CABLE IMPEDANCE IN EASYPOWER AND SKM

[MyCleveland]

In EasyPower, the ONLY impact of setting a different ambient temperature is determining the ampacity of the conductors. The program will derate the ampacity based on the NEC derating requirements. That's it. There's a different parameter for adjusting the actual conductor temperature.

Regarding the "as-delivered" default of 50 deg C for the conductor temp - the "worst case" conductor temperature depends on the study being performed. For short-circuit calcs, a lower temperature would give higher fault currents. For power flow, a higher temperature would give a higher voltage drop and power loss. Again, we're talking about small changes in the results.

One of the features I like about SKM is that you can specify two different conductor temperatures - one for short circuit and one for power flow. You could do that in EasyPower using Scenarios, it would take more work.

IMO, we're taking the NEC conductor data resistance table too seriously. It is strictly informational, AFAIK. Any conductor resistance table has to be based at a specific conductor temperature - they use 75 deg C. But nowhere does it say that you should use 75 deg C for your short circuit calculations or voltage drop calcs.

Let me first say...
JD
I am not ignoring you, I just simply cannot formulate a response. I always read your comments and appreciate the time you spend.

DC
I just ran a single test calc of 200' of THHN CU using the NEC table values at 30C & 40C.

we're talking about small changes in the results
2% difference seems important.
8% difference if I drop the loading to 100A vs the fully loaded NEC 75C value.

It is strictly informational, AFAIK
My understanding from actual users (6 years ago) was that the values where verbatim from the NEC tables for BOTH EP & SKM.

nowhere does it say that you should use 75 deg C for your short circuit calculations or voltage drop calcs.
I could not agree more, but believe the majority is without adjustment for either calc type.

We can leave it here, no one else has interest in the topic.

I thank you both for your time.

 

[D. Castor]

2% difference seems important.
8% difference if I drop the loading to 100A vs the fully loaded NEC 75C value.

Difference in what? And how is the the loading changing something?

You can see the resistance values used in the programs in the cable data. You can compare these to the NEC table to see if they match.

I just checked EasyPower data for a 400 kcmil copper conductor. At 75 deg C conductor temp, 60 Hz, steel conduit, RHH:

NEC Table 9 : 0.035 ohms/1000 ft.
EasyPower: 0.0344339 ohms/1000 ft.

So definitely not verbatim. I'm pretty sure I know the original source of the EasyPower data, and it wasn't the NEC table. It could have been updated or changed at some point, however, so I'm not going to open that can of worms. I no longer have any connection with EasyPower.

One thing I find odd in the NEC data is that they have a higher resistance for cable in AL conduit compared to steel. I'm having a hard time explaining the physics of that. In EasyPower, the resistance in AL conduit is slightly lower than in steel conduit which I suspect is due to the impact of skin effect and the increased reactance of the steel vs. aluminum.

Perhaps someone on here knows where the data in the NEC originated, but I don't know.

Since you're doing the study in SKM, you may want to compare the SKM data with the NEC table.

 

[ramsy]

Perhaps someone on here knows where the data in the NEC originated, but I don't know.

NEC Tbl.9 impedances are converted 1/(Z per foot) to "C" table results, published by IEEE Std. 241-1990, IEEE Recommended Practice for Commercial Building Power Systems.

 

[D. Castor]

....IEEE Std. 241-1990, IEEE Recommended Practice for Commercial Building Power Systems.

Thanks for the info. The old Grey Book. I don't have a copy handy to check their source data. But it's definitely hasn't changed in a while.

 

[MyCleveland]

Difference in what? And how is the the loading changing something?

Since you're doing the study in SKM, you may want to compare the SKM data with the NEC table.

I thought we were discussing SCC.
I just ran a single test calc of 200' of (3/0) (50kA SCC) THHN CU using the NEC table values at 30C & 40C.

we're talking about small changes in the results
2% difference seems important. (Changing the ambient from 30C to 40C)
8% difference if I drop the loading to 100A vs the fully loaded NEC 75C value.
** The NEC 75C is R value of a fully loaded cable to get a conductor temp of 75C. Change the loading value changes the R value.

No, I do not have SKM or Easy Power.

 

[MyCleveland]

NEC Tbl.9 impedances are converted 1/(Z per foot) to "C" table results, published by IEEE Std. 241-1990, IEEE Recommended Practice for Commercial Building Power Systems.

Ramsy
Where did you find this reference?

 

[jim dungar]

....I do not have SKM or Easy Power.

Are you making hand calculations, or do you have a different software package?

I believe SKM assumes 150°C for conductors when it performs a SC study.
It uses different values when it is doing other analyzes such as voltage drop.

I know our engineering standards team reviewed issues like this and created default values for the three major software packages, so our reports would be consistent. Our primary studies were SC, TCC, AF, and VD. I don't remember tweaking conductor temperatures on any regular basis.

 

[ramsy]

Ramsy
Where did you find this reference?

Spreadsheet note did not reference a source. Full note below:

This Point-to-Point implement of 1/Z per foot varies with Pwr Factor and conduit types, based on resistance and reactance, per NEC Tbl.9 formulas.

Setting power factor to 85% matches "C" table results, published by IEEE Std. 241-1990, IEEE Recommended Practice for Commercial Building Power Systems.

 

[D. Castor]

I thought we were discussing SCC.
I just ran a single test calc of 200' of (3/0) (50kA SCC) THHN CU using the NEC table values at 30C & 40C.

we're talking about small changes in the results
2% difference seems important. (Changing the ambient from 30C to 40C)
8% difference if I drop the loading to 100A vs the fully loaded NEC 75C value.
** The NEC 75C is R value of a fully loaded cable to get a conductor temp of 75C. Change the loading value changes the R value.

No, I do not have SKM or Easy Power.

No matter how you calculate the fault current, you have to assume a conductor temperature. If you use the data in NEC table without adjustment, you're assuming 75 deg C. Are you saying you got a 2% difference in fault current? I still don't understand your reference to changing the loading to 100A if you are calculating fault current.

I thought you were talking about differences between SKM and EasyPower resistance data.

I believe SKM assumes 150°C for conductors when it performs a SC study.

In SKM, you can select whatever conductor temperature you want. You can have one temperature for SC calcs and a different conductor temperature for Load Flow. It's very easy to change in SKM. In EasyPower it's not as convenient, but can be done.

 

[MyCleveland]

No matter how you calculate the fault current, you have to assume a conductor temperature. If you use the data in NEC table without adjustment, you're assuming 75 deg C. Are you saying you got a 2% difference in fault current? I still don't understand your reference to changing the loading to 100A if you are calculating fault current.

I am struggling to make my points, so please forgive.

Some of your prior comments...
**ONLY impact of setting a different ambient temperature is determining the ampacity of the conductors.
**we're talking about small changes in the results

I provided that simple SCC calc as example to show that in fact ambient temp does impact results.
The second part was to show that a change in the loading of the cable (my example 3/0 cu) from fully loaded 200A dropped to 100A resulted in an 8% change in result. Here we were talking about the NEC table values which reflect a jacket temp of 75C...meaning the cable is FULLY loaded or running at 200A. My design allows for selecting the actual operating amps on the cable and the jacket (or your term FIELD temp) temperature is then adjusted. Again, I cannot understand how any user could guess the jacket temp in order to change it EP other than a WAG.

I had stated my experience through discussions with many other users of both programs was that they were both using NEC table values.
Not one of these guys offered up any options of changing the cable temp internally in the programs, nor did a SKM tech support.
This all took place 6+ years ago and you have stated it can be done inside of EP. I believe you, but I am worried not many are aware or are utilizing the feature.

 

[MyCleveland]

Are you making hand calculations, or do you have a different software package?

I believe SKM assumes 150°C for conductors when it performs a SC study.
It uses different values when it is doing other analyzes such as voltage drop.

Hand calcs is where we all start I would believe.
Expanded into hiring out through friends for larger work.
On to EXCEL and point to point...then a four tier point to point...added motor contribution.

Then wanting to always be able to capture the exact X/R developed another spreadsheet using the Ohmic Method, again four tiers. Note that any node can be a motor.
Expanded that to add MV on the front end for customers that were receiving power at MV. This took some time to incorporate cable data for overhead and underground.

Somewhere in the middle I added a spreadsheet for Arc-Flash Analysis, the last IEEE formula package not the most resent.
I never used it for commercial work, building in the TCC's is time consuming but doable. In did come in handy when the clearing time was required to be included on the main. Via a different piece of software, I could capture any TCC, import it into excel sheet, and after calculating SCC at main it would interpolate the time value for me.

I few years back I decided to try and build that last design into a windows based app, and I believe I have. I get the exact same results from either one. I believe I tried to get your eyes on it. Check your PM's.

It has been a long road Jim.

You stated..."I believe SKM assumes 150°C for conductors when it performs a SC study." You did not really mean 150 did you? Were in the damage curve ballpark.

 

[MyCleveland]

Spreadsheet note did not reference a source. Full note below:

I think I have seen this, I will try and find it.

 

[jim dungar]

You stated..."I believe SKM assumes 150°C for conductors when it performs a SC study." You did not really mean 150 did you? Were in the damage curve ballpark.

Yes, short circuits damage cables, if not cleared fast enough. Did you look at the SKM reference I provided?

 

[MyCleveland]

Yes, short circuits damage cables, if not cleared fast enough. Did you look at the SKM reference I provided?

I did, but not sure how it applies here.
My thoughts were regarding the various things to keep in mind to make sure the correct variables are considered that effect the conductor temp before the fault calc is performed.

Did you not state prior that if a given conductor is protected with a UL489 OCPD, the damage curves are accounted for.

 

[jim dungar]

Did you not state prior that if a given conductor is protected with a UL489 OCPD, the damage curves are accounted for.

Yes and no.
The damage curves are pretty much irrelevant to UL listed OCPDs.
The UL standards require a length of 'normal building wire' be connected to each OCPD during short circuit and overload tests. The conductor insulation cannot be damaged and the conductor cannot pull free from the lugs.

 

[MyCleveland]

Spreadsheet note did not reference a source. Full note below:

The "C" table values from the Bussmann "SPD" books I have make no footnote reference of NEC Chap 9, table 9 credit.
It does credit multiple IEEE sources.
The values do appear to be the inverse of the NEC table 9 values for PF of 0.85.

I would think if the NEC was sourcing values from IEEE they would state as much.

 

[jim dungar]

You stated..."I believe SKM assumes 150°C for conductors when it performs a SC study." You did not really mean 150 did you?

I need to do some more searching I believe I may have misspoke.

 

[MyCleveland]

Yes and no.
The damage curves are pretty much irrelevant to UL listed OCPDs.
The UL standards require a length of 'normal building wire' be connected to each OCPD during short circuit and overload tests. The conductor insulation cannot be damaged and the conductor cannot pull free from the lugs.

I interpret your statement as, only be concerned about potential cable damage if I am in the MV realm.

 

[jim dungar]

I interpret your statement as, only be concerned about potential cable damage if I am in the MV realm.

As long as your OCPD have an Instantaneous function.

 

[jim dungar]

I have found this article on short circuits and conductors.

Short Circuit Sizing of Power Cables Explained - ELEK Software

The fault current which flows in a power cable causes its temperature to rise dramatically. Equations for minimum conductor size to withstand the short circuit are provided.

elek.com

SKM PTW uses the adiabatic method in their calculations.