Electrical – How to add 100 amp electrical service to a shop

The answer depends on the details of what loads are already connected to your 150A panel. You didn't say if this was a residence or other. Certainly a second 3-phase service would be highly unusual in a residence.

However, assuming that the sum of the existing loads on the 150A panel don't preclude adding the additional load of the guest house, then the 4AWG aluminum conductors would admit a maximum 60A feed (Unless you can find a 70A breaker… I've never seen one). Not 80A.

As to which panel to connect… if there are existing circuits in the guest house, I would be leery of connecting the SER to a different panel than these circuits… mixing circuits from two different services in the same dwelling is increasing the chance for disastrous errors in the future.

And unless you're really experienced, knowledgable, and qualified about electric power distribution, I would advise that you not touch the 3-phase stuff. There are numerous non-obvious pitfalls in that for the untrained person.

You'll have to use a cable or wires that are rated for wet locations. You'll most likely use Underground Feeder (Type UF) cable, Underground Service-entrance cable (Type USE) , or THWN wires.

You'll have to install a proper grounding electrode at the cabin, and bond the grounding conductor to it.

You'll have a grounding electrode at the pole, where you'll bond the service neutral and the equipment grounding conductor. From there you'll run four wires (two "hots", one neutral, and one ground), and you'll keep the ground and neutral separate.