# Resistance , Length calculation



## seo (Oct 28, 2008)

They do make a cable length meter but I've never seen a book that told you how to do it. You may look at table 8 conductor properties look at the resistance of the conductors use an ohm meter and do some math?


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## BryanMD (Dec 31, 2007)

I've seen charts with the data. Try google.


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## waco (Dec 10, 2007)

The problem with that method is that it is difficult to measure very low resistances very accurately. However, I suppose one could get a rough idea of the conductor's length using that method. I've never tried.


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## TxElectrician (May 19, 2008)

If I knew, I wouldn't use a spool with 187' of wire on a 190' conduit run!


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## collin.thomas (Apr 14, 2008)

Well if you had a meter that could read mΩ you could figure it out using table 8 in the NEC (2008) the table gives you Ω per 1000 feet.

so if you have 300' of 1/0 copper wire you should read .0366Ω

.122 = _X_ = .122*300/1000=.0366
1000 300

if you can get a resistance reading your equation would look like this to find length

4/0 copper @ .0453Ω

.0608 = .0453 = 1000*.0453/.0608=X 
1000 ___ X

X = 745.07'

Of course you would also have to account for temperature, since these listed values are at 75º C, which is another equation in the notes below the table.


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## calimurray (Apr 29, 2007)

> collin.thomas Well if you had a meter that could read mΩ you could figure it out using table 8 in the NEC (2008) the table gives you Ω per 1000 feet.
> 
> so if you have 300' of 1/0 copper wire you should read .0366Ω
> 
> ...


You know all I had was a 1996 code book in the truck and I looked up table 8 , and showed him the ohm/kft for coated wire and he was stumped but I got a 2002 in front of me and I see what your saying now!

Although I noticed you were using the uncoated column.How accurate is the total length using this formula?


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## collin.thomas (Apr 14, 2008)

calimurray said:


> You know all I had was a 1996 code book in the truck and I looked up table 8 , and showed him the ohm/kft for coated wire and he was stumped but I got a 2002 in front of me and I see what your saying now!
> 
> Although I noticed you were using the uncoated column.How accurate is the total length using this formula?


I'm using the uncoated column because I assume that coated means tinned copper, and aluminum is aluminum. Your insulation should not change the resistance of your wire.

I'm guessing this is one of those things that only works well in theory because the table is based on 75 C (167 F). There is a formula that you can use to adjust for temperature though. The meter you are using will also skew the result.


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## shunt trip (Jan 15, 2009)

It is a exact science, or Theory (Electrical). lots of things could skew it, connections, Temperature, moisture, I'm sure you can get close.


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## 480sparky (Sep 20, 2007)

Most day-to-day DMMs that we carry are not sensitive enough to be able to determine the length of wire on spool.

UEI makes just such a meter, and if I'm not mistaken, it needs to be at the same temp as the wire, and has a calibration bar that comes with it that needs to be used just prior to taking the measurement.

Temperature is a big factor. Imagine a spool of 500 sitting out in the cold (cold here is about 10° right now). If you take the meter out of the office or your truck, you'd need to let it acclimate for a couple hours or else your measurement would be way off.

As far as I'm concerned, weighing the wire would be a quicker, cheaper and far more accurate way to determine length.


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## markzs2001 (Jan 30, 2009)

hey I just did this in school .......

Lenght= CMA X R / k


R= resistance 
k= constant for type of wire (copper 10.4 and aluminum is 17)
L= feet
cma= circular mill area (not including insulation)


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## nick (Feb 14, 2008)

*the old one amp one volt method*

Well there is a method to measure resistance of a spool it is not accurate but will get you close enough .And yes you can buy a cable length meter greenlee has them we use greenlees at work its fine . Or use a micro ohm meter or dlr meter , but if you like to make stuff build you a regulated one volt and one amp current regulated power supply put kelvin test leads on it the voltage applied to the spool of wire is one volt the current applied to the spool is one amp now do the math 1/1= ? It will give you resistance of that spool in ohms law now for that circuit its just a simple voltage & current regulated power supply . To measure any wire you first test one foot of length of that spool then after you measure the total length just divide one foot of resistance into total ? I built my own for fun and its as accurate as greenlees only thing ive got to calulate were theres does it for you mine cost 50 dollars thers is hundreds there is no ohm meter which can measure less than one ohm comments best to ya


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## Tim (Feb 9, 2009)

Can any one tell how to meg a house. I have been told by the inspector i need to meg this house and give him all my readings. Thanks


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## brian john (Mar 11, 2007)

1. Disconnect all loads including stove and other difficult to disconnect appliances.
2. Turn off all CB's and make panel safe
3. Disconnect all neutrals.
4. Check neutral to ground for all circuits
5. Check black/red conductor to neutral and ground.

Preferable start with a lower voltage 50, 100 or 250 VDC to verify all loads are disconnected.

Document all test at 1000 VDC typically for one minute, record temperature and humidity.


If you IM me with your email I can provide standard form for recording test.

BE SAFE BE CAREFUL.


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## JohnJ0906 (Jan 22, 2007)

brian john said:


> 1. Disconnect all loads including stove and other difficult to disconnect appliances.
> 2. Turn off all CB's and make panel safe
> 3. Disconnect all neutrals.
> 4. Check neutral to ground for all circuits
> ...


Remove lamps from luminaires. (Take the light bulbs out of the lights :whistling2
Remove GFCIs, dimmers, or any other electronic device, and wire nut the circuit through.
Unplug everything.


And Tim, welcome to the forum!


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## Tim (Feb 9, 2009)

Thanks Brian My Email is [email protected] The forms will be very he helpfull.

Thanks


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