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Voltage systems, Ohms Law, and DC-DC converters

Oct 1, 2013

Voltage systems: Boats up to about 60 feet in length generally have a 12 Volt DC electrical system. This is largely due to the fact that many pieces of boating electrical gear are based on automotive/industrial electrical devices and so it is cheaper to use these mass produced items. Larger boats tend to have 24-Volt DC electrical systems for the same reason of keeping costs down and increasing savings. This may sound a bit confusing, but after we bone up on Ohms Law this seeming paradox will become clear.

Ohms Law deals with the relationship of Voltage (E), Current (I), Resistance (R) and Power (P) of an electrical circuit and if we know any two quantities we can plug them into the formula and come up with the third. The various forms of Ohms Law follow: E=IR, I=E/R, R=E/I and P=IE, I=P/E, E=P/I.

For our purposes the most important formula is the one dealing with current (I) because it will determine what size wire we need to operate safely. Notice that current in the Power formula I=P/E is a function of the power in watts divided by the system voltage, so if we double the voltage our current will be cut in half. Since electrical wire gauge is determined by its current carrying capacity, increasing voltage allows us to decrease the size of electrical cabling and save money.

Example #1: P=240 Watts, E=12 VDC so I=240/12 which equals 20 amps current, but if we go to a 24-volt system then it works out as in Example #2:  P=240 Watts, E=24 VDC so I=240/24 which equals 10 Amps current. Since a longer boat will need longer cable lengths it is doubly important to keep the cable gauge to a safe minimum or the costs will skyrocket and that is the big advantage of using a 24-VDC system versus 12 VDC.

But what if your boat has a 24-VDC electrical system and you need to install a new electronic device that is built for 12 VDC? Again the current is the problem because I=E/R and given the same resistance if you tap into 24 VDC with a device designed for 12 VDC then you will double the current and most likely smoke your electronics. This situation can be resolved in one of three ways: 1) using a device that is rated for 10 to 35 VDC, 2) tapping off 12 volts from a 24 volt system, or 3) by using a 24 Volt to 12 Volt DC-DC converter.

The simplest solution is to check the data plate on your new electronic device and hope that it is rated for 10-35 VDC in which case you could just connect it into your 24 Volt system. If this is not the case then you can tap off 12 volts from the middle of the 24-volt stack, but I suggest that this only be done on an emergency basis for the following reasons. Tapping will cause disproportionate charging of the batteries, overcharging one and undercharging the other which leads to shortened battery life. Also, you need to use the proper size wiring and fuse or circuit breaker it to protect the wiring from overheating during a short, etc. If you still want to tap off the 24-volt stack, then hook the positive conductor to where the two batteries are connected together (+ to -) and connect the ground lead to the output black lead. Again this method is not recommended.

The optimum solution to our problem is to procure and install a device known as a 24 Volt to 12 Volt DC converter. These devices are rated for peak output and continuous output current. Just make sure that the continuous output rating matches your constant load requirements. In order to cut down on any potential RF interference from a DC-DC converter make sure to buy one that is listed as FCC “Class B” which should keep interference to a minimum. A comparison price for these taken from the latest West Marine catalog is $79.99 for the 6 Amp model, $119.99 for the 12 Amp model, and $ 969.99 for the 50 Amp model. As you can see the price varies considerably depending on the continuous output current rating.

If your boat has a NMEA 2000 network it may be as easy as tapping into the 12 Volt trunk line using a tee and running a drop cable to your 12-volt device. Most newer marine electronics have provision for hooking into an NMEA network so it should be an easy installation. You can buy NMEA 2000 cables and accessories from West Marine and they even have a NMEA 2000 starter kit for only $87.99. Well, fellow sailors, enough for his time, so until we talk again: “Clear Skies and Following Seas!”

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Oct 2, 2013 02:58 pm
 Posted by  Arnold

Fine article, thank you for writing it! I've always remember Ohm's Law as W (watts) = V (volts) x A (amps).
My silly memory hook for this is War=Vetrans x Adminstration.

Oct 2, 2013 03:45 pm
 Posted by  Tim

Thanks, Arnold. Interesting memory hook!

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