CABLE IMPEDANCE IN EASYPOWER AND SKM
- Thread starter binwork91
- Start date
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
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.
Operating and ambient temperatures, and stranding were some of the differences.
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
If you run a few test calcs with the different options, what is the % difference in the results?
D. Castor
Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
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.
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
Senior Member
- Location
- Cleveland, Ohio
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?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
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
Senior Member
- Location
- Cleveland, Ohio
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%?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
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
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
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
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Software that wont allow MFG cable data assumes everybody complies with adopted standards.
D. Castor
Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
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
Senior Member
- Location
- Cleveland, Ohio
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
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Profit determines material characteristics, not physics.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I found this conductor information from SKM.
Table 1 summarizes typical allowable conductor temperature limits under short circuit, emergency overload and normal operating conditions.
Table 1 Typical conductor operating temperature limits
The ampacity landmark is located in the top decade of a TCC at 1000 seconds.Type | Insulation | Voltage | Short Circuit | Emergency | Normal |
0.01 < t < 10 sec. | 10 sec. < t < ~1-6 hrs | t > ~1-6 hrs | |||
Al or Cu | TW | 600V | 150ºC | 85ºC | 60ºC |
Al or Cu | THWN | 600V | 150ºC | 90ºC | 75ºC |
Al or Cu | THWN | 600V | 150ºC | 105ºC | 90ºC |
Al or Cu | XLP | 5-15kV | 250ºC | 130ºC | 90ºC |
Al or Cu | EPR | 5-15kV | 250ºC | 130ºC | 90ºC |
AAC | Air | All | 340ºC | 150ºC | 100ºC |
ACSR | Air | All | 645ºC | 150ºC | 100ºC |
D. Castor
Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
EasyPower allows the conductor "field" temperature to be adjusted for any conductor. This changes (slightly) the resistance of the conductor. The actual resistance and reactance values can also be changed in the conductor data. Changing the "Ambient Temperature" setting in EasyPower will derate the conductor ampacity, but has no impact on the resistance of the conductor.
I'm pretty sure SKM also allows adjustment of the conductor temperature, but I don't have access to the program at present.
The overall impact of conductor temperature on resistance is pretty small over the range of temperatures normally encountered.
I'm pretty sure SKM also allows adjustment of the conductor temperature, but I don't have access to the program at present.
The overall impact of conductor temperature on resistance is pretty small over the range of temperatures normally encountered.
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
DC
Changing the “Ambient Temperature” has NO impact on the resistance of the conductor…could you give that another try?
Granted the changes may be small depending on the rise or drop of the ambient magnitude, but it does change. You stated as much in post 11.
Could you respond to post 12, question 1?
D. Castor
Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
I'm referring to the Ambient Temperature setting in the EasyPower cable data. This only impacts the continuous current rating used for the conductor. It does not change the conductor resistance used by the software. To modify the actual conductor temperature, you must modify the "Field Temperature of Conductor" setting in EasyPower. This is just how the program works.
I'm not sure what is meant by "NEC Library" in terms of EasyPower. The standard defaults for ambient temp is 30 deg C. For the Field Temperature of conductor, the standard default is apparently now 50 deg C - I just checked. The default can be changed to whatever you want under "Tools - Equipment Defaults" The defaults can also be changed for individual conductors. The ampacities used should match up with the values in the NEC.
Hope that helps. If you have EasyPower, their tech support folks will be happy to help you. But feel free to message me if you have more questions.