Cable resistance - ohms/1000' table

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sespllc

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Just a quick question: I am compiling a spreadsheet of cables sizes and resistance values; there are numbers based upon specific temps and other variables, but I am looking for good ball park numbers for preparing conceptual designs to clients. When the actual cable is selected, then we get to the specifics; Any ideas on where I can collect this data? Just ballpark up to 2000KCMIL.
Thanks
 
Don't forget to adjust for temperature. Also, you may need the reactance and to adjust for skin effect, etc depending on what you are using the data for. See the notes & table 9 as well.
 
Be careful how you write it out...your subject line is incorrect (unless you actually mean to create a table that is only accurate for 1000' lengths of cable).

The standard insulation resistance equation will get you ohms for 1000', not ohms per 1000'. Big difference there.
 
drbond24 said:
Be careful how you write it out...your subject line is incorrect (unless you actually mean to create a table that is only accurate for 1000' lengths of cable).

The standard insulation resistance equation will get you ohms for 1000', not ohms per 1000'. Big difference there.
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Please elaborate.:-?
 
mivey said:
Please elaborate.:-?
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Ohms/1000' (ohms per 1000') as the OP wrote it would mean that the insulation resistance varies proportionally to length. Longer cables would have higher insulation resistance values, and shorter cables would have lower insulation resistance values. That might make sense, but it isn't correct.

If you double the length of a cable, you half it's insulation resistance. Length and insulation resistance are inversely proportional. Longer cables have more parallel paths to ground and therefore a lower insulation resistance.

The equation for insulation resistance is IR=k*log(D/d) where k is an insulation resistance constant and D and d are the diameters over and under the insulation, respectively. The number you get out of this equation is the resistance for 1000' of your cable at 60 degrees F. If your length or temperature are different then some scaling will need to be done.
 
drbond24 said:
Ohms/1000' (ohms per 1000') as the OP wrote it would mean that the insulation resistance varies proportionally to length. Longer cables would have higher insulation resistance values, and shorter cables would have lower insulation resistance values.
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I did not get the impression the OPs question had anything to do with insulation resistance.

I think they are asking about the conductor resistance.
 
I was under the impression he was not talking about insulation resistance but AC/DC resistance. The DC resistance being based on the cross sectional area, length, and volume resistivity (at the specified temperature) of the conductor, with the AC resistance increasing beyond the DC because of skin effect, proximity effect, and hysteresis & eddy current losses.

I also mentioned the reactance in case is was not purely the resistance he was looking for.


Add: too late.
 
mivey said:
Don't forget to adjust for temperature. Also, you may need the reactance and to adjust for skin effect, etc depending on what you are using the data for. See the notes & table 9 as well.
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This fantastic post will probally get ignored, so I am drawing attention to it again. All your results are useless without temp compensation.
 
All ideas are good ideas in this case

All ideas are good ideas in this case

1) Chapter 9 table 8 gives me a very good place to start;

2) The Insulation R gives me food for thought;

3) and the Formulae was something I considered;

as I was looking for the "gut" check numbers that comes with years of exp in this business. Those numbers from the Senior Masters and EE's with real experience lend credance to concepts.

Thanks to all :eek:)
 
090420-1029 EST

sespllc:

I would prefer a table of the 20 deg C copper values for solid wire. Then handle temperture in two parts. The first part is maximum ambient temperature and the second part is the addition of the temperature rise from self heating in the wire. The self heating rise is a function of a number of factors.

The NEC tables make some assumptions and apply some scaling factors to the basic 20 deg C values. These assumptions are not arbitrary, but are conservative and based on experience.

I believe the users should understand the basic theory and what the assumptions are even when they use a table that has generalized this information.

So putting the NEC tables into a spreadsheet and adding a discussion of the basis of the tables would be useful. The actual use for your spreadsheet would determine how you structure the tables and calculations.

.
 
iwire said:
It's funny how we read things sometimes. When this type of thing happens to me I immediately wonder what other things I have read but misunderstood.:-?
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Does this mean you'll restore some of my posts that were deleted due to your misunderstanding them? :wink:
 
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