Cable Tray Bonding Jumper Size Calculator
Find the minimum equipment bonding jumper or grounding conductor size for a metallic cable tray, sized from the rating of the overcurrent device that protects the circuits in the tray, per NEC Table 250.122. Pick a device rating and conductor material to get the size.
Minimum bonding jumper / EGC size
8 AWG
Per NEC Table 250.122, for a 100 A overcurrent device.
How this is sized
For a metallic cable tray, the equipment bonding jumper and the equipment grounding conductor are sized from the rating of the overcurrent device protecting the circuits in the tray, using NEC Table 250.122. This calculator returns that minimum size for copper or aluminum.
A few things this does not do, on purpose, so you do not misapply it:
- The bonding jumper is not required to be larger than the circuit conductors it serves (NEC 250.122(A)).
- A supply-side bonding jumper is sized differently, from the supply conductors, per NEC Table 250.102(C)(1). Use that table, not this one, on the line side.
- Using the cable tray itself as the equipment grounding conductor is a separate question governed by NEC 392.60(A), which sets a minimum cross-sectional area of tray metal based on the overcurrent device.
- For overcurrent devices above 2000 A, consult NEC Table 250.122 directly.
Values follow NEC Table 250.122. Codes are adopted and amended locally and change between editions. Confirm against the NEC edition your jurisdiction enforces and the project specifications before you build. This tool is a reference, not a stamped engineering decision.
How to size a cable tray bonding jumper
- Find the ampere rating of the overcurrent device (the breaker or fuse) protecting the circuits in the tray.
- Choose the conductor material: copper or aluminum.
- Read the minimum equipment grounding conductor size from NEC Table 250.122 for that device rating. The calculator above does this step.
- Verify against the NEC edition your jurisdiction enforces and the project specs, and confirm the jumper is not required to be larger than the circuit conductors it runs with (NEC 250.122(A)).
Frequently asked questions
What size bonding jumper does a cable tray need?
The minimum equipment bonding jumper or grounding conductor for a metallic cable tray is sized from the rating of the overcurrent device protecting the circuits in the tray, using NEC Table 250.122. For a 100 A device it is 8 AWG copper or 6 AWG aluminum; for a 200 A device it is 6 AWG copper or 4 AWG aluminum. Larger devices step up from there.
Do I size the bonding jumper from the breaker or from the wire?
From the overcurrent device, the breaker or fuse, not the circuit conductors. NEC 250.122 sets the equipment grounding conductor size by the ampere rating of the overcurrent device ahead of the equipment. One rule caps it: the jumper is not required to be larger than the circuit conductors it runs with, per NEC 250.122(A).
How much larger does an aluminum bonding jumper need to be than copper?
Aluminum is sized up one to two AWG sizes versus copper for the same device rating. At 100 A it is 8 AWG copper or 6 AWG aluminum; at 400 A it is 3 AWG copper or 1 AWG aluminum. NEC Table 250.122 lists both columns.
Can the metallic cable tray itself serve as the equipment grounding conductor?
Sometimes. A cable tray system can be used as an equipment grounding conductor where it qualifies under NEC 392.60(A), which sets a minimum cross-sectional area of tray metal based on the overcurrent device rating and requires listed, identified tray. That is a separate check from sizing a bonding jumper conductor, and not every tray or installation qualifies.
Is a supply-side bonding jumper sized the same way?
No. A supply-side bonding jumper, on the line side of the service or a separately derived system, is sized from the supply conductors using NEC Table 250.102(C)(1), not from the overcurrent device. Use this calculator for load-side equipment bonding jumpers and equipment grounding conductors only.
Does this replace an engineer or the code book?
No. The values follow NEC Table 250.122, but codes are adopted and amended locally and change between editions. Confirm the result against the NEC edition your jurisdiction enforces and the project specifications before you build. It is a fast reference, not a stamped engineering decision.
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