Estimating remaining footage from weight

Why the answer depends on construction

A reel weighs what its cable weighs plus what its drum weighs. Change the insulation wall from 175 mils to 420 mils, add a copper tape shield and a jacket, and the same 500 kcmil conductor gains tens of pounds per hundred feet. Change the conductor from copper to aluminum and the metal weight falls by roughly two thirds. This is why no honest buyer quotes footage from weight without first fixing the construction, and why a seller who says only that a reel is heavy has told the desk almost nothing.

The other half of the problem is the drum. Wooden reels of the same flange diameter differ in weight by construction and by how much water they have absorbed. Steel reels are heavier and their weight is more consistent. A reel that has stood outdoors for years can carry a good deal of water in the timber, and a soaked drum will make a short length look generous. If the tare is on the tag, use it. If it is not, weigh an empty reel of the same size and say that is what you did.

The arithmetic, and the numbers to use

Conductor weight follows from the cross-section, so it can be calculated rather than looked up. Copper weighs about 0.32 pounds per cubic inch and aluminum about 0.098, so a thousand feet of a thousand circular mils comes to roughly 1,950 lb in copper and 590 lb in aluminum. From that, any size follows by proportion: copper weighs about 1.95 lb per thousand feet per kcmil, aluminum about 0.59. The table below rounds those figures and leaves out insulation, shielding and jacket, which add a few percent on building wire and considerably more on thick-wall medium-voltage cable.

Using a scale

Any scale rated for the load and sitting on hard, level ground will do: a platform scale at a scrap yard, a crane scale under a sling, or a truck scale with the reel on the bed and the truck weighed before and after. The truck-scale method is the least accurate, because two large numbers are being subtracted to produce a small one, and a few pounds of mud or rain on the bed swamps the answer. If you use a truck scale, weigh the empty truck immediately before and after, on the same scale, with the same fuel load.

Then subtract the tare and divide by the weight per foot for that exact construction. Say 1,180 lb gross on a reel with a 180 lb tare leaves 1,000 lb of cable. A 500 kcmil copper single conductor with a 420 mil wall, a tape shield and a jacket runs to something close to 1.8 lb per foot, so that reel holds about 550 ft. If the tag says 600 ft, the two are close enough that nobody needs to argue. If the tag says 1,500 ft, one of the numbers is wrong and the reel needs a flange measurement.

Weigh the reel as it stands, not after stripping the outer wraps. The wraps are part of the length, and the first few feet on a used reel are often the dirtiest and most damaged — they still count as material until they are cut off and thrown away, at which point the footage claim should drop by the same amount.

Flange measurement, and the half-full trap

Footage on a reel is proportional to the cross-sectional area of the wound cable, which means it goes with the square of the diameter, not with the diameter. That single fact causes more bad estimates than anything else in the yard. Measure the barrel diameter, the full-flange diameter, and the diameter of the cable build as it stands now, then compare areas.

Take a 60 inch reel with a 24 inch barrel, full to the flange. The full build has an area proportional to 60 squared minus 24 squared, which is 3,600 minus 576, or 3,024. Suppose the cable now sits at 42 inches across. That build is 1,764 minus 576, or 1,188, which is about 39% of the full reel. A reel that looks two thirds full by eye is carrying well under half its original length, and a buyer who says so is not being harsh — they are doing the arithmetic.

This method needs the original full footage and the two diameters, so it works best on a reel with a tag. Without the tag it still narrows the range, and combined with a diameter measurement over the cable it will usually confirm or contradict a claimed length.

Counting wraps

The third method counts what is physically there. Cable is wound in layers, each layer running across the traverse, so the length in one layer is the circumference at that layer's diameter multiplied by the number of turns across the width. Count the turns visible on the outer layer, measure the diameter at that layer, and multiply: a layer at 40 inches diameter has a circumference of about 10.5 ft, so 24 turns across the width is roughly 250 ft in that layer. Each layer inward is shorter, which is why the total is not the outer layer multiplied by the number of layers.

Wrap counting is slow, awkward on a reel that has been standing outdoors with dirt between the wraps, and unreliable where the outer layer is damaged. Its real value is as a sanity check on a suspicious tag, or on a reel where only a rough order of magnitude is needed to decide whether a truck is justified. A count that lands within ten percent of the weight-based figure is a good outcome; a count that disagrees by half means somebody should look again.

What each method is worth, and when to stop

The table below is about method, not about the world: it describes how far each approach typically drifts when the construction is known and the reel is intact. Construction uncertainty sits on top of all of them and can be much larger.

• Weighing is worth it on any reel above a few hundred pounds, where the metal value repays the effort of getting it onto a scale.
• It is not worth it on a small part reel of building wire that will be priced as recovery material regardless.
• It is not worth it where the construction is unknown, because the divisor is a guess and the answer inherits the guess.
• It is rarely worth arguing about a hundred feet on a reel of several thousand; the argument costs more than the difference.