AC Resistance at 30*C vs 75*C

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mbrooke

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.
 
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 ?
 
mbrooke said:
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.
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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
 
winnie said:
No.

First off the factor is 0.823 (you are off by a factor of 10)
Click to expand...

My bad, typo. 😳 See this instead:

Why the Limit Here?

So isn't this essential test gear for you guys? It is for a professional electrician here in the UK. Typical values can range from high hundreds of amps to low thousands in a domestic setting. We use it in several modes - to determine the PSCC (perspective short circuit current) ie. Will a...
forums.mikeholt.com

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
Click to expand...



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.
 
mbrooke said:
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.
Click to expand...

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

[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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Julius Right said:

[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]
Click to expand...


Why does distance between phases change skin effect?
 
This is an attached list for maximum Rac [ohm/km] at 30oC
 

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