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Marine GradeT Boat Cable vs. SAE Wire

"Can I use 'regular wire' for my boat?". The answer to this common question is a qualified "yes", if the wire is SAE (Society of Automotive Engineering) J378, J1127 or J1128. These wires are designed for "surface vehicles", not for the special requirements of the marine industry, but meet the minimum standards for boats in limited circumstances, even if tinned copper, they should not be run in bilge spaces or other areas subject to moisture from spray or dripping. They should not be run in engines spaces, unless marked "oil resistant". They should not be used in applications where subjected to vibration or frequent flexing and must never be used for 110 volt applications. For safety, use only wire which is marked with size and type.

Most importantly, SAE wire is up to 12% smaller than AWG Boat Cable which means that, in many applications, larger gauge wire must be used to stay within the voltage drop limits recommended by experts (see Tables C & D, below). The wire charts found in "Chapman's Piloting" and other publications are all for "AWG" wire, not "SAE" type wire.


Table A: Marine Wire Color Code

Color Item Use
Yellow or Blk Ground Return, Negative Mains
Lt. Blue Oil Pressure Oil Pressure Sender to Gauge
Dk. Blue Cabin & Instrument Fuse or Switch to Lights
Brown Generator Armature Generator Armature to Regulator
  Alternator Charge Light Generator Terminal or Alternator Auxiliary Terminal to Regulator
  Pumps Fuse or Switch to Pumps
Green Bonding System Bonding Wires (if insulated)
Grey Navigation Lights Fuse or Switch to Lights
  Tachometer Tachometer Sender to Gauge
Orange Accessory Feed Ammeter to Alternator or Generator Output and Accessory Fuses or Switches
  Common Feed Distribution Panel to Accessory Switch
Pink Fuel Gauge Fuel Gauge Sender to Gauge
Purple Ignition Ignition Switch to Coil & Electrical Instrument
  Instrument Feed Distribution Panel Electrical Instruments
Red Main Power Feeds Positive Mains (particularly unfused)
Yellow Generator Field Generator to Regulator Field Terminal
Brn w/Yellow Bilge Blowers Fuse or Switch to Blower
Yellow w/Red Starting Circuit Starting Switch to Solenoid


Table B: Conductor Sizes

Ampacity is the ultimate safe current carrying capacity of the wire before damage occurs to the insulation, without regard to voltage drop. Because the insulation of most SAE wire types is not the same as marine grade wire, this chart should not be used for other conductor types. Use Table C & D to find proper wire size to insure adequate performance.

AWG Sq.
mm
AWG
CM area
SAE
CM Area
Ampacity
Engine Space
Outside
Ampacity
Engine Space
Inside
18 0.8 1,600 1,537 20 17
16 1 2,600 2,336 25 21
14 2 4,100 3,702 35 30
12 3 6,500 5,833 45 38
10 5 10,500 9,343 60 51
8 8 16,800 14,810 80 68
6 13 26,600 24,538 120 102
4 19 42,000 37,360 160 130
2 32 66,500 62,450 210 178
1 40 83,690 77,790 245 208
1/0 50 105,600 98,980 285 242
2/0 62 133,100 125,100 330 280
3/0 81 167,800 158,600 385 327
4/0 103 211,600 205,500 445 378

Table C: Conductor Sizes (AWG) for 3% Voltage Drop at 12 Volts

Note: Length is total length of run, i.e. include both hot and neutral wires in measuring total cable length.

Length 5 10 15 20 25 30 40 50 60 70 80 90 100
10' 18 14 12 12 10 10 8 8 6 6 6 4 4
15' 16 12 10 10 8 8 6 6 4 4 4 2 2
20' 14 12 10 8 8 6 6 4 4 4 2 2 2
25' 14 10 8 8 6 6 4 4 2 2 2 1 1
30' 12 10 8 6 6 4 4 2 2 2 1 1/0 1/0
40' 12 8 6 6 4 4 2 2 1 1/0 1/0 2/0 2/0
50' 10 8 6 4 4 2 2 1 1/0 1/0 2/0 3/0 3/0
60' 10 6 6 4 2 2 1 1/0 2/0 2/0 3/0 3/0 4/0
70' 10 6 4 2 2 2 1/0 2/0 2/0 3/0 3/0 4/0 4/0
80' 8 6 4 2 2 1 1/0 2/0 3/0 3/0 4/0    
90' 8 4 4 2 1 1/0 2/0 3/0 3/0 4/0      
100' 8 4 2 2 1 1/0 2/0 3/0 4/0 4/0      
110' 8 4 2 2 1 1/0 2/0 3/0 4/0        
120' 6 4 2 1 1/0 2/0 3/0 4/0 4/0        
130' 6 4 2 1 1/0 2/0 3/0 4/0          
140' 6 2 2 1/0 2/0 2/0 3/0 4/0          
150' 6 2 1 1/0 2/0 3/0 4/0            
160' 6 2 1 1/0 2/0 3/0 4/0            
170' 6 2 1 2/0 3/0 3/0 4/0            

Use 3% voltage drop for any "critical application" affecting the safety of the vessel or its passengers: bilge pumps, navigation lights, electronics, etc...


Table D: Conductor Sizes (AWG) for 10% Voltage Drop at 12 Volts

Note: Length is total length of run, i.e. include both hot and neutral wires in measuring total cable length.

Length 5 10 15 20 25 30 40 50 60 70 80 90 100
10' 18 18 18 16 16 14 14 12 10 8 8 6 6
15' 18 18 16 16 14 14 12 12 10 8 8 6 6
20' 18 16 16 14 12 12 10 10 8 8 8 6 6
25' 18 16 14 12 12 10 10 8 8 8 6 6 6
30' 18 16 14 12 10 10 8 8 8 6 6 6 4
40' 16 14 12 10 10 8 8 6 6 6 4 4 4
50' 16 12 10 10 8 8 6 6 4 4 4 4 2
60' 16 12 10 8 8 8 6 4 4 4 2 2 2
70' 14 12 10 8 8 6 6 4 4 2 2 2 2
80' 14 10 8 8 6 6 4 4 2 2 2 2 1
90' 14 10 8 8 6 6 4 4 2 2 2 1 1
100' 12 10 8 6 6 4 4 2 2 2 1 1 1/0
110' 12 10 8 6 6 4 4 2 2 1 1 1/0 1/0
120' 12 8 8 6 4 4 2 2 2 1 1/0 1/0 2/0
130' 12 8 6 6 4 4 2 2 1 1 1/0 2/0 2/0
140' 12 8 6 6 4 4 2 2 1 1/0 1/0 2/0 2/0
150' 10 8 6 4 4 2 2 1 1 1/0 2/0 2/0 3/0
160' 10 8 6 4 4 2 2 1 1/0 1/0 2/0 2/0 3/0
170' 10 8 6 4 4 2 2 1 1/0 2/0 2/0 3/0 3/0

Use 10% voltage drop for any "non-critical applications: windlasses, cabin lights, etc...

Important!

Length (feet): Determined by measuring the length of the conductor from the positive (+) power source connection to the electrical device and back to the negative (-) power source connection. Note that the power source connection may be either the battery, panelboard or switchboard.

Current (amps): Determined by adding the total amps on a circuit.

Conductor sizes not covered in Table C or Table D may be calculated by using the following formula:

After calculating the Circular Mil Area (CM), use Table B to determine the proper conductor size (National Fire Protection Agency and Coast Guard require that the next larger conductor be used when the calculated CM area falls between two conductor sizes).

CM=K x l x L / E

CM = Circular Mil area of Conductors
K = 10.75 (Constant representing the mil-foot resistance of copper)
I = Current - amps
L = Length - feet
E = Voltage drop at load (in volts)

For Example...

Q: For example: A bilge pump draws 10 amps. The positive run is l l feet from the power panel, including the float switch. The negative run is only 10 feet. What size wire?

A: Use the formula to reach the correct answer:

CM = 10.75 x 10 (amps) x 21 (total length of run) / 0.36 (3% of 12v) = 6,271

Table "B" shows that 12 AWG wire has a CM area of 6,500 and is the correct choice. However, SAE wire has a CM area of only 5,833. Under NFPA and USCG regulations, 10 SAE wire must be used.


 
Last modified: May 07 2014 23:44