Fault current calculation

[tx2step]

I'm installing 2 new single phase 240/120V 400A Main Circuit Breaker panels (loadcenter construction), feeding them from a 167KVA single phase 240/120V pad mount transformer. The power Co says there is 69,500 maximum fault current amps available at the secondary lugs of the xformer. I will be feeding each new panel with 2 each 30 foot long parallel #250 MCM aluminum XHHW conductors per phase & neutral, in 2 each parallel PVC conduits. I ran that info through a bussman fault current calculation program (1997, version 1) and came up with 41,500 maximum available fault current amps at the main lugs of each new panel. Does that sound about right? The new panels will be fully rated at 42K AIC. Does anyone see any problems with this arrangement? The 2 panels will be installed side by side, for a total capacity of 800A. The minimum conductor length between the xformer lugs & panel Main CB lugs will be 30 feet -- maybe a few feet longer. Metering will be done with CT's in the xformer.

 

[beanland]

Huh?

Huh?

A 167kVA 240V transformer is 695A full load. If the impedance of that transformer is 3%, the maximum fault current is 23kA. That ignores all source impedance and all secondary wire impedance. Is the POCO assuming a 1% transformer and infinite source? The POCO should be able to provide realistic fault currents.

 

[augie47]

As an Inspector, I use the same program just as a "spot check"... (I don't know which version)
No sure which of us entered differently, but I came up with 43,000 +.
With that in mind plus having no idea of the motor contribution, if the job was in this area, more detailed engineering data would be required.

 

[jim dungar]

If I determined the possible fault current was 41.5kA, I would not use 42kA devices. I would request 50kA at a minimum.

All of the simplistic fault current calculations and device selection charts, pretty much ignore the X/R (X over R ratio) of the system. Yet, IEEE standards recommend, and UL testing requires, that molded case circuit breakers have their AIC rating adjusted based on the actual X/R of the installed system.

 

[beanland]

Free Excel

Free Excel

There are great free fault current calculators using Excel.

http://www.mikeholt.com/freestuff.php?id=freegeneral

 

[bob]

Every thing depends on the %Z you use.
I have an old GE manual that gives 1.6% for a 75 kva transformer.
Using that figure and infinite fault I get 19.5 ka phase to phase and 29.2 ka phase to neutral at transformer. At the end of your cable run I get 17.2 ka and 21 ka.
If you assume an finite primary fault of 6000 amps which is high The fault at the transformer is 17 ka and 26 ka. at the end of the cable it is 16 ka and 19.5 ka.
It all depends on the transformer %Z.

 

[tx2step]

This is in rural Oklahoma. I'm told that I'm the only one that ever asks for the available fault current. This is out in the country, at the end of a very long aerial primary run, then around a 200' underground primary run to the xformer. I can't easily question the POCO "engineer", but I'm pretty sure no real effort went into their calculation. It seem VERY high to me...I don't think there's any way it can really be that high. Even so, I want to keep the rated AIC of the panels above the calculation at the panels...but I also need to keep the cost down for the owner.
This is out in very rural Oklahoma. I can't easily question the POCO engineer, but I think their calc is VERY high. The xformer is at the end of a very long, aerial primary run, then about 200' of underground primary. I don't see how 69.5K would be possible. But I still want to keep the rating of the panels under the calculated value.

This is a church gym. The only motor loads are the package air conditioning units. Most of the building load is for resistive heat - it's fully electric heat. My service calculation is in the low 600A range, with all heat load on. So I'm going with 2 each 400A panels. I'm trying to stay with loadcenter type panels to keep the cost down. Graybar tells me that the highest AIC that Square D can do with a loadcenter is 42K AIC. I can make the service conductors 35 or 40 feet long lug to lug to get the fault current calc lower.

Jim, is there really a concern with having the AIC of the panels only 1K or so above the calculated value?

 

[tx2step]

Oops! Sorry! I'm doing this on my iPhone & thought the first part had disappeared. Sorry for the duplicate info...

 

[jim dungar]

Jim, is there really a concern with having the AIC of the panels only 1K or so above the calculated value?

If you were to ask me for an official report based on the supplied data, you would not like my results.

It appears you are being given a 'design' value number from your utility (which is the norm here in WI), I feel your pain.

Once, I had a customer ask the utility for an 800A 208V service (even though their calculated load was <400A), they thought they were doing their customer a favor by allowing for some growth even though the building was 100% filled. The utility provided a design level fault current based on a possible 'emergency replacement' transformer size. The end result was 65K equipment, but if the service request had been for a 600A service the rating would only have been 25kA.

By the way, Square D makes loadcenter breakers up to 65kAIC. The 42k limit is the integral 400A main breaker.

 

[kingpb]

For two more cents, I am full agreement with Jim. You have to use what your given, and 41.5KA is too close to guarantee 42kA is good enough.

 

[tx2step]

Ok, you guys have convinced me. Does one of the other switchgear manufacturers have 400A MCB loadcenters that can be rated above 42K AIC?

The alternative would be to change to MLO panels & feed them through 400A NEMA 3R fusible disconnects...but that would add at least $1,000 more in material cost plus quite a bit more labor.

The panels will be located in a restroom wall...since this isn't a dwelling, I believe that's code compliant, right? I don't like doing that, but it's the only logical place to put them.

Thanks for the help!!!

 

[tx2step]

Ok, you guys have convinced me. Does one of the other switchgear manufacturers have 400A MCB loadcenters that can be rated above 42K AIC?

The alternative would be to change to MLO panels & feed them through 400A NEMA 3R fusible disconnects...but that would add at least $1,000 more in material cost plus quite a bit more labor.

The panels will be located in a restroom wall...since this isn't a dwelling, I believe that's code compliant, right? I don't like doing that, but it's the only logical place to put them.

Thanks for the help!!!

 

[tx2step]

Jim (or anyone else), if you can tell me which swgr mfgs make 400A MCB loadcenters that can be rated above 42K AIC, it would help me a lot. I'm working in the field right now & time is tight. Thanks for your help!!!

 

[jim dungar]

I do not spend much time on specific equipment selection.

As far as single phase 400A panels go, I believe you will need to use a MLO panel and a separate main device (I would use Class J fuses).

Or, get a professional short circuit study that includes an option for determining the length of cable needed to reduce the fault current sufficiently to allow for 42kA devices.

 

[augie47]

A quick review of Loadcenters in a couple of catalogs did not turn up any Main Breaker units rated above 42k. You might have to go to a panelboard (easy to do there)

 

[kingpb]

Rated for 240V, Eaton PRL1a has a 22kA full and a 22-100ka series rating
the PRL2a has a 65kA full and 65-200kA series rating

Both can come as MLO or with a 400A main breaker.

 

[tx2step]

Motor contribution from HVAC equipment?

Motor contribution from HVAC equipment?

Thanks for the info!!!

Do air conditioning compressors or fans add any significant motor contribution? I don't think they would, since there's almost zero rotating mass & the compressor is working against a very high head pressure, right???

There are 4 identical package heat pumps. Each one has:
1. One hermetic compressor that has 20.5A RLA & 109.0 LRA (that would be a little over 3 HP wouldn't it?)
2. One 3/4 HP fan motor, with 4.0A FLA
3. One 1/4 HP fan motor, with 1.4A FLA

2 units will be fed with #8 THHN, about 40' in length lug to lug.
2 units will be fed with #8 THHN, about 100' in length lug to lug.

Am I correct that these 4 HVAC units will have only a negligible contribution to the available fault current?

 

[tx2step]

Motor contribution from HVAC equipment?

Motor contribution from HVAC equipment?

Thanks for the info!!!

Do air conditioning compressors or fans add any significant motor contribution? I don't think they would, since there's almost zero rotating mass & the compressor is working against a very high head pressure, right???

There are 4 identical package heat pumps. Each one has:
1. One hermetic compressor that has 20.5A RLA & 109.0 LRA (that would be a little over 3 HP wouldn't it?)
2. One 3/4 HP fan motor, with 4.0A FLA
3. One 1/4 HP fan motor, with 1.4A FLA

2 units will be fed with #8 THHN, about 40' in length lug to lug.
2 units will be fed with #8 THHN, about 100' in length lug to lug.

Am I correct that these 4 HVAC units will have only a negligible contribution to the available fault current?

 

[jim dungar]

Am I correct that these 4 HVAC units will have only a negligible contribution to the available fault current?

All motors contribute short circuit current, but the IEEE standard recommendation is to ignore motors smaller than 50HP.

You can figure out motor contribution in a manner similar to that used for transformers: Isc = FLA/Xd", where Xd" is the 'subtransient reactance' of the motor.
You will never find the actual Xd" of a motor except for the ones that have been made to order, with the requirement to provide the electrical data (usually only happens with motors >>100HP). Many people use a typical value Xd" = .17 for motors over 50HP and Xd" = .25 for smaller motors.

 

[jghrist]

All motors contribute short circuit current, but the IEEE standard recommendation is to ignore motors smaller than 50HP.

You can figure out motor contribution in a manner similar to that used for transformers: Isc = FLA/Xd", where Xd" is the 'subtransient reactance' of the motor.
You will never find the actual Xd" of a motor except for the ones that have been made to order, with the requirement to provide the electrical data (usually only happens with motors >>100HP). Many people use a typical value Xd" = .17 for motors over 50HP and Xd" = .25 for smaller motors.

The IEEE recommendation to ignore motors smaller than 50HP only applies to medium- and high-voltage motors. For a group of low-voltage motors rated less than the transformer, you can use an impedance of 0.25. This would make a contribution of (20.5 + 4 + 1.4) / 0.25 = 103.6 A. I would normally consider this neglible, but it makes the 41.5 kA that much closer to 42 kA and is one more reason not to use 42 kA equipment.