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 |
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?