Two capacities, and the wrong one usually gets checked
Fill is the number people go looking for, because it is the one that resembles conduit fill and because it is easy to picture. Load is the one that quietly governs on power installations. They are independent, and the ratio between them is set entirely by what kind of cable is in the tray.
Take the default numbers on this page: twelve power cables just under an inch across, plus thirty small communications cables. The power group is a fraction of the cable count and most of the weight. Add another dozen data cables and the fill number moves and the load number barely does. Add three more power cables and the load number moves sharply. A tray that is fine on both counts can be at its load rating after a single change that looked trivial on a drawing.
The depth figure is a floor
The calculator divides the total cable cross-section by the tray width to get a depth. That is the depth the cables would sit at if they packed perfectly, filling every gap, lying flat, never crossing. They do none of those things.
Round cables lying beside each other leave voids between them — a bundle of identical circles laid in a tray occupies noticeably more depth than its cross-sectional area suggests. Cables cross over each other wherever a group leaves or joins. The pile rides up on the outside of every bend. At a point where a branch drops out, everything above it lifts over. So the computed depth is a lower bound and the real one is higher, sometimes considerably. If the number is close to your usable depth, it is already too close.
What the tray itself weighs, and what a span carries
| Figure | Where it comes from |
|---|---|
| Weight per foot of each cable | The cable data sheet |
| Weight per foot of the tray section | The tray manufacturer |
| Load rating in pounds per foot | The tray manufacturer, for that tray at that span — it drops as the span grows |
| Usable loading depth | The tray manufacturer, and it can be less than the rail height |
| Fill percentage that applies | The code your jurisdiction has adopted, varying with the cable type |
Notice that four of the five come from a manufacturer and the fifth comes from the adopted code. This page supplies none of them, which is why the rating and fill limit fields ship blank and the calculator restates whatever you enter beside the computed figure rather than passing judgement on it.
The span point is worth dwelling on. A load rating is always quoted with a span, because a tray is a beam and a beam carries less over a longer span. Reading a rating off a catalogue without reading the span it belongs to is how a tray gets supported at fourteen feet on a figure published for eight.
Heat is a separate problem entirely
None of the arithmetic here says anything about temperature, and it is the omission most worth being aware of. Cables lying in a bundle in a tray cannot shed heat the way a single cable in free air can, and the deeper the pile the worse it gets. How that affects the current a conductor may carry depends on the cable construction, the arrangement in the tray, the depth of the pile, the ambient temperature and the code adopted where you are. It is a real calculation with real consequences — it can cost you a conductor size — and it is not this one. Conductor sizing generally lives on the wire size calculator and the voltage drop calculator, and derating for a bundle in a tray is a question for a licensed electrician working from your adopted code.
Where this sits among the other pages
Tray is one way to get cable across a building; raceway is the other. For a round pipe, fill is on the conduit fill calculator when the conductors are given by insulation type and trade size, and on the low-voltage pathway calculator when the cables are given by outside diameter — the second of those also covers the jam ratio, which has no equivalent in a tray because nothing is being pulled through a bore. Where cable leaves a tray and enters a raceway, the pull itself is on the cable pull tension calculator, and the raceway material list is on the conduit run take-off calculator.
The parts this page will not touch
A cable tray is not a walkway, a ladder or an anchor point, and it is not rated as any of them. Point loads from a person, from a transition fitting or from a bundle hanging out of the tray are not in the arithmetic here and are a question for the tray manufacturer.
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 full can a cable tray be?
That depends on the code your jurisdiction has adopted and on the type of cable in the tray, and the rules differ between cable types in ways a general figure would misrepresent. This page will not name one. Enter the limit you are working to and the calculator restates it beside the computed fill so the two are visible together, and leave it blank and it simply reports the fill percentage without comment.
Why is my tray at its load rating when it looks half empty?
Because fill and load are independent, and large power cable is dense. A dozen inch-diameter power cables lying in the bottom of a tray occupy a modest fraction of the cross-section and carry most of the weight, while thirty small data cables on top add to the fill and almost nothing to the load. Whichever of the two runs out first is set by what is in the tray, not by how full it looks.
Does the calculated depth match what I will actually see in the tray?
No, and it is not meant to. The figure is the total cable cross-section divided by the tray width, which assumes perfect packing with no voids and no crossings. Round cables leave gaps between them, they cross wherever a group joins or leaves, and the pile rides higher on the outside of every bend and at every drop-out. Treat the number as a floor. If it is already close to the usable depth you entered, the real installation will exceed it.
Where does the tray load rating come from?
The tray manufacturer, for that specific tray section at a specific support span. The span matters as much as the tray does, because a tray is a beam and a beam carries less as the span gets longer. A rating quoted without its span is not usable. That is why the field is blank by default: the number depends on hardware this page has no knowledge of.
Does this account for cables getting hot in a bundle?
No. Cables packed in a tray cannot shed heat the way a single cable in free air does, and the current a conductor may carry has to be reduced accordingly. How much depends on the cable construction, the arrangement, the depth of the pile, the ambient temperature and the adopted code. It is a separate calculation that can cost you a conductor size, and it belongs with a licensed electrician working from the code that governs your installation.