Hello, I need to know how to figure the sizing of the ground wire for a 150a 3 phase sub panel, I figures 1/0 wire, but what sizw ground? & how do I find it out myself next time?
Many thanks, Julie

You can review the National Electric Code by a link found in the Read First sticky note of this forum. There are also instruction to get to the actual document.

In Article 250 find Table 250.122 that lists the min size equipment grounding conductor. You will find that a circuit protected at 150 amps will need a #6 copper or a #4 aluminum. Use the 200 amp rating, as this exceeds 150 amp.