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.