How to estimate cable footage from weight
Weight gives you a length only after you know three things: the net weight, the exact construction, and the published weight per thousand feet. Get one of them wrong and the answer is confidently wrong.
- Net weight, not gross
- Published weight per 1,000 ft
- An estimate, not a count
Every reel without a tag invites the same shortcut. Weigh it, divide by something, quote a footage. The shortcut works, but only inside a narrow set of conditions, and the people who get burned are the ones who skip the construction step. A reel of 4/0 aluminium and a reel of 4/0 copper can sit side by side at similar gross weights and hold wildly different lengths. The weight is real. The divisor is where the error lives.
So treat footage by weight as an estimating method with a stated tolerance, not a measurement. It is good for deciding whether a reel is worth listing, for sanity-checking a tag, and for testing a claimed length against a scale ticket. It is not good enough to put on an invoice as a certified length, and it should never be presented to a buyer as though a counter had run over the cable.
The method, step by step
Weigh the reel and take the tare off properly
Use a platform scale or a truck scale that is certified for trade, and record gross weight, date and the scale identifier. Then subtract the tare, which is the empty reel, not the reel plus lagging plus banding plus pallet plus whatever water the wood has taken on. The tare is stamped on many returnable reels and appears on the shipping papers. If it is unknown, weigh an empty reel of the same size and type.
Identify the construction before you divide
You need conductor count, size, voltage class, insulation, jacket, shield and armour, because the weight per thousand feet changes with every one of them. A single-conductor 600 V building wire and a three-conductor 15 kV shielded cable of the same conductor size differ by a wide margin once insulation walls, tape shields, bedding and jacket are counted. Without the construction there is no divisor, and a guess at the divisor is the whole error.
Take the divisor from published data
The number you want is the manufacturer’s approximate weight per 1,000 feet for the finished cable, published in the catalogue or the mill certification, not the conductor weight alone. Conductor data in NEC Chapter 9 gives circular mil area and strand counts, which lets you check the metal side of the arithmetic. Insulation and jacket weights come from the cable maker’s tables. If the construction is obsolete and no table exists, say so rather than borrowing a neighbour’s figure.
Work the conductor arithmetic as a check
Take a labelled example, not a catalogue figure. One thousand kcmil is one million circular mils, and one circular mil is about 0.785 millionths of a square inch, so the metal area is roughly 0.785 square inches. Over 1,000 feet that is 12,000 inches of length and about 9,400 cubic inches. Copper at roughly 0.321 pounds per cubic inch gives about 3,000 pounds of conductor per 1,000 feet. Aluminium at about 0.098 gives roughly 900 pounds, or a third.
Know what moves the answer
Compact and compressed stranding change the diameter and slightly change the metal content for the same nominal size, so use the column that matches the stranding class on the reel. Insulation wall thickness, armour, interstitial fillers and factory splices all add weight that carries no length. A reel that has been outside holds water in the wood and between the wraps. All of these push the estimate the same way, which is toward overstating footage.
State a tolerance and reconcile with the tag
Publish the estimate as an estimate, name the divisor and its source, and give the figure to a sensible round number rather than to the pound. Where a tag exists, put the tag length and the weight estimate side by side and explain any gap before a buyer finds it. Two honest numbers that nearly agree are worth more than one confident number that cannot be supported when the reel is weighed at the other end.
Before the next call
The method earns its keep in the storehouse, on the day you are deciding what to list and in what order. Run it across a hundred reels and you will find the ones whose tags are wrong, the reels that were re-spooled, and the pallet that is not what the label claims. That is worth more than the footage figure itself.
What it cannot do is substitute for a counter. If a length has to stand behind a contract, measure it or have it measured, and keep the record.
Questions on this
Can I use the weight per foot from a scrap yard instead?
Only for the metal, and only if you are pricing recovery material. A scrap factor tells you what the copper or aluminium content weighs, not what the finished cable weighs, so dividing gross weight by a metal-only factor overstates footage badly. Insulation, jacket, shield and armour are real weight and no length. Use the finished-cable figure for footage and the metal figure separately, for content.
How accurate is the result in practice?
Close enough to plan with and not close enough to invoice against. Accuracy depends almost entirely on whether the construction and the tare are known exactly. Where both are documented, the estimate usually lands near the tag. Where either is guessed, the figure can be out by enough to change the order of the reels. Write down which case you are in.
The reel holds two different cables spliced together. What then?
Treat each section as its own item and stop treating the reel as one length. A factory splice or a field joint means two constructions, two weights per foot and two potential buyers. Photograph the joint, record its position if you can see it, and list the reel with a note. Most buyers will price a spliced reel as shorter usable lengths plus recovery material.
Put the guide to work
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