Chapter 4
Alternating Current
Power
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Overview
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What is Alternating Current
AC Hazards
AC Power Requirements
Shoreside Utility System
On-Board Generators
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What is Alternating Current
• General Description
• Basic Terminology
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General Description
• Batteries provide limited amount of power
 Can’t run air conditioner or heater
 DC appliances are expensive
• Therefore need for AC on boat
 From shore power
 From on-board generator
• Need “Service Drop”
 Not an extension cord
 Safety hazards with AC power
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Basic Terminology
• Polarity
– Hot, neutral and ground
• Voltage
– 120 VAC
• Frequency – 60 Hz
• Resistance – DC opposition to current flow
• Reactance – AC opposition to current flow
• Voltage Drop – difference input & output
• Wave Form – Commercial AC is sine wave
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Sine Wave
• Plot of AC Voltage is Sine Wave
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AC Hazards
• Shock
• Fire
• Reversed Polarity
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Shock and Fire
• Shock
 AC “shock” could be lethal
 Marine environment increases this hazard
 Need GFCI protection
• Fire
 Inadequate wiring and overloaded cables
 Over fusing or no overload protection
 Heater on extension cord
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Reverse Polarity
• Reverse Polarity is extremely dangerous
- Neutral is “Hot” (120 VAC not 0 VAC)
- Produces stray current
CIRCUIT
REVERSED POLARITY
120V HOT
BREAKER
2-WIRE OUTLET
LOAD
NEUTRAL
GROUND
120V POTENTIAL
NO SAFETY GROUND
EXISTS BETWEEN
CONNECTION
BOAT NEUTRAL AND WATER
Outlet checker indicates if AC outlet is correctly wired
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Reverse Polarity - 2
• Receptacles are polarized
 Plug can only be inserted one way
• Modern receptacles are polarized & grounded
 Narrow vertical slot is Hot (120 VAC)
 Longer vertical slot is Neutral (0 VAC)
 Rounded hole is Ground
• ABYC requires a Polarity Indicator
 Normally lights on AC power panel
 If reverse polarity light is “On”
• DO NOT USE SHORE POWER
• Immediately disconnect the shore power cable
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AC Power Requirements
• Maximum Current Limitations
• Priority Usage Table
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Maximum Current Limitations
• Dockside Outlet
• Service Cord
• Boat’s Service Entrance
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Dockside Outlet
• 30A outlet protected by 30A circuit breaker
• 15 A outlet protected by 15A circuit breaker
• If 15A to 30A adapter plugged into 15A outlet and
30A cable plugged into adapter
 Current available limited to 15 Amps
• A 30A to 20A adapter permits a 15A cord to be
plugged into dockside outlet
 NOT SAFE!
• No GFCI protection
• Outlet is overprotected
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Service Cord
• 30A, 120 VAC service cord wire is AWG #10
• 50A, 120 VAC service cord wire is AWG #6
• Service cords are 25 and 50’
 Cords are rated for 50’ length
• Limit for #10 wire carrying 30A is 60’
 Ch 2 Wire Size Selection Table
 When two 30A service cords connected together,
safe current is limited to 20 Amps
• Similar problem for two 50A cords
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Boat’s Service Entrance
• Has a maximum current rating
 Size for the service cord
• Service switch is DPST
 Breaks hot and neutral wires
 Normally left “On”
• If power is lost and circuit breakers are “On”
 Check service entrance switch
• Using one 30A service cord, rather than two
 and configuring power panel for one cord
 Service switch will trip if exceed 30A total
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Priority Usage Table
• Maximum current limited by smaller of:
 Dockside outlet and circuit breaker
 Service cord
 Service inlet and service switch (circuit breaker)
• Following table gives typical AC power use
 Everything cannot be on at same time
 Prioritize demand based on limiting factor
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Priority Scheme
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Shoreside Utility System
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Utility Power Distribution
Shoreside Outlets
Dockside Service Drop
Shore Power Service Cable
Connector Standards and Adapters
Service Entrance and Distribution
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Utility Power Distribution
• Power lines are at 2,300 VAC or higher
• Output from last transformer is 120/240 V
 If loads are balanced, no current in neutral
 Hot wires are Black and Red
• 120 VAC from either wire to neutral
• 240 VAC between black and red wires
 Center tap of transformer is grounded
• No current flows in ground (green) wire
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Shoreside Outlets
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Dockside Service Drops
• Shoreside power: 120/240 V 60 Hz
 Cords and plugs are called 125 or 125/250 V
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Boats under 30’: usually one 30A service cord
Boats between 30 & 40’: may use two 30A cords
Larger boats: one or two 50A 125/250 V cords
Neutral wire is grounded only at transformer
Inadequate service (brownout)
 Line voltage under 100 VAC
 May damage motors (refrigerator, air-conditioner)
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Shore Power Service Cable
• Flexible, durable and oil/moisture resistant
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Called “Hard Service Cable”
Compliant with NEC article 555
Outlets are female; Plugs are male
Outlets / plugs are polarized with twist lock
Outlets are “keyed” to prevent mismatch
30 Amps @ 125 V
50 Amps @ 125/250 V
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Connector Standards
15A
125V
20A
125V
30A
125V
50A 125V
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Adapters
15 A to 30 A
50 A to two 30 A
50 A 125/250 to
one 30A 125 V
Reverse “Y”
Two 30A 125 V to
One 50A 125/250 V
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Service Entrance & Distribution
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Service Entrance Connector
Isolation Transformer
Galvanic Isolator
AC Distribution Panel
Branch Circuit Conductors
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Service Entrance Connector
• Normally one or two
 30A 125 V or 50A 125 V or 50A 125/250 V
 Most houses have 200A 125/250 V
• Close weatherproof cover when not in use
 Clean contacts with Electronic Cleaner
• Sequence for Shore Power service cord
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Turn “Off” all circuit breakers
Connect boat end first
Then connect shore end
Turn “On” shore circuit breaker
Check Reverse Polarity for proper connection
Turn “On” boat Main then Branch circuits
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Isolation Transformer
• No direct electrical connection
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between boat and shore power
Avoids reversed polarity hazard
Stops stray current
Big $ and weight
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Galvanic Isolator
• Solid state - large capacitor
and power diode
 In series with green ground
wire
 Blocks DC and low AC
voltage
 Stops Stray current
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Boat AC Distribution Panel
• Review of Chapter Two
• Normally combined with DC power panel
 Main (Shore Power) circuit breaker
• Not Service Switch (near input connector)
• Breaks both hot and neutral
 Branch circuit breakers
• Breaks only hot wire
• All AC circuits are 3-wire
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Branch Circuit Conductors
• Review of Chapter Two
• Wiring is stranded 3-wire UL 1426 BC cable
 Normally 14 AWG for 15A circuits
 Normally 12 AWG for 20A circuits
• All neutral & ground wires returned to panel
• Support wires every 18”
• Ring connectors on both ends of wires
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On-Board Generators
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Inverters
Propulsion-engine driven generator
Auxiliary-engine driven generator
Isolation from Shore Power Entrance
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Inverters
• Convert DC to AC
 12 VDC to 120 V 60 Hz
 Efficiency 90 to 95%
• Sizes from 50 to 3,000 watts
 1200 watts of 120 VAC draws 110A of 12 VDC
• Outputs are 120 V 60 Hz
 True Sine Wave are expensive
• Run all loads
 Modified Sine Wave are less expensive
• May damage sensitive electronic equipment
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Modified Sine Wave
• Modified Sine Wave and Sine Wave Voltage
 Either waveform will deliver the same power to a
resistive load
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Inverters - 2
• Automatically disconnect output when shore
power or generator power is connected
• May produce interference
 Mitigation is discussed in Chapter 7
• Combined Inverter/Charger (illustrated)
 Either charge batteries or generate AC
 Lighter and cheaper than separate systems
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Propulsion-Engine Driven
• AC Generators driven by propulsion engine
 Can only be used while underway
 Will not produce full output at idle
• Not as efficient as auxiliary driven generator
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Auxiliary-Engine Driven
• Directly coupled to small gas or diesel engine
 Fuel should be same as propulsion engine
• Solution if need over 2 Kw of 60 Hz power
• Big, heavy and noisy
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Isolation from Shore Power
• Inverters automatically only provide power to
AC bus when no other AC power is present
• On-board Generators must also be isolated
 Normally done by transfer switch
• Neutral and ground are connected at generator
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Summary
• Polarity reversed: Don’t use shore power
• Nominal shoreside power: 120/240 volts, 60 Hz
• Maximum current limited (smaller of):
 Dockside circuit breaker, dockside outlet
 Service cord and boat’s service entrance
• Priority scheme for AC appliances needed
• Outlets must be 3-wire polarized
 GFCI in galley, head, machinery spaces & weather deck
• Inverters: for short term loads under 2 Kw
• Generators: for long term loads over 2 Kw
• Use transfer switch with inverter or generator
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