Why the tape across the pack lies to you
A reel is a stack of concentric layers, and each layer is one cable diameter thick, so the diameter of the pack grows by twice the cable diameter with every layer. What changes faster than the diameter is the length in each layer, because every wrap goes round a circle whose circumference is growing.
Run the default numbers: a 30 inch reel with a 12 inch drum and a 16 inch traverse, holding three quarter inch cable. Twenty one wraps sit across the traverse. The first layer, going round a mean diameter of 12.75 inches, holds about 70 feet. The outermost layer, at a mean of about 24.75 inches, holds about 136 feet on exactly the same twenty one wraps — nearly twice as much. Wind the reel only to half its radial depth and it is not holding half its capacity; it is holding roughly a third.
Which is a useful thing to know in two directions. Looking at a part-used reel in the van, there is less on it than it looks. Pulling from a full one, the first few hundred feet come off fast and the last few hundred take much longer than the first did, because each layer is shorter than the one before.
The arithmetic
| Quantity | How it is worked out |
|---|---|
| Wraps per layer | Traverse width divided by cable outside diameter, rounded down |
| Layers that fit | Flange diameter less twice the clearance, less the drum diameter, divided by twice the cable diameter, rounded down |
| Mean diameter of layer n | Drum diameter plus the cable diameter times (2n minus 1) |
| Length of layer n | Wraps times pi times that mean diameter, in inches, divided by twelve |
| Total | The sum over every layer that fits |
The clearance below the flange rim is worth setting honestly. Winding a reel right out to the edge of the flange is how cable ends up over the top of the flange in transit, and once a few wraps have gone over the edge they trap the ones underneath.
What the geometry does not know
Everything above assumes cable that lies exactly where it should. It never does. The first layer sits against the drum differently from the way later layers sit on each other. Where the winding reverses at each flange, wraps cross. On a job site nothing is level wound at all — cable goes back onto a reel in whatever order it comes, and the resulting pack is loose and irregular.
That is what the random winding option is for, and it is deliberately crude: it takes a flat percentage off every layer, and the percentage is yours to choose. There is no honest way to model an untidy pack, and pretending otherwise would be worse than admitting it. Where the length genuinely matters — a run that has to reach — the answer is to measure the cable, not to calculate the reel.
Weight is the figure that catches people out
Cable weight per foot is a small number and reel capacities are large ones, so the product surprises. The default numbers here come to a little under a thousand feet at 0.42 pounds per foot, which is around 390 pounds of cable before the reel itself is counted. Scale it up to a large reel of power cable and the gross weight goes into four figures quickly, before the reel itself is counted, and wooden reels are heavier than they look.
Two hazards follow, and both are named here without any procedure attached because procedure is not calculator content. A reel is a heavy round object with a small contact patch and a high centre of gravity: on any slope it wants to roll, and once it is rolling nothing stops it by hand. Getting one off a vehicle, standing it up or laying it down are the moments people are hurt. How a particular reel is handled is a question for the supplier and for whoever is running the site.
The second use of the weight figure is the pull itself. Weight per foot is the primary input to the tension arithmetic on the cable pull tension calculator, and back tension from the reel and its payoff arrangement feeds into the same calculation — it gets multiplied by every bend downstream of it, so reel drag at the feed is not a small effect at the far end.
Related pages
Whether the cable fits where it is going: the conduit fill calculator for conductors given by insulation type and trade size, the low-voltage pathway calculator for cables given by outside diameter, and the cable tray fill and load calculator if it is going into a tray. The raceway material list is on the conduit run take-off calculator, and if the job is structured cabling by the drop rather than by the reel, the ethernet drop calculator works in boxes of cable instead.
The parts this page will not touch
A raceway you are pulling into is very often a raceway that has something else in it, and the assumption that a circuit is dead is the assumption that kills people in this trade. Whether a conductor is energised, how that gets established, and who is permitted to work on or near it are matters for a licensed electrician working to the rules adopted where you are. Nothing on this page establishes any of it and nothing here is a procedure.
This page does not supply limits and cannot judge yours. Fill percentages, support and strap spacing, minimum bend radii, tray loading rules, cover depths and every other installation limit come from the code your jurisdiction has adopted and from the manufacturer of the product you actually bought, and the two do not always agree. The arithmetic here runs on figures you type in. Whether the result is acceptable is a conversation with a licensed electrician and with the inspector who signs the job off.
Questions people ask
How do I tell how much cable is left on a part-used reel?
Measure the diameter across the wound pack and enter it. The calculator works out how many complete layers that diameter represents and adds up their lengths. It counts complete layers only, so a partly finished outer layer is not included and the reel usually holds slightly more than the figure shows. Measure at more than one point across the pack, because a reel that has been pulled from is rarely a clean cylinder and the widest measurement is the one that reflects the outer layer.
Why is a half-full reel not holding half the cable?
Because the layers are not equal. Every wrap in a layer travels round a circle whose diameter is two cable diameters larger than the layer beneath it, so the outer layers hold considerably more than the inner ones on the same number of wraps. On the default reel here the outermost layer holds roughly twice what the innermost does. Wound to half its radial depth, such a reel is carrying about a third of its capacity, not half.
What is the difference between level wound and random wound in the calculator?
Level wound assumes neat wraps lying side by side, which is the geometric best case and what a factory reel approximates. Random wound applies a flat percentage reduction you choose to every layer, standing in for cable that went back on the reel in whatever order it came. The allowance is deliberately crude because there is no honest way to model an untidy pack. Where the length genuinely matters, measure the cable rather than calculating the reel.
Why does the calculator ask for clearance below the flange edge?
Because winding out to the rim is how cable ends up over the top of the flange during transport, and once a few wraps have crossed the edge they trap everything under them. Leaving the pack short of the rim keeps the cable contained. The figure is yours — it depends on the reel and on how it will be moved — and it directly reduces the number of layers that fit, so it changes the capacity.
Does the reel weight matter for anything other than lifting it?
Yes, twice over. Weight per foot is the primary input to pulling tension, because a straight length of raceway costs weight times length times friction and a vertical rise costs the full weight of the cable in it. And the drag of the reel and its payoff shows up as back tension at the feeding end of the pull, which then gets multiplied by every bend in the run. A little resistance at the reel is not a little resistance at the far end.