Marine Electrical & Electronics Articles
NMEA 2000 Spit Power Insertion
Article 03/02/2026
A common question from technicians and DIYers - Why split power for the NMEA 2000 bus
There are many reasons for this and we will look at each in turn: But first let's explain a little about what is split power insertion.
What is split power insertion?
The NMEA bus is essentially a cable with five cores. There are two data wires that are uninterrupted along the entire length with a 120 Ω resistor at each end. As the resistors are in parallel of course this gives us a resistance of 60Ω between them. These 2 resistors 'light up' with each pulse of data and stop the signal from rattling around the cable like a loud clap in an echoey room,. These data wires are not interrupted anywhere along the bus. They are simply tapped into via the drop links. They are protected by interference from the a dedicated shield wire that keeps outside interference out, and inside interference in. This is uninterrupted.
Next we have the 2 power cables, and these are the only conductors out of the 5 that can be interrupted in such a way as to be fused and switched separately, or even powered from a completely different sources, with no risk of ground loops or power loops. This is done via special power insertion points, some provide power out of one side only, and do nothing to the the otter side, allowing it to be fead from elsewhere. Some are designed to provide 2 feeds. one for one side and one for the other.
Why would we want split power insertion ?
If you use a single mid point split power insertion T piece you can fuse each half of the bus independently. So if the bus cabling could carry 4A with single power insertion you would have to fuse with a 4 A fuse the whole bus. With split power insertion you can fuse each half of the bus with a dedicated 4 amp fuse. The cable better protected and you now have double the ampacity effectively. You also have redundancy too, if theres a short inside one of the devices or sections of cable, one fuse will blow, and the other will be fine, so you still have some instruments.
NMEA 2000 Split power insertion examples:
Twin helm yachts
We may have 3 or 4 MFI's at each helm, none of which have any analogue data going to them, just NMEA 2000. A system could have each end of the bus terminating at each helm position, and the last few meters fed with a split power point, so each helm position is fused and switched separately. These instruments are vulnerable to damage so if any of them short you only lose that helm position, the rest of the bus is powered, and the other helm position has data.
It's not just about redundancy. you may be down below and not need the cockpit instruments running, so with split power insertion you can have dedicated panel for zones of the bus. Port helm, Stb Helm, etc .
One end up the mast?
Not on my installations.
I have never been a big fan of terminating the bus all the way a the top of the mast. I prefer to use anemometers with 0813 data running down the mast and then convert this to NMEA 2000 inside the boat. If you are going to run N2K masthead gear then - at least split the power off so the mast wiring is fused separately.
Flybridge motor cruisers
The instruments on the flybridge are vulnerable to the elements, and if you're operating from down below you may not want all you MFIs powered up, so split this section of the bus off with a split power insertion point and you can fuse it separately so if theres a short you dont loose the entire bus power, and you can switch off these instruments when not in use. Note that you can leave power to the MFD's if you dont want alarms going off, or you can switch power off to this too and have the master down below.
Voltage drop
Marine electronics will hold on to way below their nominal operation voltage. Bus power is rated at 12v but will work anywhere between 9v and 16v. The problem is current, and unlike resistive loads that draw less current with less voltage, electronic instruments pull MORE current with lower voltage, more current means more voltage drop and soon enough on bigger systems it pulls the voltage below the 9v lower limit near each end of the bus so those instruments shut down, whilst those near the power insertion points are fine. After the devices reach under voltage, their shut down triggers the voltage to shoot back up and they come back on!
On longer buses this leads you to believe there is an issue with some section of the bus when it's just Ohms law kicking in. With split power insertion resistance is futile. you can beat the voltage drop by reducing the current on each section.
Summary
You can make your NMEA bus more reliable, and safe by controlling how and where power is inserted. You can beat voltage drop on longer busses by running a thicker cable to take the weight further down stream. You can keep your instruments running longer if power is running out, and battery voltage is starting to drop, and if theres a cable fault, connector fault, or instrument fault that results in a shorted power circuit within the bus, only one section is effected, keeping essential systems alive.
Next step
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Marine Electrical Hazards
News Article 02/06/2025
What are the most hazardous parts of the marine electrical system?
Most boat & yacht fires are caused by the electrical system.
We are often asked at MECO training how can I make my boat safer, and the answer is really simple and inexpensive. Unfortunately many boat owners feel that their boat is safe because it complies with the relevant standards, or is brand new, however this is not the case, even boats that comply fully with the relevant standards have a few achilles heels.
1: Starter motor circuit to fuse or not to fuse…
There are no requirements under any standard to fuse the starter motor circuit and there are still people out there with no faith in modern fuses! There is absolutely no reason why you can not add an oversized fuse to a starter circuit. If the starter sticks, or the starter / cabling shorts out then a slow blow fuse could prevent a serious fire, and if the fuse is 150% oversized and it DID blow then you would have had much more to worry about! Modern starters are more efficient, and modern fuses can cope with the high inrush currents from starters. Believe it or not a 200 AMP ANL Fuse can cope with 150% of its rated current for an hour, and double its rated current for a whole minute. It’s the one circuit on board where you can oversize a fuse above the ampacity of the cables to prevent nuisance tripping. Why is this still safe? Simply put, the faults you get on starters are usually very high short circuit currents rather than slow overloads.
2: Starter and Alternator cabling.
If you’re still not convinced about point 1 above, then at the very least do this! Make sure the starter positive and starter or engine negative cables do NOT run clipped together, place each cable in its very own protective flexible conduit. Use the side split type so there’s no need to un wire anything.
Check the entire cable lengths for chaffing, and fit chaff protection.
3: Shore power
This is a very common cause of fires. There is a very common misconception that fuses and circuit breakers will prevent all fires. The truth is most boat and yacht fires are caused by loose connections, these can actually result in less current going through the circuit protection rather than more. In a poor connection the loose or dirty connection actually consumes some of the energy it should be conveying and coverts this into a highly localised heat source, enough to start a fire. This kind of fault is not detectable by normal fuses, or circuit breakers. What can you do?
3.1 After removing both ends of the shore power cable, each time check for any degradation in the terminals or plastic from heat.
3.2 Do not connect or disconnect the shore power under load. Not all connectors are designed to make or break heavy loads, and each time you do so you effectively loose material from the conducting pins. It’s best to make sure all heavy AC loads are off when connecting or disconnecting the shore power.
3.3 Every year check the terminals inside each end of the shore power connectors to make sure there is no corrosion, if the terminals have a screw that touches the strands of wire then you will need bootlace ferrules on the ends of the cable.
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