B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness; Time Pressure; Troubleshooting
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
During boarding and preflight; just after to FO completed the walk around; a ground crew personnel came up to the cockpit to inform us of a large fuel leak near the right wing tip area.  The FO went outside to investigate and I informed ramp of the fuel leak and they responded that they were already aware of the situation and made the calls for the cleanup.  The FO quickly came back and informed me that indeed it was a large leak and suggested we stop boarding and deplane the passengers.  I immediately stopped the boarding process and made a PA informing the passengers that I wanted them to gather all their belongings and deplane the aircraft and wait in the gate area.  The Number 1 FA; who did a great job; then got on the PA and instructed the passengers through the deplaning process.  The Agent came on board and I informed her of the situation and that I was deplaning the passengers.  I then informed maintenance; dispatch; and IOC Duty Pilot of  the situation.  The FO and I went outside multiple times to assess the situation.  One of the fire chief personnel then came to the cockpit and said that fuel was still leaking and wanted to transfer a small amount of fuel from the right tank to the center to try and stop the overflow.  I told him to do want he needed to do; and that is when I instructed the Fight Attendants that the entire crew was going to deplane now too.  Just prior to me leaving the aircraft; maintenance personnel came to the cockpit and informed me the leak was stopped but they didn't have any idea of what the problem was or even how to start troubleshooting.  I asked them to keep us informed as we were leaving the aircraft.  I informed the gate agents of what informations I knew at that time and told them I was heading down to operations.  Shortly after arriving in operations I received a phone call from one [of] the Chief Pilots in ZZZ asking for information and informed me that he received a call from Ramp and they were uninformed of the situation and needed more information and he would like me to call ramp.  So I called ramp and told them that maintenance was trying to determine the problem and we were waiting for maintenance to inform us.  I went back up to the aircraft and went outside to talk with maintenance and they said the leak continued and the aircraft was out of service.  All agencies were then informed.As Captain I communicated with Ramp; Maintenance Control; Maintenance personnel; Passengers; Agents; Flight Attendants; Fire Chief; and IOC Duty Pilot all prior to the call from the Chief Pilot in ZZZ asking me to call Ramp and inform them of the situation.  It seems reasonable to me that after a large fuel leak with all the response from fire trucks and cleanup crews that Ramp and Maintenance could have some dialog directly without the need for the Captain to be the go-between.  Also; the answers do not come as quickly as some may like and waiting for the answers from maintenance in this case is all we can do.The refueling panel had a malfunction and did not stop the right wing tank from filling after it was full.  Monitoring would only have prevented the larger spread of the leak not the leak itself.Testing the refuel panel itself may have identified this issue.
Pilot reported a fuel leak while on the ground taking the aircraft out of service.