The GTN extends fwd into the subpanel and it is necessary to open a segment of the subpanel. I measured the center of the GTN from each side of the panel and then applied that measurement to the subpanel. I then opened this segment to allow the GTN and its wiring to fit into this space. I cleaned up the edges including round filing the 90 degree edges. I added plastic edge trim to prevent wire chafing. I will now need to support the edges with angle iron to make a secure subpanel.
