The builder can only accept 50k per side. Regardless of the cable you use you need to run more than one power connection to it.
From the wiki emphasis mine:
"
The more complicated a dimension is, the more power it will require for maintenance. This should be taken into consideration, as a Dimensional Builder can only accept 50000 RF/tick from each side of the block, making it 300000 RF/tick total. If a dimension requires more power per tick to stay up, an identical Realized Dimension Tab should be made and then placed into another Dimensional Builder."