Cable Tray Fill Calculator — NEC 392.22

Q: Sum of diameters or fill area — which do I use?

Cable size decides it: 1/0 and larger power cables use sum-of-diameters in a single layer; anything smaller uses the allowable fill area from Table 392.22(A).

Q: Does tray type change the answer?

Yes. Ladder and ventilated trays get the standard allowances; solid-bottom trays are reduced because they trap heat.

Q: Do control and signal cables count?

They occupy area too. Mixed power and control installations have their own combined rules in 392.22(A); size the power cables first, then fit the rest into the remaining area.

Q: Can I stack cables in layers?

Large power cables (≥1/0) must be a single layer. Smaller cables can fill by area up to the limit, which effectively allows some layering within the area budget.

Q: Is fill the only tray limit?

No — you also derate cable ampacity for tray fill per 392.80. This tool checks the physical fill; ampacity is a separate step.

Short answerFor multiconductor power cables 1/0 AWG and larger in a ladder or ventilated tray, install in a single layer and keep the sum of the cable diameters ≤ the tray width. For smaller or mixed cables, use the allowable fill area from NEC Table 392.22(A).

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How it's calculated

NEC 392.22 governs how much cable a tray may carry. The rule depends on cable size and tray type. Large power cables (≥1/0) go in a single layer so heat escapes, limited by the sum of their outside diameters. Smaller cables are limited by total cross-sectional area, which varies with tray inside width and whether other large cables share the tray.

Cables ≥ 1/0 (ladder/ventilated): Σ (cable outside diameters) ≤ tray inside width, single layer Cables < 1/0: Σ (cable cross-section areas) ≤ allowable fill area for the tray width (Table 392.22(A))

Worked example

12 in ladder tray; six 4/0 cables at 1.20 in Ø each.
Σ diameters = 6 × 1.20 = 7.2 in.
7.2 in ≤ 12 in → compliant, single layer.

Reference tables

Allowable fill area — cables < 1/0 (NEC Table 392.22(A), ladder/ventilated ≤ 6 in usable depth)

Tray inside width	Max fill area (in²)
6 in	7.0
12 in	14.0
18 in	21.0
24 in	28.0
30 in	35.0
36 in	42.0

Method by cable size

Cable size	Tray type	Fill rule
1/0 and larger	Ladder / ventilated	Sum of diameters ≤ width, single layer
Smaller than 1/0	Ladder / ventilated	Sum of areas ≤ Table 392.22(A)
Any	Solid bottom	Reduced allowances — see 392.22(A)(1)(d)

A number the tables do not give: a 12 in ladder tray of 4/0 power cable at 1.20 in each fits about 10 cables in a single layer before you reach the width limit.

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FAQ

Sum of diameters or fill area — which do I use?

Cable size decides it: 1/0 and larger power cables use sum-of-diameters in a single layer; anything smaller uses the allowable fill area from Table 392.22(A).

Does tray type change the answer?

Yes. Ladder and ventilated trays get the standard allowances; solid-bottom trays are reduced because they trap heat.

Do control and signal cables count?

They occupy area too. Mixed power and control installations have their own combined rules in 392.22(A); size the power cables first, then fit the rest into the remaining area.

Can I stack cables in layers?

Large power cables (≥1/0) must be a single layer. Smaller cables can fill by area up to the limit, which effectively allows some layering within the area budget.

Is fill the only tray limit?

No — you also derate cable ampacity for tray fill per 392.80. This tool checks the physical fill; ampacity is a separate step.

This page is a reference aid. Always verify against the current edition of the NEC (NFPA 70) and the authority having jurisdiction. Figures shown are for orientation, not a stamped design.

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