Metal Wireways — Fill and Ampacity

Wireways make a great gutter for terminating and distributing lots of circuits at a panel. Keep the conductors to 20 percent of the cross-section, and once you pass 30 current-carrying conductors the derating kicks in.

The removable cover tempts people to treat it as unlimited space. It is not — the same heat physics apply as in any raceway.

When it applies

Metal wireways serve as accessible troughs for grouping many conductors with removable covers for easy changes. Conductor cross-section is limited to twenty percent of the interior, and once more than a set number of current-carrying conductors ride together, ampacity derating applies.

Common mistakes

• Overfilling the wireway because the cover still closes. Twenty percent fill exists for heat, not for the lid.
• Ignoring derating once you exceed thirty current-carrying conductors bundled in the trough.
• Counting a neutral as non-current-carrying when the load makes it carry.

In the field

On a plant control-room job I inherited a wireway stuffed near solid with conductors. The thermal load was pushing insulation past rating and the terminations discolored. I re-engineered the fill, split conductors across two wireways, and applied the correct derating factor. The trough runs cool now. Wireways are generous, but the fill and derating math still governs the heat.