01What this calculator tells you
This calculator checks whether your cables fit inside a cable tray within the limits of Article 392 of the National Electrical Code (NFPA 70). Choose the tray type and inside width, enter the overall outside diameter of your cable and how many you are running, and it returns the total cable area, the maximum area the NEC allows, the spare area left, and a clear pass or fail.
It follows the area method of NEC Table 392.22(A) for multiconductor cables rated 2000 volts or less. If your cables leave the tray and continue in a raceway, pair this with our conduit fill calculator for the raceway section.
02The NEC allowable fill areas
Unlike conduit, cable tray fill is not a single percentage — the NEC gives a fixed maximum cable area (in square inches) for each tray width. Ladder and ventilated trough trays share one column; solid-bottom trays are lower because they trap more heat. These values are from NEC Table 392.22(A) for multiconductor cables smaller than 4/0 AWG.
03How many cables fit — quick reference
A common question is “how many of this cable fit in that tray?” Divide the tray’s allowable area by one cable’s area (area = π × (OD ÷ 2)²). The table below shows a few examples for a ladder / ventilated trough tray; read it as a sanity check, then use the calculator for your exact cable.
Notice how sensitive the count is to diameter: going from a 0.75″ to a 1.00″ cable in a 24″ tray drops the capacity from about 63 cables to 35, because area grows with the square of the diameter.
A “pass” here means the cables fit mechanically. It is necessary, but not sufficient — several other checks still apply.
- Ampacity derating is separate. Once a tray carries several current-carrying cables, NEC 392.80 can require you to derate their ampacity, which may force larger cables (and then a wider tray) even when fill looks fine.
- Single-conductor cables differ. Single-conductor cables follow NEC 392.22(B): 1000 kcmil and larger must lie in a single layer with the sum of diameters not exceeding the tray width. This calculator covers multiconductor cables.
- Cables 4/0 and larger. When multiconductor cables 4/0 AWG or larger are present, a single-layer diameter rule and the Column 2 adjustment apply instead of the plain area table.
- Mixed cable sizes. This calculator assumes one cable OD and a count. For a mix, work out each group’s area and add them (see the Formula tab).
- Construction sites also fall under OSHA’s wiring rules (29 CFR 1926.405), which reference NEC wiring methods.
- Pick the tray type (ladder / ventilated trough, or solid bottom) and the inside width.
- Read the overall outside diameter (OD) of your cable from its datasheet — not the conductor size.
- Enter that OD and the number of same-size cables in the tray.
- Press Calculate to see the fill percentage against the NEC allowable area.
If the run fails, step up to the next wider tray and recalculate, or move some cables to a second tray. Want your own number? Enter your values in the calculator and hit Calculate.
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01The cable tray fill formula
Cable tray fill compares the total cross-sectional area of your cables against a fixed allowable area from NEC Table 392.22(A). Each cable is treated as a circle, so its area comes straight from its outside diameter.
Where:
- OD= the overall outside diameter of the cable (not the conductor size).
- Σ A= the sum of every cable’s cross-sectional area in the tray.
- allowable area= the NEC Table 392.22(A) maximum fill area for the tray type and inside width.
02Worked example
Take ten multiconductor cables of 0.75″ outside diameter in a 24-inch ladder tray. Work it one line at a time:
That spare area leaves room for roughly 53 more of the same cable before the tray reaches its NEC limit — though leaving 15-20% unused is smart practice so future pulls stay easy.