transformer Terminals

That checks out with the transformer specifications given.

This:

Your 800 amp panel is probably series rated with many different fuse types.......I also would guess your 800 amp panel is likely series rated if you have RK-5, fuses ahead of it, or at very least class J fuses.

Check the series ratings from the manufacturer.

 

[jim dungar]

I also would guess your 800 amp panel is likely series rated if you have RK-5, fuses ahead of it, or at very least class J fuses.

Neither Class RK5 nor J fuses are available above 600A. 601A and larger fuses are going to be Class L.

In my experience, 800A and larger fuses almost always require a minimum of 22kAIC devices for series ratings.

 

[Jraef]

... Check the series ratings from the manufacturer.

Just to be clear, this is the ONLY practical way this concept works, but even then there are catches that can trip you up. For a temp service on a construction site, you may not run up against those, but then again as you observed, I have never run up against this entire concept for termporary services before at all, so given that your AHJ is already looking, there's no predicting how detailed he wants to get.

So for example one catch (240.86.C.2) is that your motor loads cannot be greater that 1% of the "protected breaker" base kAIC rating. So in this case because yours is 14kA, the total motor load down stream cannot be greater than 140A, regardless of the Series Rating of the panel itself.

 

[dkidd]

Be sure to review 230.95. It looks like you need ground fault protection.

 

[infinity]

Be sure to review 230.95. It looks like you need ground fault protection.

Good point.

 

[electrofelon]

Be sure to review 230.95. It looks like you need ground fault protection.

He has 700 amp fuses....It could be argued that his service disconnect is rated over 1000 regardless of what it is fused at, but I dont imagine that is the intent. Wonder why it is 1200, maybe just what they had lying around?

 

[infinity]

He has 700 amp fuses....It could be argued that his service disconnect is rated over 1000 regardless of what it is fused at, but I dont imagine that is the intent. Wonder why it is 1200, maybe just what they had lying around?

The service disconnect size is determined by the largest fuse that will fit.

230.95 Ground-Fault Protection of Equipment. Ground-
fault protection of equipment shall be provided for solidly
grounded wye electric services of more than 150 volts to
ground but not exceeding 1000 volts phase-to-phase for
each service disconnect rated 1000 amperes or more. The
grounded conductor for the solidly grounded wye system
shall be connected directly to ground through a grounding
electrode system, as specified in 250.50, without inserting
any resistor or impedance device.
The rating of the service disconnect shall be considered
to be the rating of the largest fuse that can be installed or
the highest continuous current trip setting for which the
actual overcurrent device installed in a circuit breaker is
rated or can be adjusted.

 

[electrofelon]

The service disconnect size is determined by the largest fuse that will fit.

Thanks, I guess I never read that far!

 

[infinity]

Thanks, I guess I never read that far!

Initially I thought the same thing as you.

 

[Jraef]

The service disconnect size is determined by the largest fuse that will fit.

That's why if you read the fine print or foot notes in FDS catalogs, you find that 1200A fused disconnect switches are not SUSE listed for 480V Wye systems. 800A max.

To get SUSE labels on 1000A and up switches, you have to go with a fused BPS (Bolted Pressure Switch, aka Pringle Switch, Bolt-Loc switch, etc.) because they can have a shunt trip attached to them, which can be tied to a GF sensor/ relay.

 

[dkidd]

Thanks, I guess I never read that far!

I made sure to read it through before posting.

 

[kwired]

Neither Class RK5 nor J fuses are available above 600A. 601A and larger fuses are going to be Class L.

In my experience, 800A and larger fuses almost always require a minimum of 22kAIC devices for series ratings.

Excuse my ignorance, I seldom see over 600 amp fuses, and even when I do, I am usually not the one selecting what is correct for the application.

 

[petersonra]

I do not use fuses over 600A for this reason. It is less expensive to use a MCCB in virtually all cases above 600A. really above 200A in most cases. depends on the AIC requirements but usually a MCCB is the way to go.

 

[electrofelon]

Excuse my ignorance, I seldom see over 600 amp fuses, and even when I do, I am usually not the one selecting what is correct for the application.

I admit I know very little off the top of my head about fuse sizes, classes, applications, etc. I hardly ever use safety switches outside of solar. There is only one time I recall using a greater than 200 amp switch for a non solar application, and it was 1000 amp. That was a job someone else had started and ordered the gear for, and I wouldnt have done it that way if I had designed it.

 

[Electric-Light]

I have a temporary 1000 kVA "medium volt" transformer (25K/480/277) that is feeding a new construction job site. It has an impedance rating of 3.81% and according to the EOR he has based the calculated fault current at 31,570 amps assuming an infinite buss. The transformer feeds a 1200 amp disconnect that is fused at 700 amps supplying an 800 amp, 3-phase 480/277 volt panel used for the temporary on a new building under construction.

The worst possible calculation is the simplest and if the calculated results do not cause a road block, you move on. Otherwise, calculations are made to see if costly upgrades are needed.

Attach a doorbell transformer and short the secondary and you won't see measurable drop to your house. If you were to do the same with a 240 to 24v 1kVA transformer attached to a 15A branch and place a dead bolt short across the secondary very briefly, you'll see a significant voltage drop at your primary terminals due to impedance of wiring to the transformer and recalculating with this taken into effect might let you squeeze into the lower bracket. This same thing applies to everything else. When you have a larger transformer, the power line and substation gears are not always considered infinite source.


Check the bottom of bottom left of page 2. SCCR rating on your setup?
http://www.ul.com/wp-content/uploads/2014/04/ul_PanelboardShortCircuitRatings.pdf