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I have a 150 amp 120/240 and a three phase panel that are connected by 4 wires. two of the 120/240 is shared between two separate meters/panels. The 3 phase panel is also connected to an extra wire that I believe is High leg delta.

A neutral/ground bare wire enters the 120/240 meter/panel. In resume total of 4 wires come from the pole, two insulated black and one bare goes to 150 amp single phase. The same two black are also shared with the 3 phase plus another wire that I believe is a high leg delta. I want to wire a 60 amp subpanel in an attached guest house of 500 square feet, about 70 feet from these panels by feeding a 60, 70 or 80 amp breaker.

Does it make a difference in which panel I should install the feeder breaker? The guest house is wired for 24" electric stove. An electric Ac already is connected to the main 150 amp panel. The main house has a 24" electric stove and electric dryer. The wire that brings service to the guest house is #4 ALU SER service entrance.

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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.

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  • Keep in mind that a dwelling unit requires a minimum of a 100A service...
    – ThreePhaseEel
    Jun 20, 2016 at 0:49
  • Ah. that's interesting. In my jurisdiction it's still possible to obtain a 60A service if the dwelling total above-ground floor area is less than 80 m^2 (861 sq ft). But 60A is quite limiting… However, the guest house in this instance would not be served from a utility service, but as part of the 150 A service… not sure if what the rules would be around that in Mike's jurisdiction.
    – AndyW
    Jun 20, 2016 at 1:31
  • I can see that my use of the word "service" was misleading, so I've edited my answer to clarify...
    – AndyW
    Jun 20, 2016 at 1:37

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