Question, I am running new service to a new garage. 50 amp breaker off the main panel in the house. About 30 feet ran in basement. Then approx 150 feet to the subpanel in the garage. Could we go with #6 THHN ran underground to the subpanel? I was planning on running 6-3 Romex with ground to a junction box before it exits the basement. I was also deciding about possible UF cable underground and not in conduit. Last what size ground do I need to run can it be smaller than #6 THHN or not. Thanks.
First of all you are not running a new “service” to your outbuilding. The utility would do that and since you stated " 50 amp breaker off the main panel in the house" it could not ever be a “service” panel in the new garage based on your statements. You are looking to run a feeder to that remote building.
As for the THHN, if you are using a dual rated THHN/THWN-2 and installing it inside a raceway then you would be fine but not if you plan on direct burial as THHN/THWN-2 is not rated for such use and would need to be in a complete raceway system but I am sure you are aware of that little fact.
Now we can only assume you know the loads in question and that you have calculated that you only need 50 amperes to serve your load. Since you did not ask about disconnection means and so on at the remote building or the grounding electrode system required at that remote building we will assume you are all square on those requirements without any need for assistance.
However, you can put that 6 AWG on a 60A OCPD as long as the calculated load doesn’t exceed the ampacity of the conductors at the 60 degree value which is 55 amps. We will assume your load is 55 amps or less.
You could run that Type NM-B inside the building (in a dry location) to a accessible junction box and then continue on to your remote buildings with a 6/3 W ground Type UF-B Cable if you so desire. Just keep in mind your depth of cover requirements found in Table 300.5.
You could also run that Type NM-B inside the main building and then hit a junction box and transition to a PVC Raceway and run the raceway out to the detached structure. The required disconnection means at that structure can be inside the panel enclosure as well as separate if you desire.
As for the Equipment Grounding Conductor [EGC], that is taken care of within the Type UF-B and Type NM-B. However, if you choose to use a raceway then size your EGC based on the size of the circuit breaker feeding that building so in your case a 60A or 55A or even a 50A if you so choose. You would visit section 250.122 for that sizing which in your case would be a 10 AWG CU.
Now I can’t tell you how to install your system but those are some good suggestions for you to consider. Have Fun !
And Again…Don’t for get you have to establish a Grounding Electrode System at that new garage…