The water pump discharge and the 'city water' connection BOTH feed the same plumbing system.
Depending on which one has higher pressure will determine which source feeds system IF using both.
Water pump has an internal check valve to prevent 'city water' source to backfeed thru pump and vice versa 'city water' connection has a check valve to prevent water pump to feed thru connection.
Typical standard rv water pump has discharge pressure of 45-55 psi.
'City water' sources can have various pressures.......30 to over 100 psi depending on source. Because of the various pressures that can be found it is a 'safe' idea to use a pressure regulator when connected to 'city water' source.......Watts makes a several good adjustable ones.
Ours is set for 50 psi (static)
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2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31