TECHNOLOGY BRIEF

A Method to Calculate the Capacity of a Reel or Spool

It is often necessary to determine how much of a given cable or wire will fit onto a spool or reel. This article provides a relatively simple method to calculate the approximate maximum length if you know the cable's diameter and the reel's dimensions.

Providing wire or cable on reels or spools is the most common method of packaging used today. The package is easy to ship or transport, it protects the cable, it is easy to store, and allows the product to be partially unspooled as it is consumed or cut to the lengths needed. Reels also provide many of the same benefits to the manufacturer, plus they permit packaging or spooling finished cable at very high production speeds.

Cable is most often supplied in specific lengths such as 500 feet or 1000 feet per package or reel. However, longer random lengths may also be provided if requested by a customer. In either case manufacturers or customers must select the most economical reel for the given cable and length required. This article proves a simple calculation method that approximates the maximum cable length that may be wound onto a given sized reel or spool. Knowing this length, you may also estimate the package weight by applying the cable's unit weight.

Since various cable designs have very different dimensions, you must first determine the cable's diameter, then calculate a “reel factor”. This reel factor is determined by the following dimensions:

Note from the above drawing that there is usually a small amount of unused flange (Dimension U) because cable is seldom wound to the top of the reel. This allows a cardboard or other protective layer to be applied over the product.

Dimensions for shipping reels normally used by Quabbin Wire are listed in the table below with the calculated reel factor.

The factor for a given reel or spool is calculated using the following equation. All dimensions must be in inches.

Factor = (H + B) X (H) X (T) X (0.262)

Using the reel factor and the cable diameter, you may now calculate the approximate maximum cable length in feet that will fit on that reel. Simply divide the reel factor by the diameter of the cable in inches squared.

Length of Cable = Reel Factor / (Cable Diameter)2

For example, Quabbin Wire's plastic 10 ½ inch spool can contain about 500 feet of a 0.25 inch diameter cable. As a comparison, almost 700 feet of the same cable will fit on the plywood 12 inch reel.