With various standards around, it is important to know how to convert proper cable sizes from one standard to another. Also, voltage drop and power losses depend on the cable length, thickness and current flowing through the cable.
Warning: no math in this article, seriously :)
AWG to mm2 Conversion Calculator
Note: Calculator DON'T accept comas ',' - instead, when needed, use dots '.'.
AWG to mm2 Conversion Table
Resistivity of copper is 1.68 Ωm at 20°C. Other materials can be used as well, but most of the time, wires are made out of copper. Keep in mind that metal resistance increases with temperature, not by much, but if you can't hold your trolling motor cables in your hands due to the heat, decrease the power and get thicker cables as soon as possible.
- AWG #: American Wire Gauge cable thickness
- Diameter mm: diameter of cable, given in millimeters. So, if you need, for example, AWG 5 cable, but you are offered cables in millimeters only, you should buy cable 4.62mm in diameter (with 16.77 mm2 cross section area), or next thicker cable.
- Diameter inches: diameter of cable, given in inches.
- Area mm2: cross section area of the cable, given in mm2.
- Area in2: cross section area of the cable, given in inch2.
- Resistance Ω per m, Copper, t=20°C/68°F: this is resistance of ideal copper cable (of corresponding thickness), 1m in length, at 20°C/68°F. Pure copper cable has resistivity coefficient of 1.68x10-8 Ωm at 20°C/68°F and temperature coefficient of 0.003862 K-1, but copper in wires/cables is not always 100% copper. In fact, to be more manageable, even copper is annealed (kind of heat treatment) and that change resistivity coefficient at 20°C/68°F to 1.72x10-8 Ωm and temperature coefficient to 0.00393 K-1. In this table, data are provided for pure copper wire at 20°C/68°F - 1.68x10-8 Ωm and 0.003862 K-1.
Long story short - don't go cheap on wires and cables.
- Max. Current @U 5% Drop, 10m (2x5m) Wire: This is calculated maximum current trough 10m wire (2x5m cable) at given voltage, with allowed 5% drop (loss) of voltage. Keep in mind that trolling motors usually have shorter cables and they usually tolerate 5% voltage drop on cables. Even so, such cables can get very warm, even hot, if trolling motor is pushed to the limit and cable is NOT dimensioned accordingly.Bow and stern thrusters usually tolerate larger voltage drops (although, these losses should be avoided, if possible), but they usually operate for shorter period of time, when compared with electric trolling motors. Engine starters operate for few seconds only, but draw huge currents, and their cables are much thicker.
- Power Loss @U 5% Drop, 10m (2x5m) Wire: this is power loss in the cable when there is 5% voltage drop. Very thick cables are used for engine starters and they draw several hundred Amperes. Not a problem for a really thick cables. On the other hand, electric boats - having electric main motor(s) - use complex DC/AC converters/controllers and have high-voltage, multiphase motor(s), thus decreasing the currents and hence, losses in cables. Again, these are calculated values for 10m (33 feet) wire - 2x5m (2x16 feet) cable. Trolling motors usually have shorter cables. If you double the length of the cable, you should use cable that has half the resistance to keep voltage and power losses at the same level. Keep in mind that longer cable, at the same power loss, will generate less heat per meter of the cable.
- Power Transfered @U 5% Drop, 10m (2x5) Wire: this is calculated transferred power to the motor (or to anything else that is electrically powered on the boat), tolerating 5% voltage drop at given voltage.
Note: all these values are calculated having in mind ideal battery - with NO internal resistance. Internal resistance of the batteries changes with many factors and it would complicate calculations even further :)
- Voltage and Power Loss, I=60A, 10m (2x5) Wire: electric trolling motors usually have maximum current in 50-55A range and have 60A circuit breaker recommended for protection. If there is need for more power, current is kept below 60A and voltage is increased from 12V to 24, 36 or even more volts. That is why we have I=60A limit in this example. Remember, this is 10m wire or 5m cable with two wires.
I promised no formulas, so, here is the table :)
|Diameter mm||Diameter inches||Area mm2||Area in2||Resistanse
Ω per m, Copper, t=20°C/68°F
|Max. Current @U 5% Drop, 10m (2x5m) Wire
||Power Loss @U 5% Drop, 10m (2x5m) Wire
||Power Transfered @U 5% Drop, 10m (2x5m) Wire
||Voltage and Power Loss, I=60A, 10m (2x5m) Wire
|6/0||14.7333||0.5800||170.4854||0.2643||98.5x10-6||@72V, 3654A||@72V, 13.1kW||249kW||0.059V, 3.6W|
|5/0||13.1203||0.5165||135.2010||0.2096||124.3x10-6||@72V, 2896A||@72V, 10.4kW||198kW||0.074V, 4.5W|
|4/0||11.6840||0.4600||107.2193||0.1662||156.7x10-6||@72V, 2297A||@72V, 8.27kW||157kW||0.094V, 5.6W|
|3/0||10.4049||0.4096||85.0288||0.1318||197.6x10-6||@72A, 1821A||@72V, 6.55kW||124kW||0.118V, 7.1W|
|2/0||9.2658||0.3648||67.4309||0.1045||249.1x10-6||@72V, 1455A||@72V, 5.23kW||99.5kW||0.149V, 8.9W|
|0||8.2515||0.3249||53.4751||0.0829||314.2x10-6||@48V, 763A||@48V, 1.83kW||34.7kW||0.188V, 11.3W|
|1||7.3481||0.2893||42.4077||0.0657||396.2x10-6||@48V, 605A||@48V, 1.45kW||27.5kW||0.237V, 14.2W|
|2||6.5437||0.2576||33.6308||0.0521||499.5x10-6||@48V, 480A||@48V, 1.15kW||21.8kW||0.299V, 17.9W|
|13||1.8278||0.0720||2.6240||0.0041||6.4025x10-3||@12V, 9A||@12V, 6W||102W||-|
|14||1.6277||0.0641||2.0809||0.0032||8.0734x10-3||@12V, 7A||@12V, 5W||79W||-|
|15||1.4495||0.0571||1.6502||0.0026||10.1804x10-3||@12V, 5A||@12V, 4W||57W||-|
|16||1.2908||0.0508||1.3087||0.0020||12.8372x10-3||@12V, 4A||@12V, 3W||45W||-|
|17||1.1495||0.0453||1.0378||0.0016||16.1874x10-3||@12V, 3.7A||@12V, 2.2W||42W||-|
|18||1.0237||0.0403||0.8230||0.0013||20.4120x10-3||@12V, 2.9A||@12V, 1.7W||33W||-|
Cables thinner than AWG 12 should not be used on boats, IMHO, they are very thin and mechanically weak. If used, AWG 12-18 cables should be used for LED lights only. Anyway, always check manuals for any given device and if in doubt for some reason, use thicker cables.
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