Cable reel capacity calculator

The four measurements, and where to take them

Measure the empty reel with a tape, in inches. The flange is the round side; take its diameter edge to edge through the arbor hole. The traverse is the inside width between the two flanges, where the cable actually sits, not the out-to-out width across the bolts. The drum is the barrel the first layer wraps around; measure its diameter, or wrap a tape around it and divide the circumference by 3.14.

The cable's outside diameter is on its spec sheet, usually in inches or millimetres; divide millimetres by 25.4. For a cable already in hand, a pair of calipers across the jacket is close enough. If you are reading a size off a reel stencil, makers usually paint it as flange x traverse x drum, the same order this calculator asks for.

The formula behind the number

NEMA WC 26, the shipping reel standard for wire and cable, gives an approximate capacity formula that Anixter reprints in its Wire and Cable Technical Information Handbook: footage = 0.262 x T x (H - C) x (D + H - C) / d squared. T is the traverse, D the drum diameter, d the cable outside diameter, C the clearance left empty below the flange rim, and H the height of the flange above the drum, which is half of flange minus drum.

The calculator writes the same thing a little differently. It takes A, the depth actually available for winding, as (F - D) / 2 - C, then multiplies (A + D) x A x T x 0.2618 and divides by d squared. Wire rope makers such as Kulkoni and Paducah Rigging publish exactly this form, with the constant K = 0.2618 / d squared. Both constants are pi divided by 12, which turns inches of wrap into feet of length; NEMA rounds it to 0.262, so the two forms agree to within about a tenth of a percent.

A quick check from Newfoundland and Labrador's hazard alert on drum capacity: a 16 inch drum with 2 inches of flange depth and 24 inches between the flanges holds (16 + 2) x 2 x 24 x 0.262 = 226 feet of 1 inch rope. Put 20 x 24 x 16 with zero clearance and a 1.0 inch cable into the calculator above and it returns the same 226 feet.

How much clearance to leave

Clearance is the empty band between the last layer of cable and the outer rim of the flange. It protects the cable when reels are rolled, chocked or stacked, and it is the input people most often forget. In its Table 10.2 Anixter uses 1 inch for flanges of 16 to 28 inches, 2 inches for 30 to 48 inch flanges, and 3 inches for 54 to 96 inch flanges. Superior Essex bases its copper reel charts on 2 inches. Industrial Rope suggests about 2 inches for wire rope unless the end fittings need more, and Kulkoni asks for at least one rope diameter.

The calculator starts at 2 inches. Raise it for a large reel, a reel that will be rolled over rough ground, or a cable with a pulling eye or fitting on the outer end; lower it only for a small reel of light wire that will be handled by hand.

Capacities from the NEMA WC 26 table

These are the published footages from Anixter's Table 10.2, Capacities of Typical Shipping Reels per NEMA WC 26, for cable of 0.50 inch and 1.00 inch outside diameter. Each one reproduces from the formula above with the clearance shown, which is a useful way to check that a calculator, or a quote, is reading the reel the same way.

Why real reels hold less

The formula assumes round cable laid in perfectly even layers from flange to flange. Real winding is rarely that tidy: the turns ride over each other at the ends, the last layer is often part full, and stiff or large cable does not sit tight against the layer below. The buyawg.com engineering guide says its version of the NEMA method builds in a 5% factor and 95% traverse use for this reason. Treat the result as a ceiling for planning, and order reels with some room to spare.

The formula also does not cover paralleled or triplexed assemblies, flat cable, or cable with a jacket that is far from round. For those, ask the cable maker for its own reel table.

Weight can run out before space does

A reel that has room for the footage may still be the wrong reel if the cable is heavy. Okonite's reel table lists a cable weight capacity for each size beside its dimensions; for example a 36 x 24 x 17 reel is rated for 2,000 pounds of cable, a 50 x 32 x 24 for 3,000 or 4,800 pounds depending on construction, and a 96 x 54 x 56 for 6,000 or 12,000 pounds. Okonite also notes that each cable maker checks the gross cable weight against the chosen reel's loading weight. Multiply the footage by the cable's weight per foot and compare it with the reel's rating before you plan a wind.

Drum size rules for fiber and shielded cable

Footage is not the only limit; the drum must also be big enough that the cable is not bent too tightly. NEMA WC 26, as reproduced in Anixter's Table 10.1, sets minimum drum diameters as a multiple of cable outside diameter: 20 times for fiber optic cable, and never less than 12 inches, with a note that some makers recommend 30; 10 times for nonshielded cable up to 2,000 volts; 14 to 20 times for tape shielded cable; 16 times for wire armored cable.

Corning's re-spooling procedure for fiber optic cable says the drum should be at least 8 inches or 30 times the cable diameter, whichever is larger, and warns that a drum smaller than recommended can cause increased attenuation and permanent cable damage. If you are re-spooling, check the drum first, then the footage.

From a footage to a reel lot

1. 01

   Find the size that fits

   Try a few flange and drum combinations until the footage clears your run with some margin, and the drum meets the bend rule for your cable.

2. 02

   Press Price this reel

   The calculator copies the size, the cable diameter and the footage into the request form, so the reel lot ticket starts with the numbers already filled in.

3. 03

   Add the count and the ZIP

   Tell us how many reels and where they go. We answer with matching empty wooden reels, their condition, and the freight to your site.