We recommend graphite for large round and specially shaped holes (such as square and hexagon)  These special aluminum holders are designed for these larger electrodes.

Refer to the following charts for Graphite electrode sizes and graphite electrode sizes:

https://electroarc.com/wp-content/uploads/2021/11/Graphite-electrode-holder-chart.pdf 

https://electroarc.com/wp-content/uploads/2021/11/Graphite-Electrodes-standard-sizes.pdf