# wire length help

#### pntdxtr

##### Member
Please help , i am trying to figure out some leftover wire on reels i have on the jobsite . The reels are #6 thhn and want to calculate the length to see if i need to order more ..Thanks

#### pntdxtr

##### Member
thanks

thanks

I have seen this before but was looking for the formula so i don't have to spend the \$350.00

#### jusme123

##### Senior Member
did you look at the wire to see if it has the length numbers on it (I know its obvious, but just checking)?

#### nhfire77

##### Senior Member
Cheaper version would be the triplett 3271 TDR. (\$152 on amazon) You have to calibrate it to the wire you are measuring based on a known length of that cable.

Megger has a nicer \$900 tdr as well

#### ActionDave

##### Moderator
Staff member
Weigh it.

I have also stopped by the supply house and asked them to measure it for me.

#### LEO2854

##### Esteemed Member
Please help , i am trying to figure out some leftover wire on reels i have on the jobsite . The reels are #6 thhn and want to calculate the length to see if i need to order more ..Thanks
You can always pull them off the reels and see how log they then just roll them back up again ,or have your helper roll them up..:lol:

#### infinity

##### Moderator
Staff member
Weigh a new full reel of the same size and manufacturer, then weigh the used reel. Find the weight to feet ratio and do the math.

#### ActionDave

##### Moderator
Staff member
Weigh a new full reel of the same size and manufacturer, then weigh the used reel. Find the weight to feet ratio and do the math.
You may not even need to get too deep into the math.

A new 500' reel of #8 weighs 31 pounds. So if my partial reel more than fifteen pounds I know I have at least 200' available. That may be all the information I need.

#### tkb

##### Senior Member
Measure the circumference of the reel and count the amount of times it wraps the core.

Wraps x circumference = length on reel

#### Hv&Lv

##### Senior Member
Ohm it out, then divide by .00051 :roll:

#### ptonsparky

##### Senior Member
Ohm it out, then divide by .00051 :roll:
\$2200 to \$3200 for a Micro ohm meter makes the other options pretty reasonable.:jawdrop:

Weight or measure once and count the wraps. If we aren't sure, it is cheaper to have new rolls on hand than to come up short on a long pull.

You haven't kept track of what you have pulled in so far?

#### scook56

##### Member
Approximate Length

Approximate Length

As tkd stated, Measure the circumference of the reel and count the amount of times it wraps the core.

Wraps x circumference = length on reel

Also: C = Pi * D: Circumference = 3.14 * Diameter. Approximate length for hand coils, too.

#### Hv&Lv

##### Senior Member
\$2200 to \$3200 for a Micro ohm meter makes the other options pretty reasonable.:jawdrop:

Weight or measure once and count the wraps. If we aren't sure, it is cheaper to have new rolls on hand than to come up short on a long pull.

You haven't kept track of what you have pulled in so far?
Yea, I was just throwing that out there for a chuckle. Didn't think it would even be considered...
Seriously, I have taken my reels to the supply house and had them spool it off through their counter for the lengths. Usually less than 100 feet though.

Of course, I bought these same reels from them, and they know it.

#### handy10

##### Senior Member
Without being too specific about what is wrong with the information given so far, I suggest that the problem is more complex than simple formulas will give. An exact formula would be quite difficult, but there is some possibility of a reasonable approximation. Here is some notation:

t=radius of the wire and its insulation
Ri = inner radius of the spool
Ro = outer radius of the spool

First assume that the flanges of the spool are so close together that the wire will look like the way sailors coil rope on the deck of a ship(flat coil). To make such a coil, one would start at the core of the spool (the inner radius Ri) and and make n turns to get to the outer edge of the flange (the outer radius, Ro). Then we can find the number of turns to be

n=(Ro-Ri)/t

The first time around the core requires 2pi(Ri+t) amount of wire, since the radius of the second turn has been increased by t, the second turn will require 2pi(Ro+2t) amount of wire. The last time around will require 2pi(Ri + nt) wire. If all of the turns are added, the formula I get is

Wire on one flat coil = 2n(pi)(Ri + n(n+1)t)

To complete the calculation, you must know how many flat coils are on the spool. If D is the distance between flanges then

m=D/t where m is the number of flat coils.

Therefore, the total length is

Total = m(wire on one flat coil).

Of course, wire does not wrap on a spool exactly like a bunch of flat coils. The second layer on the spool will fall in the space between coils of the first layer, think of three tangent circles. For this and some other reasons, the formula is only an approximation. However, all of the information required can be obtained with a ruler or tape measure.

#### K8MHZ

##### Senior Member
Don't forget Murphy's Constant.

In applied Murphology, Murphy's Constant is 0.97.

That makes the math simple. You just multiply the amount of spooled material you need by 0.97 and that is how much will be on the spool. So if you need 100 feet, there will only be 97 on the spool.

#### Johnmcca

##### Senior Member
How true? But I never had it happen to me.:lol:

#### ptonsparky

##### Senior Member
So Handy10s very fine formula times K8s fudge factor & you got the answer...to short again.

#### Cow

##### Senior Member
So Handy10s very fine formula times K8s fudge factor & you got the answer...to short again.
And now the crew is standing around waiting to pull the wire back out and start over. Meanwhile, you could have spent the same amount of money on a meter and had something to show for it....

Just saying....it doesn't take too many man hours for one of these meters to pay for itself. We used to count wraps, but it doesn't make sense financially, when you figure how much time you spend doing it in say, a year or so. Not to mention a lot more of our spools got used up because we knew what we had on them. When we used to rough guess how much was on it, we'd get scared we'd be short and order new spools instead. You can imagine how many leftover spools end up in inventory this way....we've really trimmed back now using a meter.

#### handy10

##### Senior Member
Truth compels me to admit that my wonderful formulas are wrong. For starters, the number of wraps and the number of layers, must be obtained by dividing by the diameter of the wire, not the radius. In retrospect, I like the Murphy's law the best. A variant is to measure the length of wire required and subtract 10' to find out how much is on the spool.