AC Resistance at 30*C vs 75*C

[Julius Right]

Attached is a list for AWG or kcmil[MCM] at 30oC.I did for 75oC also and is 1-3% different from NEC Table 9 [pvc conduit].May be because at NEC the result is rounded.

 

[mbrooke]

Attached is a list for AWG or kcmil[MCM] at 30oC.I did for 75oC also and is 1-3% different from NEC Table 9 [pvc conduit].May be because at NEC the result is rounded.

Did you get anything for 150*C? For 250*C I'm being told 1.9039 multiplier.


I can't thank you enough!

Water mark these and send them to the NFPA!

 

[Julius Right]

I am sorry. The list of #21-RAC 30oC AWG_mcm is wrong. For the
kcmil 250-2000 I put the 25oC dc resistance in ohm/km instead of
ohm/kft. I APOLOGIZE for my mistake and here attached the corrected list.

 

[mbrooke]

I am sorry. The list of #21-RAC 30oC AWG_mcm is wrong. For the
kcmil 250-2000 I put the 25oC dc resistance in ohm/km instead of
ohm/kft. I APOLOGIZE for my mistake and here attached the corrected list.

No need to apologize, I've done the same. I can't thank you enough!

 

[Julius Right]

I think I found why NEC is 1-3% different. NEC team followed Neher and McGrath way calculation. This way is based on tables and curves rather than formulae. It seems to me this system is a little more accurate-a bit-but it is not so practical. Here attached it is the 150 oC copper conductor a.c. resistance table.

 

[mbrooke]

I think I found why NEC is 1-3% different. NEC team followed Neher and McGrath way calculation. This way is based on tables and curves rather than formulae. It seems to me this system is a little more accurate-a bit-but it is not so practical. Here attached it is the 150 oC copper conductor a.c. resistance table.

Ok, maybe I'm seeing it wrong but the table still says DC? Flat and trafo are AC resistance values?

 

[Julius Right]

Up to 6 awg it is no difference between dc and ac indeed. However ,up to 3/0 the difference it is less than 1%.Up to 600 mcm the difference increases up to 10% and for 2000 mcm it is more than 50%.

 

[MyCleveland]

Up to 6 awg it is no difference between dc and ac indeed. However ,up to 3/0 the difference it is less than 1%.Up to 600 mcm the difference increases up to 10% and for 2000 mcm it is more than 50%.

JR
Can you explain the method you are manipulating the values.
We discussed this briefly 18 months or so ago and with your help I was able to also get close to CHAP9 Table 9 values using N-M formulas.

I am getting lost trying to go from one AMB temp to another.

If I start with with a Southwire table of R values at 25C. (NO LOAD)
How to get to 60 or 75 or 90 at same ambient T.
How to shift a R value, at operating temp or no load to another ambient.

I have asked numerous factory engineers, and ONLY one has stated they have all the formulas, but they are proprietary and can't be shared.

I have a SCC spreadsheet that I want to have an option to change ambient temp for a given BUS if desired.

 

[Julius Right]

If you intend to follow N-McG method it is difficult to calculate since here for ys [skin effect] and yp [proximity effect] functions F(x) and F(p) are in tables using about 500 numbers. I think the base of this table calculation is using Bessel function but I not yet found how.

I follow IEC 60287-1-1 method-there using formulas which may introduce an error of 1 to 4%. That, in my opinion, is enough accurate.

I have no problem to do an excel sheet where you may chose conductor temperature and ambient and other for ac resistance calculation. The problem is I cannot attach an excel spreadsheet in this forum-only jpg file may be attached here.

 

[mbrooke]

Up to 6 awg it is no difference between dc and ac indeed. However ,up to 3/0 the difference it is less than 1%.Up to 600 mcm the difference increases up to 10% and for 2000 mcm it is more than 50%.

No difference? I'm confused.

Table 8 and Table 9 show a difference between AC and DC.

For example, solid #12 copper is listed as having a DC value of 1.93 ohms vs AC of 2 ohms.

 

[mbrooke]

If you intend to follow N-McG method it is difficult to calculate since here for ys [skin effect] and yp [proximity effect] functions F(x) and F(p) are in tables using about 500 numbers. I think the base of this table calculation is using Bessel function but I not yet found how.

I follow IEC 60287-1-1 method-there using formulas which may introduce an error of 1 to 4%. That, in my opinion, is enough accurate.

I have no problem to do an excel sheet where you may chose conductor temperature and ambient and other for ac resistance calculation. The problem is I cannot attach an excel spreadsheet in this forum-only jpg file may be attached here.

Shoot me an Email.

 

[MyCleveland]

If you intend to follow N-McG method it is difficult to calculate since here for ys [skin effect] and yp [proximity effect] functions F(x) and F(p) are in tables using about 500 numbers. I think the base of this table calculation is using Bessel function but I not yet found how.

I follow IEC 60287-1-1 method-there using formulas which may introduce an error of 1 to 4%. That, in my opinion, is enough accurate.

I have no problem to do an excel sheet where you may chose conductor temperature and ambient and other for ac resistance calculation. The problem is I cannot attach an excel spreadsheet in this forum-only jpg file may be attached here.

Thank you for the reply.
I will pass on the excel sheet although others may ask. If I am adding code to a program, I need to understand the formulas.
Not familiar with any IEC documents. Does this specific paper go through a detailed method of what I am looking to achieve ?
You stated 1-4% error....this comes from the IEC document ?

 

[Julius Right]

[TD valign="bottom"]Up to 6 awg it is no difference between dc and ac indeed. However ,up to 3/0 the difference it is less than 1%.Up to 600 mcm the difference increases up to 10% and for 2000 mcm it is more than 50%.

If you intend to follow N-McG method it is difficult to calculate since here for ys [skin effect] and yp [proximity effect] functions F(x) and F(p) are in tables using about 500 numbers. I think the base of this table calculation is using Bessel function but I not yet found how.
I follow IEC 60287-1-1 method-there using formulas which may introduce an error of 1 to 4% That, in my opinion, is enough accurate.
I have no problem to do an excel sheet where you may chose conductor temperature and ambient and other for ac resistance calculation. The problem is I cannot attach an excel spreadsheet in this forum-only jpg file may be attached here.

It is not an absolute error compared with a 100% correct value [unknown], but only compared a way to another. Using a formula -as in IEC way-one get continue values. In the N_McG method, due to the fact that the table is used in this way, the results are in a discrete form - which means that x is obtained between a and b and it does not know whether to choose a or b. Then an interpolation is required.[/TD]

[TD valign="bottom"][/TD]

 

[mbrooke]

So can we say Table 8 and 9 is more accurate than the IEC?

 

[mbrooke]

Shoot me an Email. And I'd also message the mods about making this forum do PDFs and Excels. This is a pro forum, not a meme chat forum.

 

[Julius Right]

I'm not sure, but since NEC is the law, I have to say it's the right value.

 

[mbrooke]

I'm not sure, but since NEC is the law, I have to say it's the right value.

Well, truth be law isn't always correct. Much the opposite actually, case in point...

Table 250.122 Inadequate?

Is 250.122 Inadequate in high fault current applications or where devices have been selectively coordinated? Some eye opening statments from the IEEE- 1) 2) 3)

forums.mikeholt.com

 

[Julius Right]

Usually, in catalogue it is written the maximum resistance as per standard.
In IEC 60228 standard we get the maximum diameter of circular copper conductor, the number of strands and the maximum conductor resistance at 20oC .The maximum diameter is connected with minimum resistance and maximum resistance with minimum diameter.
For instance, 120 sqr.mm copper conductor stranded class 2 -it is close to 4/0 [107 sqr.mm], of 37 strands, maximum diameter 14.5 mm [2.07 mm each strand]. That gives the maximum 124.7 sqr.mm cross section area and the minimum resistance of 0.1383 ohm/km. The maximum conductor resistance at 20oC it is 0.153 ohm/km.
In DOHA catalogue it is written only 0.153 ohm/km.
If we take 120 sqrmm, the nominal area, then we get 0.1437 ohm/km.
120 sqr.mm is 236826.5 CI and as per N_McG Rdc at 20oC is 1.02*10.371/236826.5=0.04467 ohm/kft. =0.14655 ohm/km.
If we take 120 sqrmm, the nominal area, then we get 0.1437 ohm/km.
120 sqr.mm is 236826.5 CI and as per N_McG Rdc at 20oC is 1.02*10.371/236826.5=0.04467 ohm/kft. =0.14655 ohm/km.
So, what is our resistance?
120 sqr.mm 0.1437 ohm/km, maximum 0.153 [6.47% more], minimum 0.1383 [3.76 less] or else.
There are clients which prefer standard off [waste] material if it is cheaper. So ±10% ?
In this case an error of 1-4% it does not matter.

 

[mbrooke]

I'm not sure, but since NEC is the law, I have to say it's the right value.

Do you know a multiplier that will work with chapter 9 table 9 for 150*C, ohms/kft?

I know you have the DC numbers for IEC-60287 at 150*C, but I'm now reasoning like you that chapter 9 table 9 might be more conservative or accurate.

Ideally it wouldn't matter for VD, however I'm working on the final short circuit temperature of 150*C starting at 75*C for tap conductors.

 

[Carultch]

Can you explain when to use coated or uncoated ?

Coated means tin-coated-copper, rather than the conductive core of the wire being entirely copper. Uncoated is the most common, unless you have a specific reason for using tin-coated-copper wires.