A purchase order asks for 500 meters of cable on a drum, and the supplier answers with a drum size. Both sides treat the question as settled. What has actually been settled is the last of three decisions, and it is the one with the least influence on whether the cable arrives usable.
A drum is not a container that happens to hold cable. Its dimensions are the outcome of a bending limit, a protection margin and an arithmetic capacity, applied in that order. Read the capacity first and the drum gets designed around the wrong constraint.
The parts of a drum, and what each one governs
Four measurements describe a shipping drum, and only one of them is about quantity.
Part | What it is | What it governs |
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Flange diameter | The total size of the reel. The side discs that guard each end of the core | The outer protection, and most of the capacity |
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Barrel diameter | The drum core, the circular surface the cable is wound onto | The smallest bend the cable will be held at for months |
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Traverse | The inner width between the two flanges | How many turns sit side by side in each layer |
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Free-board | The gap between the outermost usable winding surface and the flange edge | Whether the outer layers survive rolling and shipment |
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The trade-off between the first two is stated plainly in the trade literature: a larger barrel offers a broader safe bending radius for the cable but also reduces the capacity of the reel. There is also a degenerate case worth knowing. The barrel diameter cannot be larger than the flange diameter, because at that point there is no radial space left to wind into and the capacity is zero.
The four measurements of a shipping drum
The capacity formula, and the assumptions printed inside it
A published engineering form of the drum capacity calculation, from a wire rope supplier’s technical page, reads:
L = (A + D) × A × B × K
Where A is the depth of the winding space on the drum, B is the width of the drum between the flanges, D is the barrel diameter, d is the cable diameter, K is a constant equal to 0.2618 divided by the square of the cable diameter, and L is the length in feet when the other measurements are in inches.
The constant is the interesting part of that expression. 0.2618 is pi divided by twelve, so K carries both the circumference factor and the conversion from inches to feet. Take it apart and the formula is three quantities multiplied together.
Factor | Expression | What it counts |
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Turns per layer | B ÷ d | How many cable diameters fit across the traverse |
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Layers | A ÷ d | How many cable diameters fit in the radial winding space |
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Mean circumference | pi × (A + D) | The circumference at the middle of the wound mass |
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Written in metric units, with every length in millimeters, the same expression is:
L = pi × (A + D) × A × B ÷ d²
Start with the published version and the metric one looks unfamiliar. Start with the three factors and both versions are the same sentence: the length on a drum is the turns per layer, times the layers, times the average circumference of a turn.
Capacity as three multiplied factors
Worked once, so the sensitivity is visible
Numbers make the shape of the expression obvious. Take a cable of 12 mm outside diameter on a drum with a 400 mm barrel, a 1,000 mm flange and a 600 mm traverse.
The radial winding space is (1,000 − 400) ÷ 2, which is 300 mm. Turns per layer are 600 ÷ 12, which is 50. Layers are 300 ÷ 12, which is 25. The mean circumference is pi × (300 + 400), or pi × 700 mm. Multiplied out, that is 2,748,894 mm, or about 2,749 meters.
Now change one thing at a time and watch what the expression does.
Change, one at a time | Resulting length | Effect |
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Flange 1,000 mm to 1,200 mm | 4,189 m | Up 52 percent |
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Traverse 600 mm to 750 mm | 3,436 m | Up 25 percent |
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Barrel 400 mm to 500 mm | 2,454 m | Down 11 percent |
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Cable diameter 12 mm to 15 mm | 1,759 m | Down 36 percent |
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What each single change does to the length
Two conclusions fall straight out of the arithmetic, and both matter commercially.
The first is that capacity falls with the square of the cable diameter, because the diameter appears in the turns per layer and again in the layers. A cable half as thick carries four times the length on the same drum. That is why a quotation offering a drum of a given size means very little until the cable size is attached to it.
The second is that flange diameter is the strongest single lever available. Raising it by 20 percent adds half again to the length, because a taller flange increases both the radial winding space and the mean circumference at the same time.
The assumption that does not survive contact with a real drum
The published formula arrives with two conditions attached, and it states them itself. It is based on uniform winding, and it assumes the same number of wraps in every layer. The source then adds a qualification worth reading twice: the second assumption is not strictly correct, but there is no appreciable error unless the traverse is small relative to the flange diameter.
That is a statement about when to trust the arithmetic, and it is easy to read backwards. The approximation holds on a drum that is wide relative to its flange, where each layer settles into much the same count of turns as the one below it. It holds less well on a narrow, deeply flanged drum, where the count of turns per layer drifts as the winding builds and the outer layers do not sit as neatly as the inner ones. On that shape, treat the result as an estimate with a margin rather than a number to promise.
The barrel is decided first, and it is decided by the cable
Here is the decision the capacity formula is not allowed to make. The barrel is the surface the cable is held against for months, including the weeks between the factory and the site. Its diameter fixes the radius of the innermost layer, which is the tightest bend the cable will ever be asked to hold.
If a construction’s minimum bend radius is k times its own diameter, then the smallest barrel that respects it has a diameter of 2k times the cable diameter, since a diameter is twice a radius. The value of k belongs to the cable’s own specification, and it is the one input to the drum design that the drum supplier cannot obtain by looking at the reel.
The innermost layer deserves the attention, because it is the one nobody inspects. The outer wraps are visible every time the drum is rolled, and the cable end gets checked when the reel is opened. The first two or three layers sit against the barrel, at the tightest radius on the reel, carrying the compressive load of everything wound above them, for as long as the drum exists. By the time the drum is unwound at site, that layer has been held in a bend that nothing about the finished installation will reverse.
So the order is fixed. The cable sets a minimum barrel. The minimum barrel removes radial space and with it some capacity. Only then does the arithmetic say how much length remains. Working the calculation in the other direction, picking a drum for its capacity and discovering later that the barrel is too tight, produces a reel that looks correct on the packing list and arrives with cracked insulation.
The flange decides whether the cable arrives intact
Capacity is not the same as usable capacity, and the difference is the free-board.
Some reels have round-edged flanges, which leave a gap between the flange edge and the usable flange surface, and the winding can only be stacked up to that surface. That empty space is the free-board or edge clearance. Its purpose is protection: if a reel is filled completely, the cable can be damaged in shipping or storage, and in some countries the free-board dimension is regulated rather than left to the supplier.
There is a second discount on top of it. Where winding is done by hand rather than by machine, the tightness of the turns and layers varies, so suppliers commonly quote a recommended capacity that is a rough estimate of eighty percent of the maximum. A recommended figure is not a limit. It is a supplier telling you what they are willing to stand behind.
Put the two together and the honest statement of a drum’s capacity is the geometric figure, less a free-board allowance, less a fill margin, quoted as a recommendation. Anyone assessing a quotation should ask which of those three numbers they are being given.
What to exchange before the drum is built
Item | Why it is needed |
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Cable outside diameter over the jacket | It enters the calculation squared |
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Total length, and whether it must be a single length | Splitting a length changes the drum, and the joint changes the cable |
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Minimum bend radius, and the document it comes from | It sets the barrel, which sets everything else |
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Maximum flange diameter | Handling, transport, and whether the reel fits the equipment on site |
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Maximum drum gross weight | A drum of dense cable can reach a handling limit before a capacity limit |
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Required free-board or edge clearance | Decides whether the arithmetic answer may be used as it stands |
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Machine winding or hand winding | Determines whether the ninety-percent answer is available |
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Whether the drum is returnable or one-way | A returnable drum is a different purchase from packing |
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When the drum is chosen by something other than capacity
Capacity is not always the binding constraint, and on dense cable it often is not. A drum of power cable can reach a gross weight the site cannot lift long before the winding space is full. The question then changes from how much fits to what the drum and its contents weigh, which carries three separate limits: the lifting equipment at the factory, the equipment at the receiving site, and the vehicle in between. Where that happens, the answer is more drums holding less cable each, not a bigger drum.
FAQ
Does a larger drum always hold more cable?
No. Capacity rises steeply with flange diameter and in direct proportion to traverse, but it falls with the square of the cable diameter, and enlarging the barrel to satisfy a bending limit reduces it further. A large drum holding a thick cable can hold less than a smaller drum holding a thin one.
Why does doubling the cable diameter cut capacity to a quarter?
Because the thickness is counted twice. Twice the diameter means half as many turns across the traverse and half as many layers in the same radial space, and two halvings is a quarter.
Can a drum be wound right up to the flange edge?
Practice leaves free-board, the gap between the outer winding surface and the flange edge, because a reel filled flush can be damaged in shipping or storage, and in some countries that dimension is regulated. Where winding is done by hand, a recommended capacity of about eighty percent of the maximum is commonly quoted.
Conclusion
A drum’s capacity is a real number and a useful one, but it is an output. The barrel comes from the cable’s bending limit, the flange comes from a protection margin and a handling limit, and only then does the arithmetic report how much length is left over. Read in that order, the calculation is a check. Read in the other order, it is a way of ordering the wrong drum with confidence.
Cable on drums, sized to the installation
ZONG Cable can state the drum dimensions, the barrel diameter, the traverse, the free-board and the gross weight for a given construction and length before an order is placed, so the drum and the site equipment agree. Send us the cable type, the outside diameter, the length and the minimum bend radius you are working to, and we will set out the drum that satisfies all three. The range sits under cable, the drum is part of the packing so the options are set out under packaging solutions, the bend limit during installation rather than winding is covered in the guide to pulling tension and bend radius, winding limits on flexible constructions are covered under industrial flexible cable, the batch evidence that travels with a drum is explained under test and inspection, and for OEM support drum marking and packing are fixed with the label program.
Sources
- Samm Teknoloji, Cable Reel Capacity Calculator, clarification of cable reel or cable drum elements. Read directly for this article: the definition of flange diameter as the total size of the reel, the side circular guards covering each end of the flat edges of the drum core; the definition of barrel diameter as the size of the drum core, together with the statements that a large barrel offers a broader safe bending radius for the cable but also reduces the capacity of the reel, and that the barrel diameter cannot be larger than the diameter of the two flanges or the capacity would be zero; the definition of traverse as the inner width of the reel, which determines how many rolls of cable can be stacked next to each other in each circular layer, and that a longer traverse means more capacity; the definition of cable diameter as the thickness of the cable on the reel, and that a smaller diameter allows more rolls horizontally and more layers vertically; the definition of edge clearance as free-board, the distance between the flange edge and the usable flange surface, together with the statements that some countries regulate the free-board dimension because a completely full reel can suffer damage in shipping or storage, and that where winding is done manually the tightness of rolls and layers may differ so that a recommended capacity of roughly eighty percent of maximum is used. https://telecom.samm.com/cable-reel-calculator
- Industrial Rope, Calculating Reel Capacity. Read directly for this article: the statement that it is virtually impossible to calculate the precise length that can be spooled on a reel while the method given provides a sufficiently close approximation; the variable definitions for A as the depth of the space on the drum, B as the width of the drum between flanges, D as the drum barrel diameter, K as a constant for a given diameter, and X as clearance; the expression of K as 0.2618 divided by the diameter squared; the table of K factors by diameter; and the accompanying note stating that the formula is based on uniform winding on the reel, that it will not give correct results if the winding is non-uniform, that it assumes the same number of wraps in each layer, that this assumption is not strictly correct, and that there is no appreciable error unless the traverse of the reel is quite small relative to the flange diameter. https://industrialrope.com/wire-rope/calculating-reel-capacity/
- ZONG Cable range. https://zh-zong.com/cable/
