Estimating conductor footage from reel weight

What the method actually measures

A scale measures mass, not length. The method converts one to the other through a published figure: the weight of one thousand feet of that exact construction. Almost everything that goes wrong with an estimate goes wrong at that figure. Almost nothing goes wrong at the scale.

So the order is fixed. Identify the construction. Look up its weight per 1,000 ft. Find the empty weight of the reel. Take the gross weight from a scale ticket. Subtract, divide, multiply by a thousand. Done out of order, the same arithmetic still produces a number with four significant figures, and that number means nothing.

The four numbers

Each of the four has a source, and the source is worth writing beside the result. An estimate that shows its working can be checked by the person receiving it. One that does not is a guess with a decimal point.

• Construction. Size in AWG or kcmil, stranding, conductor material, insulation and jacket, read from the tag or the jacket print.
• Weight per 1,000 ft. The manufacturer’s published figure for that exact construction. A bare-conductor table will not do for shielded cable.
• Tare. The empty weight of that reel. Many wood reels carry it stencilled on the flange; failing that, weigh an identical empty reel.
• Gross. A certified platform scale. On a small lot, weigh the loaded vehicle and the empty vehicle and take the difference.

Published weight per 1,000 ft

Every standard construction has a published weight, and the tag usually names the construction closely enough to find it. Where the conductor is bare there is a short cut worth carrying in your head. One thousand circular mils of copper weighs about 3.03 lb per 1,000 ft. The same thousand circular mils of electrical-grade aluminum weighs about 0.92 lb.

Multiply the kcmil by those figures and the answer lands close to the published number for stranded bare conductor. The values below are that arithmetic, rounded. They are an illustration of the method, not a substitute for the table for the reel you are holding.

The arithmetic, worked twice

Two reels of bare conductor, both on wood. The figures are round so the sum can be followed. They are examples, not quotations, and they say nothing about what any lot is worth.

Weighing in practice comes down to the scale available. A platform scale reads the vehicle and the reels together, so the loaded and the empty ticket have to come from the same scale on the same day, and the difference is the payload. A floor scale weighs reels one at a time, which is slower and more accurate. Either way, note the graduation on the ticket: a scale that reads to the nearest 20 lb cannot support a footage figure quoted to the nearest foot.

Where the estimate breaks, and how to report it

Insulated cable is heavier than bare conductor by the weight of its semiconductive layers, insulation, metallic shield and jacket. Divide an insulated reel by a bare figure and the footage comes out high. On medium voltage cable the error is not small, because the insulation and shield are a serious share of the weight per foot.

Wet wood, water standing in the barrel, wrappers, battens, lagging and banding all appear in the gross and none of them is conductor. A reel that has sat in a Georgia yard through a summer carries several of those at once. Look at the tare as a fraction of the gross: on light aluminum conductor the tare can be more than half the ticket, so an error there is an error in the answer, not a rounding difference.

A reel carrying two constructions defeats the method — a remnant wound over a longer length, a splice joining two sizes, a tag that describes the original wind while the barrel holds something else. Estimate per construction, or state plainly that the reel is mixed.

Then check the estimate against the reel itself. Sequential footage marks at the exposed end are direct evidence of remaining length, and a wrap count across the barrel is a reasonable second opinion. When weight and marks disagree badly, the marks are right about the cable and the weight is right about the reel, and something between them has not been accounted for.

Writing it down is part of the method. Reel number, construction, gross, tare, the weight per 1,000 ft used, and the result marked estimated. Four lines and a photograph of the scale ticket will carry a reel through a review faster than a page of description, because the reader can check the arithmetic instead of trusting it.