score:8
Firstly I think you have your terminals a little mixed up. Although United has on occasion used terminal B for international arrivals, it's very rare. Almost always flights like UA51 will arrive in terminal C.
Your outbound flight will also probably depart from terminal C, so you won't need to change terminals. There is a chance it will depart from terminal A, but in that case there is an air-side bus that goes from terminal C (around near gate 70) to terminal A. As with all US international arrivals you will need to clear immigration, collect your baggage, clear customs, and then re-check your baggage and clear security.
How long this takes will depend mainly your citizenship and how busy the immigration and security areas are, however 2 hours 45 mins should be plenty. I did a similar connection at EWR two days ago and it took less than 20 mins from when the plane door was opened until I was through security - although we were the first plane to land for the day, I had no checked luggage, and I do have TSA PreCheck so getting through both immigration and security were fast.
If you do happen to miss the connecting flight, just go to any of the United service desks/check-in counters and they will assist you. If you are flexible and could handle going to Washington Dulles IAD instead of BWI then you should talk to them before you drop your bags after customs - there is a check-in counter just behind where you would normally drop your bags (next to the escalator). Once you've dropped your bags, changing airports will be almost impossible.
Upvote:1
Why do you think the air train will not be operating? This airport info says it's 24x7.
I travel to Nashville on United several times a year, usually via Newark, but sometimes via Chicago. I never book a ticket with a connection of less than 2hrs 30 min. I've never (yet) missed a connection.
As has been observed, wait times at immigration are unpredictable, but your 2h 45m should usually be fine.
On passing immigration you then collect your checked bags and clear customs, immediately on exit from customs there are drop-off points for your checked bags. You do not need to take your checked bags over to terminal A, the drop-off procedure is very quick.
However, you are now ground-side so you will need to clear security again in terminal A and on occasions this can be a little slow, so don't delay take the air-train and get yourself through security in terminal A.
My understanding is that as you are on a single ticket covering the connection then should you miss the connection United will rebook you, so you will get a connection eventually. However you would need to cover any accommodation costs incurred.