Estimating cable footage by weight.

The arithmetic, worked through

Say a reel of 3/C 500 MCM copper feeder at 15 kV comes off a truck. Gross weight on the scale is 4,180 lb. An empty reel of the same size weighs 310 lb, so net cable is 3,870 lb. Manufacturer data for that construction gives roughly 4.4 lb per foot. Divide 3,870 by 4.4 and the answer is about 880 feet on the drum.

Then round down. Take something off for the wrap left on the reel, for termination stubs cut back at each end, and for the fact that published weight per foot is nominal rather than exact. Call it 850 feet. That figure is close enough to start a review and honest enough to be corrected at inspection, which is where the last of the doubt gets resolved anyway.

• Gross weight, tare and net weight, recorded separately
• Weight per foot from the manufacturer data sheet
• Feet equals net weight divided by weight per foot
• Round down for nominal weights and cut back ends

Weigh a similar empty reel instead of guessing the tare. On large steel and timber reels it runs to several hundred pounds.

Where the weight per foot comes from

Every manufacturer publishes a weight for each construction, usually per 1,000 feet or per metre. The figure you want is the one for the exact cable: conductor size and material, number of conductors, voltage class, insulation, shield and jacket. Take it from the data sheet if the part number survives, and from a cable chart if it does not, then check the chart is in the same units as your scale.

Do not borrow a figure from a neighbouring size. Weight per foot climbs quickly with conductor size and again with voltage class, because the insulation wall and the metallic shield add material that carries no current at all. A 600 V cable and a 15 kV cable of the same conductor size are nowhere near close enough to substitute for each other.

Units cause more errors than anything else. A data sheet in pounds per 1,000 feet, a scale reading in kilograms and a reel measured in metres will all produce a number, and three of them will be wrong. Convert once, write the converted figure down, and keep the same unit through the calculation so somebody else can check it.

Why the estimate breaks down

The first problem is the reel. Gross weight includes it, and the tare varies with reel size, material and how wet the timber has become. A wooden reel that has sat through two hurricane seasons can weigh far more than the figure stamped on its flange, and the error goes straight into the footage as if it were cable.

The second is the cable itself. Water in an open end, dirt packed into a coil and a jacket that has taken up moisture all add weight that is not length. Copper and aluminium also differ by a factor of roughly three in conductor weight for the same size, so misreading the metal throws the estimate further than any weighing error ever will.

• A wet or mud filled coil, weighed as if it were dry cable
• A reused reel carrying something other than the tag describes
• Mixed cut lengths weighed together and divided by one figure
• Armour, interlocked armour or lead sheath adding weight without length
• Very short remainders, where the tare error swamps the answer

Weight is a good estimate and a poor measurement. It will tell you whether a reel holds 200 feet or 2,000. It will not tell you whether it holds 780 or 830.

When to weigh and when to count wraps

Weigh when the reel is large, the tag is missing and the construction can be looked up. A platform scale and a data sheet will get within a few percent on a full reel, which is normally enough to decide whether a collection is worth arranging. Weigh in the morning if the reel has been out in the rain, and again if it has drained.

Count wraps when the reel is small or the cable is expensive to misjudge. Counting the visible wraps across the drum and multiplying by the circumference at that layer gives a reasonable figure for a part used reel, and it needs nothing more than a tape and a ladder. On medium voltage, where the drum is deep and the layers are hard to see, weight is usually the better route.

Whichever method you use, write the working down. Gross, tare, net, weight per foot, the arithmetic and the rounded result. A buyer who can see how a figure was reached will accept a range. One who is handed a bare number with no basis will discount it, and rightly, because there is nothing in it to check.