Figuring wire length with a ohmmeter?

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J.P.

Senior Member
Location
United States
I know they make a tool for this, but I have a ohmmeter and a immediate need to know how much wire is on a spool.

The wire is #3 copper and I'm hoping for 600+ FT. How do I figure the resistance?
 

Nom Deplume

Senior Member
Location
USA
Measure the distance around the outside of the reel, count the wraps and multiply.

Quicker and just as accurate as trying to use an ohm meter.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
140429-0942 EDT

J.P.:

An ordinary ohmmeter won't do it. From the ITT "Radio Engineers Handbook" the resistance of #3 Annealed copper wire at 20 C (68 F) is about 0.197 ohms per 1000n ft. Thus, your resistance for 600 ft would be about 0.118 ohms.

To measure this you need a 4 terminal ohmmeter. This you can make by connecting a known DC current of about 10 A thru the wire. Then measure the voltage drop from two points just on the wire directly, not on the connector to the wire. At 10 A the voltage drop over 600 ft is 0.118*10 = 1.18 V.

See my photos at P19 thru P21 at http://beta-a2.com/EE-photos.html .

.
 

kwired

Electron manager
Location
NE Nebraska
The larger the conductor the lower the resistance, the more accurate of a meter you will need (that can display ohms in at least two decimals to the right - three would be better. Most general purpose meters will not measure to this low of a level.


Gar mentions you need a 4 lead meter - which is probably correct for accuracy, but a two lead meter that is otherwise pretty accurate to the levels I mentioned may still get you fairly close, but his alternate idea sounds like a good plan, but you seem to be in a hurry and it could take you longer to find what you all need (this first time anyway) then to just unreel the conductor and measure it with a tape measure.
 

J.P.

Senior Member
Location
United States
I ended up counting loops and sort of guessing at layers. It started off as a 5000 ft roll and had been in the shop for a while now. Seems to be less than 1000 ft left.

It's some grounding wire left over from a fairly large job. Have a chance to use it and some 500 up.

Thanks for all of your advice. If anybody is free tomorrow we are pulling in 2000 ft of 3ph 3/0 with 5 LB's in it........... I would be happy to give out directions to the jobsite:)
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I ended up counting loops and sort of guessing at layers. It started off as a 5000 ft roll and had been in the shop for a while now. Seems to be less than 1000 ft left.

It's some grounding wire left over from a fairly large job. Have a chance to use it and some 500 up.

Thanks for all of your advice. If anybody is free tomorrow we are pulling in 2000 ft of 3ph 3/0 with 5 LB's in it........... I would be happy to give out directions to the jobsite:)

I think we are busy today, and tomorrow or I would be glad to offer a couple guys to help you.
 

Ragin Cajun

Senior Member
Location
Upstate S.C.
I ended up counting loops and sort of guessing at layers. It started off as a 5000 ft roll and had been in the shop for a while now. Seems to be less than 1000 ft left.

It's some grounding wire left over from a fairly large job. Have a chance to use it and some 500 up.

Thanks for all of your advice. If anybody is free tomorrow we are pulling in 2000 ft of 3ph 3/0 with 5 LB's in it........... I would be happy to give out directions to the jobsite:)

Unless I am mistaken, if it's bare wire the resistance measurement will be invalid.

RC
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I know they make a tool for this, but I have a ohmmeter and a immediate need to know how much wire is on a spool.

The wire is #3 copper and I'm hoping for 600+ FT. How do I figure the resistance?

As others have indicated, a standard ohmmeter is not suitable for this purpose due to the extremely low resistance values involved. Here is a document that will help you understand low resistance measuring: http://www.testequipmentdepot.com/megger/pdf/low-resistance-testing.pdf
 

kwired

Electron manager
Location
NE Nebraska
The larger the conductor being measured the lower the resistance per foot will be, making it more difficult to be accurate with a standard ohmmeter.
 

kwired

Electron manager
Location
NE Nebraska
I do have a Fluke 29 (IIRC that is what it is) that will display to .01 ohms, not entirely certain of accuracy. I do measure 1000' reels of 12 and 14 NM cable often with it, but you need to have both the meter and reel @ same stable temp or you can take readings just a few seconds apart that will vary. You also need to zero the meter before taking the reading, again if temp is not stabilized you can get a different reading or need to re-zero just a few seconds later. It seems to be accurate enough for what I need, and is more accurate then trying to weigh these reels from past experiences doing it that way.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
You could try the low-tech method: Have a helper grab the end of the wire and start walking. When the reel is completely run out, have the helper measure the length with a tape measure. :lol:
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You could try the low-tech method: Have a helper grab the end of the wire and start walking. When the reel is completely run out, have the helper measure the length with a tape measure. :lol:

Or the helper could do it on-line by having a surveyor's wheel with them while he/she walked. But wouldn't that require a Cat III certified helper?

Oh wait, it's ground wire and not energized, so probably not...
 

Jon456

Senior Member
Location
Colorado
You could try the low-tech method: Have a helper grab the end of the wire and start walking. When the reel is completely run out, have the helper measure the length with a tape measure. :lol:
Another low-tech way that is neater and requires less space is to get a second, empty spool and set both spools up side-by-side on cable pulling stands. Have an assistant wind the wire off the filled spool and onto the empty spool, while you use a measuring wheel pressed against the wire on the spool to measure its length. ;)
 

Besoeker

Senior Member
Location
UK
Another low-tech way that is neater and requires less space is to get a second, empty spool and set both spools up side-by-side on cable pulling stands. Have an assistant wind the wire off the filled spool and onto the empty spool, while you use a measuring wheel pressed against the wire on the spool to measure its length. ;)
Or weigh the empty spool and the full spool. The difference gives you the weight of the wire on the full spool.
Cut one metre (39.37 inches) of the wire from the full spool and weigh that. Divide the weight of wire on the full spool by that, subtract one metre (for the bit you cut off) and you have the length in metres.
Simples....
:p
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I am sorry, that procedure will not work here in the States for two reasons:
1. We don't know how to measure a meter.
2. We don't know how to divide Stones by Stones when reading the scale.
:)

Tapatalk!
 

Besoeker

Senior Member
Location
UK
I am sorry, that procedure will not work here in the States for two reasons:
1. We don't know how to measure a meter.
2. We don't know how to divide Stones by Stones when reading the scale.
:)

Tapatalk!
Cheeky blighter............
First, it's a metre not a meter. You measure amps and volts with a meter, not length.
And stones are painful and best avoided....
:p
 
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