1 Main Circuit (1MC) is the term for the shipboard public address circuits on United States Navy vessels. This provides a means of transmitting general information and orders to all internal ship spaces and topside areas, and is loud enough that all embarked personnel are able to (normally) hear it. It is used to put out general information to the ship’s crew on a regular basis each day. The system consists of an amplifier-oscillator group which is located in the IC/gyro room, a microphone control station, portable microphones at each control station and loudspeakers located throughout the ship. Control stations for the 1MC announcing system are located at the pilot house, OOD stations on the quarterdecks, after steering and Damage Control Central area.
During an incident involving a casualty, the 1MC is a communication tool used by DCA (damage control assistant) to keep ship members alerted and informed of casualty location area, status, and incident response efforts by the DC organization.
Other circuits 
- 2MC - Propulsion plant
- 3MC - Aviators
- 4MC - Damage control
- 5MC - Flight deck
- 6MC - Intership
- 7MC - Submarine control
- 8MC - Troop administration and control
- 9MC - Underwater troop communication
- 10MC - Well Deck
- 18MC - Bridge
- 19MC - Aviation control
- 21MC - Captain's command
- 22MC - Electronic control
- 23MC - Electrical control
- 24MC - Flag command
- 26MC - Machinery control
- 27MC - Sonar and radar control
- 29MC - Sonar control and information
- 30MC - Special weapons
- 31MC - Escape trunk
- 32MC - Weapons control
- 35MC - Launcher captains
- 39MC - Cargo handling
- 40MC - Flag administrative
- 42MC - CIC coordinating
- 44MC - Instrumentation space
- 45MC - Research operations
- 46MC - Aviation ordnance and missile handling
- 47MC - Torpedo control
- 50MC - Integrated operational intelligence center
- 51MC - Aircraft maintenance and handling control
- 53MC - Ship administrative
- 54MC - Repair officer's control
- 55MC - Sonar service
- 58MC - Hangar-deck damage control
- 59MC - SAMID alert
Usage examples 
- General Quarters - "General Quarters, General Quarters! All hands man your battle stations. Up and forward to starboard, down and aft to port. General Quarters, General Quarters!" The reason for General Quarters is generally given (such as "Fire in Main Space 2").
- Sweepers - "Sweepers, Sweepers, man your brooms. Give the ship a good clean sweep down both fore and aft! Sweep down all lower decks, ladder backs and passageways! Dump all garbage clear of the fantail! Sweepers." Most ships actually discourage throwing of trash over the side but instead use mulch/pulp rooms. In port, "Dump all garbage clear of the fantail" is replaced with "Take all trash to the proper receptacles provided for on the pier."
- 8 O'Clock Reports - "Lay before the mast (if the Captain is on board) all eight o'clock reports. Eight o'clock reports will be received in the wardroom by the Commanding Officer."
- Personnel Working Aloft - "There are personnel working aloft on board (ship), do not rotate, radiate, or energize any electric or electronic equipment, start gas turbines, or operate ship's whistle while personnel are working aloft on board (ship)." Passed every fifteen minutes and by all adjacent ships.
- Divers Over the Side - "There are divers over the side, do not rotate screws, cycle rudders, take suction from or discharge to the sea, blow flood or vent any tanks, or operate any underwater equipment without first contacting the Chief Engineer and the diving supervisor." (Passed every fifteen minutes)
- Men in the Sail (RE: sub tender workers from the Repair Department R3 Division, IC (Interior Communications Electricians) - "There are men in the sail, do not raise or lower, rotate or radiate from any mast or antenna, there are men in the sail." Passed every fifteen minutes.
- Reveille - "Reveille! Reveille! Reveille! All hands heave out and trice up. Reveille!"
- Taps - "Taps! Taps! Out all white lights. All hands turn in to their racks and maintain silence about the decks. Taps."
- Darken Ship - "Darken Ship! Make Darken Ship reports to CCS."
- Fire - "Fire, Fire, Fire, Class (A, B, C, or D) Fire in Compartment (Compartment Number and Nomenclature if known). Away the (at sea fire party (generally called "Flying Squad"), IET (In-port Emergency Team), or ship's fire fighting team) provide from Repair 2 (or closest Repair Station not inside the fire boundary)"
- Breakaway music - Popular music played at the conclusion of an underway replenishment evolution. It is used to motivate the crew.
- Submarine diving - "Dive, dive." (followed by two klaxon blasts then) "Dive, dive."
- Security Alert - "Security Alert! Security Alert! Away the Security Alert team! Away the Back-up Alert force! All hands not involved in Security Alert stand fast! Reason for Security Alert: (state reason)"
- Flight Quarters - "Flight Quarters! Flight Quarters! Set condition 1-alpha for flight operations! Set condition 1-alpha for hoisting and lowering of boats, port boat davit! The smoking lamp is out aft of frame 1-2-niner; all personnel not involved in flight operations stay forward of frame 1-2-niner! All personnel remove soft hats and refrain from throwing FOD material over the side!"
- Testing shipwide alarm systems - "The following is a test of the helo crash alarm from the flight deck - disregard this alarm. (Test alarm sounds) Test complete, test satisfactory, all alarms back in effect!"
In popular culture 
- Crimson Tide, a 1995 submarine film, demonstrates in some detail the usage of the 1MC with regard to command instructions and ship-wide orders.
- NAVEDTRA 14325 p.4-14
- NAVEDTRA 14325 p.4-15
- United States Navy (February 2002). "Chapter 4 Communications" (PDF). Basic Military Requirements NAVEDTRA 14325. Naval Education and Training Professional Development And Technology Center. pp. 4–13 to 4–17. NAVSUP Logistics Tracking Number 0504-LP-101-1377. Archived from the original on 6 December 2006. Retrieved 2006-12-16.
- Authentic Navy Alarm Sounds from policeinterceptor.com
- "Non-Resident Training Course: Boatswain's Mate (NAVEDTRA-14343)". United States Navy. pp. 1–10—1–11. Retrieved 1 October 2010. List of standardized 1MC phrases