CABLE IMPEDANCE IN EASYPOWER AND SKM

[binwork91]

Hi, I am new to SKM.
Anyone has idea why skm and Easypower have different impedance of cable?
And which manufacture will you pick in skm? typical or IEEE or NEC or others?

 

[jim dungar]

I recall SKM had several different cable libraries, so you had to understand which each one did. For example one library may have been based on IEEE Red book whiIe a different one used the NEC.

Operating and ambient temperatures, and stranding were some of the differences.

 

[MyCleveland]

If you run a few test calcs with the different options, what is the % difference in the results?

 

[D. Castor]

The differences should be small. Conductor temperature assumed will have the most impact on resistance. If you look on the "Specification 2" tab of the EasyPower data, it will show the "Field Temperature of Conductor". I think the default is 75 deg C and SKM uses 90 deg C.

EasyPower also has options for conductor lay and spacing that will also change the reactance. The insulation thickness used will also impact the reactance. You can open the EasyPower library to see all of the factors at play in the cable data. It's one of the most complicated parts of the EasyPower model. I would trust EasyPower more than SKM, but I'm biased because I used to work for EasyPower. But I've used SKM as well.

I would not expect any major differences in short circuit calculations or power flow between the two programs as a result of differences in cable data. Your length estimate for the cable will have a much bigger impact.

 

[MyCleveland]

The differences should be small. Conductor temperature assumed will have the most impact on resistance. If you look on the "Specification 2" tab of the EasyPower data, it will show the "Field Temperature of Conductor". I think the default is 75 deg C and SKM uses 90 deg C.

EasyPower also has options for conductor lay and spacing that will also change the reactance. The insulation thickness used will also impact the reactance. You can open the EasyPower library to see all of the factors at play in the cable data. It's one of the most complicated parts of the EasyPower model. I would trust EasyPower more than SKM, but I'm biased because I used to work for EasyPower. But I've used SKM as well.

I would not expect any major differences in short circuit calculations or power flow between the two programs as a result of differences in cable data. Your length estimate for the cable will have a much bigger impact.

DC
Last part of your first paragraph, are you stating the base parameters for the operating temp of the conductors are 75C & 90C?
If yes, then the assumption in both cases are the conductors are running at 100% loading. Is this not ending in erroneous results?

 

[jim dungar]

Last part of your first paragraph, are you stating the base parameters for the operating temp of the conductors are 75C & 90C?

Remember there is ambient temperature and ultimate operating temperature, but there is also the reference temperature at which the conductor resistance is based.
I have never seen power study software assume, as standard, conductors were operating at the maximum insulation temperature rating.

 

[MyCleveland]

Remember there is ambient temperature and ultimate operating temperature, but there is also the reference temperature at which the conductor resistance is based.
I have never seen power study software assume, as standard, conductors were operating at the maximum insulation temperature rating.

JD
Is that not implied in “Field Temperature of Conductor”?

I believe we discussed this issue before Jim, but forgive me, the specifics and result I do not recall.

I had someone that has an SKM license run this past the companies tech support and they had no response other than the values are taken from the NEC.
The table states 75C at 30C ambient. Does this not result in the cable is operating at 100%?

 

[jim dungar]

It has been quite a few years since I used SKM so I don't remember all the specifics. I do remember being able to select conductor information from several different libraries which resulted in slightly different resistances. Then there was the choice of which ambient was used, the two common choices were 25°C and 30°C.

 

[ramsy]

Does this not result in the cable is operating at 100%?

Perhaps more accurately stated as, 75C is assumed, unless a different temperature column applies.

NEC ampacity tables show max ampacity associated with the temperature column.

NEC temp. correction tables, or equation 310.15(B) is the only method the NEC allows to check intermediary temperatures, without engineering supervision.

 

[ramsy]

I would trust EasyPower more than SKM

Software that wont allow MFG cable data assumes everybody complies with adopted standards.

 

[D. Castor]

Software that wont allow MFG cable data assumes everybody complies with adopted standards.

The point is that all conductors of similar material and construction have basically the same electrical characteristics. This is determined by physics, not standards. The idea that #1/0 copper THHN from one manufacturer will have consistently different electrical characteristics from another manufacturer's similar cable just isn't realistic.

Regarding the conductor temperature, as others noted, it is important to distinguish between ambient temperature and actual conductor temperature. The conductor temp will always be an educated guess or an assumption. The 75 deg C default can be changed for any cable. For short circuit calcs, the maximum current would occur at a very low temperature. For voltage drop calcs, the worst case is a fully loaded cable. During short circuits, the conductor temperature is not a constant but is rapidly increasing. So there are a lot of simplifying assumptions required regardless of how the calculation is done.

 

[MyCleveland]

The point is that all conductors of similar material and construction have basically the same electrical characteristics. This is determined by physics, not standards. The idea that #1/0 copper THHN from one manufacturer will have consistently different electrical characteristics from another manufacturer's similar cable just isn't realistic.

Regarding the conductor temperature, as others noted, it is important to distinguish between ambient temperature and actual conductor temperature. The conductor temp will always be an educated guess or an assumption. The 75 deg C default can be changed for any cable. For short circuit calcs, the maximum current would occur at a very low temperature. For voltage drop calcs, the worst case is a fully loaded cable. During short circuits, the conductor temperature is not a constant but is rapidly increasing. So there are a lot of simplifying assumptions required regardless of how the calculation is done.

Are we in agreement that if you select the NEC library…the base starting point is 30C ambient and 75C conductor temperature?
I believe at this point SKM allows ambient adjustment, but do not have an adjustment for conductor temperature. Are you stating EP has this adjustment option?

 

[ramsy]

This is determined by physics, not standards.

Profit determines material characteristics, not physics.