AC Resistance at 30*C vs 75*C

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mbrooke

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Does multiplying by the 75*C AC resistance values in chapter 9 table 9 by 0.0823 give the accurate resistance at 30*C? I can't find a 30*C AC table.
 

JEFF MILLAR

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There is a simple temperature conversion formula. The data in NEC Table 8 & 9. is great. info. for 75 C rated resistance values, for coated and uncoated copper. I want to know where the 30 C requirement comes from.
OKONITE. publishes resistance values at 20 C. and. 25 C, coated and uncoated. Can you explain when to use coated or uncoated ?
 

winnie

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Does multiplying by the 75*C AC resistance values in chapter 9 table 9 by 0.0823 give the accurate resistance at 30*C? I can't find a 30*C AC table.

No.

First off the factor is 0.823 (you are off by a factor of 10)

Second is that this gives the change in DC resistance, not AC resistance. This will be approximately correct for small conductors where DC resistance dominates.

Jon
 

mbrooke

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No.

First off the factor is 0.823 (you are off by a factor of 10)

My bad, typo. 😳 See this instead:


Second is that this gives the change in DC resistance, not AC resistance. This will be approximately correct for small conductors where DC resistance dominates.

Jon



Do you know by how much that change would be for AC resistance? I'm assuming reactance stays about the same, but both AC and DC resistance go up as the conductor temp goes up.
 

winnie

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Do you know by how much that change would be for AC resistance? I'm assuming reactance stays about the same, but both AC and DC resistance go up as the conductor temp goes up.

My educated guess is that the bulk of AC resistance is simply DC resistance scaled for skin effect, so it will change almost in proportion to DC resistance. (As I recall, skin depth depends on resistivity, so there will be a second order effect of the skin depth changing as temperature changes. ) some AC 'resistance' is probably insulation dissipation with a different tempco, but that will also be a minor effect.

And I concur that reactance will remain essentially unchanged.

Jon
 

mbrooke

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My educated guess is that the bulk of AC resistance is simply DC resistance scaled for skin effect, so it will change almost in proportion to DC resistance. (As I recall, skin depth depends on resistivity, so there will be a second order effect of the skin depth changing as temperature changes. ) some AC 'resistance' is probably insulation dissipation with a different tempco, but that will also be a minor effect.

And I concur that reactance will remain essentially unchanged.

Jon


Thanks

So, er, do you know numbers fr AC resistance or how we can find them? I know I am really curious about this value.

Ideally I'd like to make a table for 30*C, 75*C, 150*C and 250*C.
 

JEFF MILLAR

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AC resistance values are published in the nec table 9. Check out the formula how to calculate voltage drop using the NEC method.
 

Julius Right

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[TD valign="bottom"]

The conductor resistance in d.c. system for different temperature depends on the temperature only.
For example, for copper if we have the conductor temperature at 20oC for 30oC we have to multiply this resistance by Kt=(234.5+30)/(234.5+20).
In a.c. system you have to add skin effect and proximity effect .Rac=RdcT*(1+ys+yp) RdcT =d.c. resistance at T temperature.
Follow the IEC 60287-1-1 equations in
order to calculate ys and yp. You need d.c. resistance at 30oC, the frequency, the conductor diameter and the distance between the phases.
[/TD]
 

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  • IEC 60287-1-1 Proximity effect.jpg
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  • IEC 60287-1-1 Table 2.jpg
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mbrooke

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[TD valign="bottom"]

The conductor resistance in d.c. system for different temperature depends on the temperature only.
For example, for copper if we have the conductor temperature at 20oC for 30oC we have to multiply this resistance by Kt=(234.5+30)/(234.5+20).
In a.c. system you have to add skin effect and proximity effect .Rac=RdcT*(1+ys+yp) RdcT =d.c. resistance at T temperature.
Follow the IEC 60287-1-1 equations in
order to calculate ys and yp. You need d.c. resistance at 30oC, the frequency, the conductor diameter and the distance between the phases.[/TD]


Why does distance between phases change skin effect?
 

Julius Right

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Electrical Engineer Power Station Physical Design Retired
This is an attached list for maximum Rac [ohm/km] at 30oC
 

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    AC Resistance per km.jpg
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