A tray is the one pathway where the cable count is not set by a ratio. Ask how many cables fit in a 300 mm tray and the honest answer is that the question is incomplete: the number depends on which of five tray constructions you have, on whether the cables are power or signal, and on whether any single cable is large enough to move the calculation into a different rule. The one figure most often quoted for this job, 50 percent, is real, but it belongs to a narrow case that a tray full of power cable never meets.
The reason is in the definition, and it is worth reading before any of the numbers.
The tray is defined as a structural system
Article 392 does not describe cable tray as a raceway. It defines a cable tray system as a unit or assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways. That word is doing the work. A raceway encloses conductors and its fill rules are ratios of interior area; a tray supports them in the open, and its rules are written as a structural and thermal problem instead.
The scope clause names the constructions the article covers: ladder, ventilated trough, ventilated channel, solid bottom, and other similar structures. Those are not cosmetic variations. Each one changes the fill number, which is the subject of the next two sections.
Provision | What it says |
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Uses permitted | Service conductors, feeders, branch circuits, communications circuits, control circuits and signaling circuits — and tray installations are expressly not limited to industrial establishments |
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Identified for the use | Cable trays and their associated fittings must be identified for the intended use |
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Exposure to sun | Where exposed to direct rays of the sun, insulated conductors and jacketed cables must be identified as sunlight resistant |
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Nonmetallic tray | Permitted in corrosive areas and in areas requiring voltage isolation |
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Uses not permitted | Not in hoistways, and not where subject to severe physical damage |
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The five tray constructions named in the scope
Where the 50 percent figure actually comes from
There are exactly two percentage limits in the fill rules, and both carry conditions that are easy to drop.
The 50 percent figure sits in the rule for ladder or ventilated trough trays whose usable inside depth is 150 mm (6 in.) or less and which contain multiconductor control and/or signal cables only, with nothing else in the tray. Under those conditions the sum of the cross-sectional areas of all cables at any cross-section must not exceed 50 percent of the interior cross-sectional area.
The companion figure for the same cable mix in a solid bottom tray is 40 percent, not 50.
Both rules then add a provision that matters when someone tries to gain capacity by buying a deeper tray: a depth of 150 mm is used to calculate the allowable interior cross-sectional area of any tray with a usable inside depth greater than 150 mm. A 200 mm deep tray with signal cable in it is calculated as if it were 150 mm deep.
Tray construction | Cable mix | Limit |
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Ladder or ventilated trough, usable depth 150 mm or less | Multiconductor control and/or signal cables only | 50 percent of interior cross-sectional area |
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Solid bottom, usable depth 150 mm or less | Multiconductor control and/or signal cables only | 40 percent of interior cross-sectional area |
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Either, usable depth greater than 150 mm | Same | Calculated at a depth of 150 mm |
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So a tray carrying power and lighting cable is not governed by either row. That case is governed by an area, and the area is read from a table.
Two routes to a fill limit
Fill for power and lighting cable is an area
Where a ladder or ventilated trough tray contains multiconductor power or lighting cables, or any mixture of power, lighting, control and signal cables, the limit is a maximum allowable fill area taken from a table indexed by the tray’s inside width. The published areas are in square millimeters, and the useful ones are these:
Inside width of tray | Ladder, ventilated trough or wire mesh | Solid bottom |
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150 mm | 4,500 mm² | 3,500 mm² |
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300 mm | 9,000 mm² | 7,100 mm² |
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600 mm | 18,000 mm² | 14,200 mm² |
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Two things follow from reading the table rather than a rule of thumb. The sum of the cross-sectional areas is what gets compared, so cable count is an output and never an input — two cables of different diameters consume a tray at different rates. And the solid bottom column is consistently lower, by roughly three quarters to four fifths of the ladder figure, because a solid bottom tray has no openings to release heat.
There is a third configuration that is neither. A ventilated channel or solid channel tray is narrow enough that the rules are written per tray rather than per area: where only one multiconductor cable is installed the cross-sectional area must not exceed one published value, and where more than one is installed the sum must not exceed a lower one.
Two size thresholds, and they are not the same number
This is where most of the confusion lives. There are two size thresholds in the fill rules, they appear in different clauses, and they are not the same size.
For multiconductor cable the threshold is 4/0 AWG. For single-conductor cable it is 1000 kcmil. A tray holding only 500 kcmil single conductors is below the single-conductor threshold and is filled by area; a tray holding only 350 kcmil multiconductor cable is below the multiconductor threshold by a wider margin. Nothing about one threshold tells you anything about the other.
Case | Rule |
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Multiconductor, all cables 4/0 AWG or larger, ladder or ventilated trough | Sum of the cable diameters must not exceed the tray width; single layer |
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Multiconductor, all cables smaller than 4/0 AWG | Sum of cross-sectional areas within the published fill area |
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Multiconductor, 4/0 AWG and larger mixed with smaller cables | The 4/0 and larger cables go in a single layer with nothing placed on top; the smaller cables are limited to the published area minus 30 mm² for each millimeter of diameter of the larger cables present |
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Multiconductor, all cables 4/0 AWG or larger, solid bottom | Sum of the cable diameters must not exceed 90 percent of the tray width |
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Single-conductor, all cables 1000 kcmil or larger | Sum of the diameters must not exceed the tray width; single layer |
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Single-conductor, 250 kcmil through 900 kcmil | Sum of cross-sectional areas within the published fill area for single conductors |
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Single-conductor, any cable from 1/0 through 4/0 AWG | Sum of the diameters must not exceed the tray width |
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Single-conductor, any size | Evenly distributed across the tray |
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Type MV or Type MC, 2001 volts or over | Sum of the diameters must not exceed the tray width; single layer |
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The two size thresholds are not the same point
The subtraction in the mixed case is the part that catches people, because it is a deduction measured in length against an allowance measured in area, and it grows with the diameter of whatever large cable is in the tray. Add a 4/0 cable to a tray of small control cable and the remaining area for the small cable is not the table figure; it is the table figure less a penalty proportional to that cable’s diameter.
Where single conductors are run in parallel, they must be grouped so that each group contains no more than one conductor per phase, neutral or grounded conductor, to prevent current imbalance from inductive reactance — and single conductors not cabled together have to be bound into circuit groups to stop them moving under fault-current magnetic forces.
The tray’s support span is not in the code
The securing and supporting clause opens by saying that cable trays shall be supported at intervals in accordance with the installation instructions. There is no footage figure. The span is a property of the tray system and its load, which is why the number comes from the manufacturer’s documentation and the structural design rather than from the code book.
That is not the same as saying the cables are unregulated, and the clause goes on to handle them:
Situation | Requirement |
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In other than horizontal runs | Cables must be fastened securely to transverse members of the cable runs |
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Where cables enter a raceway from a tray | Supports must be provided to prevent stress on the cables |
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Support of the cables themselves | In accordance with their own articles |
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Cable tray to cable tray, or tray to raceway or equipment, for individual conductors | The distance must not exceed 1.8 m (6 ft) |
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At that transition | The conductors must be secured to the tray and protected from physical damage by guarding or by location |
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So the practical answer has two halves. The tray’s span comes from the installation instructions. The cables’ support comes from whatever article governs those cables — and if the cable is a type whose own article sets a support interval, the tray is how that interval is met. The 1.8 m figure is not a support spacing; it is the maximum gap permitted where a conductor leaves one support system and has not yet reached the next one.
Ampacity: the bundle adjustment does not apply to the tray
Two clauses, and they point the same way.
Cable type | Do the bundle adjustment factors apply? |
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Multiconductor | Only to cables with more than three current-carrying conductors, and only by the count within the cable, not the count in the tray |
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Single-conductor | No. The adjustment factors shall not apply to the ampacity of cables in cable trays |
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The instinct to count every conductor in a full tray and derate against that total is therefore wrong in both cases. What replaces it for single conductors is a set of direct caps on the tabulated free-air ampacity:
Configuration | Cap on the tabulated ampacity |
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600 kcmil and larger, uncovered tray | 75 percent |
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600 kcmil and larger, tray continuously covered more than 1.8 m with solid unventilated covers | 70 percent |
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1/0 AWG through 500 kcmil, uncovered tray | 65 percent |
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1/0 AWG through 500 kcmil, continuously covered more than 1.8 m | 60 percent |
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1/0 AWG and larger, single layer in an uncovered tray, maintained space of at least one cable diameter between conductors | 100 percent of the free-air value |
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Single conductors in a triangular or square group in an uncovered tray, with free airspace of at least 2.15 times one conductor diameter of the largest conductor in the group | Free-air value for conductors supported on a messenger |
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For multiconductor cable, continuously covering the tray for more than 1.8 m with solid unventilated covers caps the ampacity at 95 percent. And where a tray carries a combination of multiconductor and single-conductor cable, both sets of ampacities apply, on the condition that the multiconductor fill area as a percentage of its allowance plus the single-conductor fill area as a percentage of its allowance totals not more than 100 percent.
The tray as equipment grounding conductor has a rating limit
A metallic tray can serve as the equipment grounding conductor, which is one of the reasons tray is chosen over conduit on large projects. The permission is conditional in ways that are worth knowing before the tray is ordered.
The conditions are cumulative, and each one leaves a trace on the bill of materials.
Condition | Requirement |
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Sections and fittings | Identified as an equipment grounding conductor |
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Metal cross-sectional area | Must meet a minimum; for a ladder or trough tray the figure is the total cross-sectional area of both side rails, and for a channel tray or a one-piece construction it is the minimum metal cross-sectional area |
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Marking | All sections and fittings legibly and durably marked to show that cross-sectional area |
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Bonding | Bolted mechanical connectors, or bonding jumpers sized and installed accordingly; mechanically discontinuous segments need a bonding jumper as well |
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Two limits sit in the footnotes and they are absolute. Steel cable tray must not be used as the equipment grounding conductor for circuits with ground-fault protection above 600 amperes. Aluminum cable tray must not be used for circuits with ground-fault protection above 2000 amperes. A project that pushes past those figures needs a separate grounding conductor regardless of the tray’s metal area.
There is also an easier case that is easy to miss. A metal tray containing only non-power conductors needs no grounding conductor treatment at all: it must be electrically continuous through approved connections, or connected with a bonding jumper not smaller than 10 AWG. The article’s own note gives nonconductive optical fiber cable as an example, alongside Class 2 and Class 3 remote control signaling and power limiting circuits.
The other numbers worth carrying
A few provisions do not fit the sections above and are the ones most often discovered at inspection.
Provision | The number |
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Trays containing conductors rated over 600 volts | A permanent, legible warning notice reading DANGER — HIGH VOLTAGE — KEEP AWAY, in a readily visible position on all trays, spaced not more than 3 m (10 ft) |
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Cables over 600 volts sharing a tray with cables rated 600 volts or less | Either the higher-voltage cables are Type MC, or they are separated from the lower-voltage cables by a solid fixed barrier of compatible material |
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1/0 AWG through 4/0 AWG single-conductor cable in a ladder tray | Maximum allowable rung spacing 225 mm (9 in.) |
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Single conductors used as equipment grounding conductors in an industrial establishment | Insulated, covered or bare, and 4 AWG or larger |
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Cable splices inside a tray | Permitted where made and insulated by approved methods, provided they are accessible; permitted to project above the side rails where not subject to physical damage |
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Cables or conductors in bushed conduit used for support or protection from physical damage | No box is required |
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Trays passing through partitions and walls | Permitted where the installation, complete with cables, complies with the requirements for firestopping |
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FAQ
Does the NEC limit cable tray fill to 50 percent?
Not as a general rule. The 50 percent limit applies to ladder or ventilated trough trays with a usable inside depth of 150 mm or less that contain multiconductor control and/or signal cables only. A solid bottom tray with the same cable mix is limited to 40 percent, and a tray carrying power or lighting cable is limited by a fill area taken from a table rather than by a percentage.
Can a cable tray be used as the equipment grounding conductor?
Yes, where the sections and fittings are identified as an equipment grounding conductor, meet a minimum metal cross-sectional area, are marked with that area, and are bonded. Steel tray is excluded for circuits with ground-fault protection above 600 amperes, and aluminum tray for circuits above 2000 amperes.
Conclusion
Cable tray is a structural system that holds cables in the open, so three of the numbers an installer looks up behave differently than they do in a raceway. Fill for power cable is an area from a table, and the table has a different column for each tray construction. Ampacity ignores the bundle count. Support spacing for the tray comes from the installation instructions, and only the cables’ own rules and the 1.8 m transition gap come from the code.
The one figure worth discarding is 50 percent as a general tray maximum. It is a real limit, for one cable mix on one tray construction.
Cable and the trays it goes into
ZONG Cable supplies tray-rated and industrial constructions, and quotes the cable outside diameter with every reel so that a fill calculation can be run against the tray on the drawing rather than against an estimate. Send the tray type, the inside width and the cable list, and we will return the diameters and the cross-sectional areas you need to check the fill. The range sits under cable, the support and spacing rules for the pathway that usually feeds a tray are in the guide to J-hook spacing and cable tray fill, pulling forces and minimum radius through the tray run are covered under pulling tension and bend radius, the cable types built for continuous movement are set out in industrial flexible cable, the firestop requirement where a tray crosses a rated wall is covered in cable penetration firestopping, and tray-ready private-label construction is handled under OEM support.
Sources
- NFPA 70, National Electrical Code, 2011 edition, Article 392, Cable Trays. Read directly for this article: 392.1 Scope, naming ladder, ventilated trough, ventilated channel and solid bottom constructions together with the informational note citing ANSI/NEMA VE 1-2002, NECA/NEMA 105-2007 and NEMA FG 1-1998; 392.2 the definition of a cable tray system as a structural system; 392.10 Uses Permitted, including the list of circuit types, the statement that installations are not limited to industrial establishments, the sunlight-resistant identification requirement, and the wiring methods table; 392.10(B) for industrial establishments, including 392.10(B)(1)(a) on 1/0 AWG single conductors and the 225 mm ladder rung spacing, 392.10(B)(1)(c) on 4 AWG equipment grounding conductors and 392.10(B)(2) on Type MV cable; 392.10(C) on hazardous locations; 392.10(D) on nonmetallic tray in corrosive areas and for voltage isolation; 392.12 Uses Not Permitted; 392.18 Cable Tray Installation, including the complete-system requirement, mechanically discontinuous segments, covers, penetration of partitions and walls under 300.21, the exposed-and-accessible requirement, adequate access, support of raceways and boxes from tray systems, and 392.18(H) on the high voltage warning notice and its 3 m spacing; 392.20 Cable and Conductor Installation, including 392.20(A) on multiconductor cables rated 600 volts or less, 392.20(B) on mixing cables over 600 volts with cables rated 600 volts or less, 392.20(C) on grouping paralleled single conductors and binding circuit groups, and 392.20(D) on single-layer installation of 1/0 through 4/0 single conductors; 392.22(A) Number of Multiconductor Cables, including (A)(1) for ladder and ventilated trough trays with its 4/0 AWG threshold across subparagraphs (a), (b) and (c) and the Sd deduction, (A)(2) for control and signal cables only at 50 percent of interior cross-sectional area with the 150 mm depth provision, (A)(3) for solid bottom trays with its 90 percent width limit and the Sd deduction in Column 4, (A)(4) for control and signal cables only in solid bottom trays at 40 percent, (A)(5) for ventilated channel trays and (A)(6) for solid channel trays; Table 392.22(A) Allowable Cable Fill Area for Multiconductor Cables in Ladder, Ventilated Trough, or Solid Bottom Cable Trays for Cables Rated 2000 Volts or Less; 392.22(B) Number of Single-Conductor Cables, including the even-distribution requirement, the 1000 kcmil threshold, the 250 kcmil through 900 kcmil range, the 1/0 through 4/0 AWG diameter rule, the ventilated channel rule and 392.22(C) for Type MV and Type MC cables rated 2001 volts or over; Table 392.22(B)(1) for single-conductor fill areas and its Sd definition; 392.30 Securing and Supporting, including the requirement that trays be supported at intervals in accordance with the installation instructions, the fastening of cables to transverse members in other than horizontal runs, supports at raceway entries, support of cables in accordance with their own articles, and the 1.8 m limit at transitions; 392.46 on bushed conduit and tubing; 392.56 Cable Splices; 392.60 Grounding and Bonding, including the metallic tray permission, the 10 AWG bonding jumper for trays containing only non-power conductors, the informational note naming nonconductive optical fiber cables and Class 2 and Class 3 circuits, the conditions on steel and aluminum trays used as equipment grounding conductors and the marking requirement; Table 392.60(A) Metal Area Requirements for Cable Trays Used as Equipment Grounding Conductor and its footnotes excluding steel tray above 600 amperes of ground-fault protection and aluminum tray above 2000 amperes; and 392.80 Ampacity of Conductors, including (A)(1)(a) limiting adjustment factors to the current-carrying conductors within the cable rather than the tray, (A)(1)(b) on the 95 percent reduction under continuous solid unventilated covers, (A)(1)(c) on single-layer spacing, (A)(2) stating that the adjustment factors do not apply to cables in cable trays together with the 75, 70, 65 and 60 percent caps, the 2.15 times diameter rule for triangular and square configurations, and (A)(3) on combinations of multiconductor and single-conductor cable where the two fill percentages must total not more than 100 percent. Source text read from a full-text copy of the 2011 edition.
- ZONG Cable range. https://zh-zong.com/cable/
